M4 Junction 3 to 12 Managed Motorway All Lane Running
Preliminary Sources Study Report HA GDMS No. 27280
Report No: 514451-MUH-00-ZZ-RP-GE-200005
Revision / Status 0B
October 2013
Highways Agency — M4 J3-12 MM-ALR
M4 Junction 3 to 12 Managed Motorway All Lane Running
Preliminary Sources Study Report
HA GDMS No: 27280
October 2013
REPORT NO: 514451-MUH-00-ZZ-RP-GE-200005 REVISION SCHEDULE
Rev Date Details Prepared by Reviewed by Approved by
0A 17th June Submitted for Approval CW PCM ALC 2013
1F 18 October Final CW EM ALC 2013
Status Code and Description I Information D Draft R Review and Comment A Submitted for Approval F Final C For Construction
Highways Agency URS Infrastructure & Environment UK Ltd Major Projects Royal Court The Cube Basil Close 199 Wharfside Street Chesterfield Birmingham Derbyshire West Midlands S41 7SL B1 1RN
Highways Agency — M4 J3-12 MM-ALR
Limitations All limitations in line with the Highways Agency Project Support Framework (Consultancy) 2011 – 2015 URS Infrastructure & Environment UK Limited (“URS”) has prepared this Report for the sole use of the Highways Agency (“Client”) in accordance with the Agreement under which our services were performed [PO1149]. No other warranty, expressed or implied, is made as to the professional advice included in this Report or any other services provided by URS. This Report is confidential and may not be disclosed by the Client nor relied upon by any other party without the prior and express written agreement of URS.
The conclusions and recommendations contained in this Report are based upon information provided by others and upon the assumption that all relevant information has been provided by those parties from whom it has been requested and that such information is accurate. Information obtained by URS has not been independently verified by URS, unless otherwise stated in the Report.
The methodology adopted and the sources of information used by URS in providing its services are outlined in this Report. The scope of this Report and the services are accordingly factually limited by these circumstances.
Where assessments of works or costs identified in this Report are made, such assessments are based upon the information available at the time and where appropriate are subject to further investigations or information which may become available.
URS disclaim any undertaking or obligation to advise any person of any change in any matter affecting the Report, which may come or be brought to URS’ attention after the date of the Report.
Certain statements made in the Report that are not historical facts may constitute estimates, projections or other forward- looking statements and even though they are based on reasonable assumptions as of the date of the Report, such forward-looking statements by their nature involve risks and uncertainties that could cause actual results to differ materially from the results predicted. URS specifically does not guarantee or warrant any estimate or projections contained in this Report.
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Highways Agency — M4 J3-12 MM-ALR
TABLE OF CONTENTS 1 INTRODUCTION ...... 8 1.1 Scheme Title ...... 8 1.2 Scheme Proposals and Current Studies ...... 8 1.3 Location and Extent of Study Area ...... 11 1.4 Previous Geotechnical Studies ...... 12 2 SOURCES OF INFORMATION ...... 13 2.1 Highways Agency Geotechnical Data Management System (HA GDMS) ...... 13 2.2 Connect Plus (DBFO Area 5) ...... 13 2.3 EnterpriseMouchel (Area 3 Managing Agent) .... 13 2.4 Utility Service Providers ...... 14 2.5 British Geological Survey ...... 14 2.6 Records of Mine and Mineral Deposits ...... 15 2.7 Landmark Information Group ...... 15 2.8 The Environment Agency ...... 16 2.9 Archaeological Information ...... 16 2.10 Aerial Photographs (historical and recent) ...... 17 2.11 MAGIC & Natural England ...... 17 2.11.1 MAGIC ...... 17 2.11.2 Natural England ...... 18 2.12 Other data sources ...... 18 3 FIELD STUDIES ...... 19 4 SITE DESCRIPTION ...... 20 4.1 Location, Description and Topography ...... 20 4.1.1 Section 1 ...... 20 4.1.2 Section 2 ...... 21 4.1.3 Section 3 ...... 23 4.1.4 Section 4 ...... 23 4.2 Historical Development ...... 25 4.2.1 Section 1 ...... 25 4.2.2 Section 2 ...... 25 4.2.3 Section 3 ...... 25
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4.2.4 Section 4 ...... 26 4.3 Hydrology and flood risk ...... 26 4.3.1 Section 1 ...... 27 4.3.2 Section 2 ...... 28 4.3.3 Section 3 ...... 28 4.3.4 Section 4 ...... 28 4.4 Man-made Features ...... 29 4.4.1 Overbridges ...... 29 4.4.2 Underbridges ...... 31 4.4.3 Cross carriageway ducts (CCDs) ...... 32 4.4.4 Other man-made features ...... 34 4.5 Geomorphology ...... 35 4.6 Geology ...... 36 4.6.1 Section 1 ...... 36 4.6.2 Section 2 ...... 38 4.6.3 Section 3 ...... 41 4.6.4 Section 4 ...... 44 4.7 Hydrogeology ...... 48 4.7.1 Section 1 ...... 50 4.7.2 Section 2 ...... 50 4.7.3 Section 3 ...... 51 4.7.4 Section 4 ...... 51 4.8 Mining and Quarrying ...... 51 4.8.1 Section 1 ...... 52 4.8.2 Section 2 ...... 52 4.8.3 Section 3 ...... 53 4.8.4 Section 4 ...... 53 4.9 Landfills, geo-environmental Issues and possible contamination Issues ...... 53 4.10 Natural Cavities ...... 55 4.11 Unexploded ordnance ...... 55 4.12 Earthworks Instability ...... 55 4.13 Environmental Considerations ...... 56
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5 GROUND CONDITIONS ...... 59 5.1 Introduction ...... 59 5.2 Section 1 ...... 59 5.2.1 Made Ground ...... 59 5.2.2 Superficial deposits ...... 59 5.2.3 Bedrock ...... 60 5.2.4 Groundwater ...... 61 5.2.5 Summary of predicted engineering properties .. 61 5.3 Section 2 ...... 61 5.3.1 Made Ground ...... 62 5.3.2 Superficial deposits ...... 62 5.3.3 Bedrock ...... 63 5.3.4 Groundwater ...... 64 5.3.5 Summary of predicted engineering properties .. 65 5.4 Section 3 ...... 65 5.4.1 Made Ground ...... 65 5.4.2 Superficial deposits ...... 66 5.4.3 Bedrock ...... 67 5.4.4 Groundwater ...... 68 5.4.5 Summary of predicted engineering properties .. 68 5.4.6 Section 4 ...... 69 5.4.7 Made Ground ...... 69 5.4.8 Superficial deposits ...... 69 5.4.9 Bedrock ...... 71 5.4.10 Groundwater ...... 72 5.4.11 Summary of predicted engineering properties .. 73 6 PRELIMINARY ENGINEERING ASSESSMENT ... 74 6.1 Introduction ...... 74 6.2 Location ...... 74 6.2.1 Section 1 ...... 74 6.2.2 Section 2 ...... 74 6.2.3 Section 3 ...... 74 6.2.4 Section 4 ...... 74
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6.3 Soils ...... 75 6.3.1 Made Ground ...... 75 6.3.2 Alluvium (and old Alluvium) ...... 75 6.3.3 Langley Silt member (Brickearth) ...... 76 6.3.4 River Terrace Deposits ...... 76 6.3.5 Thames Group (London Clay Formation) ...... 76 6.3.6 Lambeth Group ...... 77 6.3.7 Upper Chalk ...... 77 6.4 Groundwater ...... 78 6.5 Cuttings ...... 78 6.6 Embankments ...... 79 6.7 At-grade ...... 79 6.8 Pavement Foundations ...... 79 6.9 Drainage ...... 80 6.10 Replacement/widening of restrictive Overbridges and Underbridges ...... 80 6.10.1 Introduction ...... 80 6.10.2 Old Slade Lane Overbridge (948) ...... 81 6.10.3 Langley Interchange East & West Underbridges (952 & 953) ...... 83 6.10.4 Riding Court Overbridge (960) ...... 87 6.10.5 Recreation Ground Overbridge (962) ...... 90 6.10.6 Datchet Road Overbridge (963) ...... 92 6.10.7 Windsor Railway Underbridge (966) ...... 95 6.10.8 Wood Lane Overbridge (971)...... 99 6.10.9 Oldway Lane Overbridge (972) ...... 102 6.10.10 Huntercombe Spur Overbridge (973) ...... 104 6.10.11 Huntercombe Lane Overbridge (974) ...... 107 6.10.12 Marsh Lane Overbridge (977) ...... 109 6.10.13 Thames Bray Underbridge (978) ...... 112 6.10.14 Monkey Island Overbridge (979) ...... 115 6.10.15 Ascot Road Overbridge (986) ...... 118 6.11 Structure Foundations ...... 121
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6.11.1 Introduction ...... 121 6.11.2 Super cantilever sign and signal gantries, cantilever sign gantries and cantilever (MS3, MS4 “hockey stick”) signal gantries ...... 121 6.11.3 Portal Frame Gantries (incl. Superspan) ...... 121 6.11.4 Emergency Refuge Areas – Retaining Walls .... 122 6.11.5 Widening of reduced width hardshouder / hard strip – Retaining Walls ...... 122 6.11.6 Central Reserve Vertical Concrete Barrier (VCB) ...... 122 6.11.7 Culverts and subways ...... 123 6.11.8 Environmental barrier ...... 123 6.12 Cross Carriageway Ducts (CCD’s) ...... 123 6.13 Contaminated Land/Soil Chemistry ...... 123 6.14 Existing Geotechnical Problems ...... 124 6.15 Effects of Man-made Obstacles/Site History .... 125 7 COMPARISON OF PROJECT OPTIONS AND RISKS ...... 126 7.1 Introduction ...... 126 8 FIGURES, DRAWINGS AND PHOTOGRAPHS .. 134 9 REFERENCES ...... 135 ANNEX A TO PRELIMINARY SOURCES STUDY REPORT
Appendix A Statement of Intent and Summary of relevant previous investigations Appendix B Historical exploratory hole logs Appendix C Utility Plans Appendix D Envirocheck reports (CD) Appendix E Summary of historical development Appendix F Summary of highway structures along the scheme Appendix G Summary of landfills along the scheme Appendix H HA GDMS instability tables Appendix I Summary of predicted engineering properties for strata along the scheme Appendix J Summary of geotechnical constraints Appendix K Figures and drawings Appendix L HA GDMS reports reviewed during this study
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1 INTRODUCTION
1.1 Scheme Title The Scheme title is ‘M4 Junction 3 to 12 Managed Motorway’ and this Preliminary Sources Study Report (PSSR) has been produced by the Halcrow Hyder Joint Venture in accordance with the proposals set out in the Statement of Intent (Issue 3) prepared by Mouchel in November 2011 a copy of which is included within Appendix A. Section 5.3 of the Statement of Intent details the geotechnical reports that will be produced in line with Highways Agency (HA) Standard HD22/08, Managing Geotechnical Risk. This report presents the findings of the Preliminary Sources Study Report (PSSR) for the scheme which covers the geological, geotechnical, geomorphological, hydrological and geoenvironmental aspects of the scheme and the contamination risks together with a preliminary engineering assessment and likely hazards to construction.
1.2 Scheme Proposals and Current Studies The M4 forms part of the Trans-European Network and is a key strategic route linking London and the M25 to the M4 corridor and South Wales and the South West. The M4 between Junction 3 and Junction 12 carries up to 16,000 vehicles per day has a total length approaching 31.75 miles (51.2km) comprising 27.9 miles of dual three lane rural motorway and 3.8 miles (6.2km) of dual four lane motorway between Junction 4 and Junction 5. The Scheme area has several sections of carriageway with discontinuous hard shoulder (mostly due to bridges) and regularly experiences very high traffic flows and severe congestion, especially around peak times. The schematic scheme plan (Figure 1 included in section 8 of this report) shows the main facets of the scheme, including the junctions, main structures, environmentally sensitive areas and local authority boundaries.
In December 2011 the outcome of a technical review for the M4 managed motorway scheme was reported to PICG / HIB based on the operational principle of Controlled All Lane Running (CALR). The review recommended staged construction, based on affordability, value for money and environmental considerations. It was estimated that the CALR scheme would deliver up to 28% peak period flow increases and would be 8% safer than the baseline. It was reviewed at SGAR1 in December 2011 and achieved green status.
In February 2013 RPSG determined that:
The scheme shall be based on the operational principle of Managed Motorways All Lanes Running (MM-ALR), in accordance with IAN161/12; with, Through Junction Running (TJR) to be provided at all junctions except those excluded by IAN161/12 (i.e. motorway-to-motorway with free-flowing link roads, and terminal junctions.); and, At Junction 4 if operational or air quality issues necessitates its exclusion.
The application of IAN161/12 to this scheme has significant implications in terms of cost and programme, predominantly as a result of the increases in structures works required to achieve TJR.
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The objective of the Scheme is to reduce congestion and delays by increasing the capacity of the motorway by making the existing hard shoulder suitable for use as a running lane and by introducing Managed Motorway Technology. The overall objectives of the Managed Motorway scheme can be summarised as follows:
The scheme should be developed in line with current Highways Agency Managed Motorway Guidance and Through Junction Running philosophy, IAN 161/12 To alleviate any existing transport and safety problems identified on the Motorway links. Once open to traffic, the project should not detrimentally affect traffic on the surrounding road network. To enhance the role of these major Motorway arteries by improving the currency and quality of information provided to drivers about the state of the motorway. To relieve congestion, improving journey time reliability, reducing the number of fatalities, casualties and incidents. To maximise the effectiveness of mitigation of environmental impacts. To result in no worsening of the overall existing environment and to consider enhancement where feasible. To provide journey time reliability benefits to traffic on these links in comparison to the “do nothing” baseline set for economic appraisal of the project. To minimise the impact of the severance of local thoroughfares for non-motorised users. To ensure that the project takes into account the capacity improvements planned on adjoining routes. To support local and regional development plans and government policy. The Scheme should make best use of existing infrastructure where possible, and aim to provide additional capacity within the existing highway boundary, where possible within the existing paved area; The scheme should be designed to suit the requirements of ongoing maintenance, the needs of Highways Agency Network Operations and aim to minimise whole life costs; The project shall aim to provide high value for money against its whole-of-life costs in accordance with the Department’s WebTAG guidance.
The key aspects of the scheme are:
All Lane Running: The Scheme will convert the existing hard shoulder into a running lane in accordance with IAN 161/12.
The existing motorway has a number of hard shoulder discontinuities resulting from structural constraints. Provision of All Lane Running (ALR) will require these constraints to be removed. Some under bridges, including the Bray Bridge over River Thames, will need to be widened and some over-bridges will need to be demolished and re-constructed. Emergency Refuge Areas will be provided in lieu of the hard shoulder.
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The existing links from Jct 4 to 4b and from Jct 4b to 5 are already widened to 4 lanes with hard shoulder. From Jct 4 to 4b the hard shoulder will be converted to a 5th running lane. From Jct 4b to 5 the hard shoulder will be retained.
Through Junction Running: Through Junction Running (TJR) of the converted hard shoulder will be provided where appropriate. Establishing the optimum TJR strategy is one of the objectives of the current design stage (Stage 2: Options). The current recommendation is to provide TJR at Junctions 4, 5, 6, 7, 8/9 and 11. TJR is not recommended at:
Junctions 3 & 12 as they are terminal junctions with standard D3M layout beyond the scheme extents with no plans to increase the number of lanes; Junctions 10 and 4b as they are motorway to motorway interchanges with high turning movements unsuitable for TJR operation
Managed Motorway: Operation of the scheme will be managed via mandatory variable speed limits. This will require construction of gantries with lane specific, variable message signs supported by motorway incident detection and automatic signalling (MIDAS) and comprehensive closed circuit television (CCTV). At present the Scheme layout is under finalisation but it is currently anticipated that the Active Traffic Management System will comprise in the order of 20 No. large verge mounted direction signs, 29 No. full span portal gantries, 61 No. MS4 cantilever gantries, 40 No. cantilever sign gantries and 66 No. ERA’s. Where appropriate and feasible existing gantries with be reused and adapted.
Structures Regime: Where existing structures have sufficient width for standard ALR and meet the requirements of IAN 161/12 then no structural alterations are proposed. Where the shortfall in width is only marginal then relaxations or departures will be proposed to avoid any structural alterations. Otherwise underbridges will require widening and overbridges will require demolition and reconstruction.
There are 11 overbridges and 14 underbridges (including some culverts) that require either demolition and replacement or significant works. A documented optimisation process has been used to derive the most appropriate structural design for each bridge taking account of as-built information, land availability, buildability, cost, diversion routes and local authority requirements. This has resulted in significant usage of retaining walls, use of partial off-line designs to enable traffic flow during construction and potential re-usage of some sub- structures.
The inclusion of a central reserve vertical concrete barrier (VCB) and associated drainage works also forms a significant part of the works. VCB is required to meet the road worker safety objective.
The signs and gantries will require new foundations whilst the ERA’s and overbridge and underbridge structures are likely to require amendments to the existing earthworks depending on their location. A general approach taken to developing the options for carriageway and embankment widening has been to use retaining walls to avoid land take. The provision of TJR requires more alteration and widening of merges, diverges and sliproads. For this reason more extensive retaining walls are required for TJR. The main purpose of the current study is therefore
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Highways Agency — M4 J3-12 MM-ALR to establish the ground and groundwater conditions along the Scheme such that the geotechnical risks can be accurately identified and managed in a staged and documented approach in accordance with HD22/08, Managing Geotechnical Risk.
1.3 Location and Extent of Study Area The eastern extent of the Scheme starts 800m east of Junction 3 of the M4 which is located at National Grid Reference (NGR) TQ 1046, 7821 and Scheme Chainage 10000. The western extent of the Scheme ends at Junction 12 of the M4 which is located at NGR SU 6511, 7148 and Scheme Chainage 62900. The scheme location is shown on Figure 1 below.
Figure 1. Scheme Location
The location of the Scheme is also shown on drawings M4MM-MUH-ML-ZZ-DR-GE-200001 to 200030 included in Section 8 of this report.
The M4 runs in a westerly direction from Scheme Chainage 10000; running north of Heathrow Airport to Scheme Chainage 16800 (Junction 4b) where it intersects with Junction 15 of the M25. The M4 continues in a westerly direction, passing north of the towns of Datchet and Eton and to the south of Slough. From Scheme Chainage 34100 (Junction J8/9) the M4 then passes south of the town of Reading; trending south westerly past Winnersh to Scheme Chainage 46150 (Junction 10) and then north easterly to Scheme Chainage 62100 (Junction 12).
Due to the length and complexity of the Scheme and variations in the horizontal and vertical alignment the Scheme has been split into four smaller more manageable sections for ease of reporting.
The breakdown of the Scheme into four sections is summarised in Table 1.1 below and is shown on Drawings M4MM-MUH-ML-ZZ-DR-GE-200001 to 200030.
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Table 1.1: Summary of sections
Scheme Section Location Section Junctions Scheme Chainage Section 1 Heston to Langley Junction 3 to Junction 5 10000 to 20150
Section 2 Langley to Holyport Junction 5 to Junction 8/9 20150 to 34100
Section 3 Holyport to Winnersh Junction 8/9 to Junction 10 34100 to 46150
Section 4 Winnersh to Theale Junction 10 to Junction 12 46150 to 62900
1.4 Previous Geotechnical Studies Data obtained from the Highways Agency Geotechnical Data Management System (HA GDMS) has shown that numerous previous investigations have been undertaken along this stretch of the M4 for various upgrade schemes. Each has been tagged with a Scheme HHJV (Halcrow/Hyder Joint Venture) 3 digit numeric identifier. Chapter 9 of this report provides a comprehensive list of all identified HA GDMS reports sorted by HA GDMS number and HHJV number. Table 1.2 (Appendix A) comprise a list of those HA GDMS studies where review has identified relevant geotechnical information for use in the design and construction of this Scheme. The table includes a brief description of the contents of each report along with the stretch of the Scheme covered.
In addition to the information made available on HA GDMS, the British Geological Survey (BGS) has made archive borehole logs available on its website (http://www.bgs.ac.uk/data/boreholescans/). A review of this website has found that a number of ground investigations that have been undertaken along the scheme are not available on HA GDMS. Table 1.3 (Appendix A) comprises a list of those ground investigations undertaken and includes a brief description of the investigation along with the stretch of the Scheme each ground investigation covers. Each has been tagged with a Scheme HHJV 3 digit numeric identifier. Chapter 9 of this report provides a comprehensive list of all identified ground investigations recorded on the BGS website sorted by HHJV number.
Copies of the relevant exploratory hole records from these investigations are included in Appendix B and the locations of the exploratory holes are shown on Drawing M4MM-MUH-ML-ZZ- DR-GE-200001 to 200030 included in Section 8 of this report. Each hole is identified on the drawings by it’s original identifier suffixed with the Scheme HHJV number defined above. For example borehole W567 included in HHJV report 102 will be noted on the drawing as W56: 102. Also included in Appendix A is two key tables for the historic exploratory hole records one (Key B1) for those sourced from HA GDMS references and one (Key B2) for those sourced from the BGS.
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2 SOURCES OF INFORMATION A detailed list of all of the sources of information consulted for this study is included in Section 9 whilst the following paragraphs summarise the most pertinent information. Interpretation of the information is provided in Chapters 4, 5 and 6 of this report.
2.1 Highways Agency Geotechnical Data Management System (HA GDMS) Section 1.4 of this report lists all reports that have been retrieved from HA GDMS which are relevant to this Scheme. Many of these reports contain detailed information on the ground and groundwater conditions along this section of motorway. They are discussed in greater detail in Chapters 4, 5 and 6 of this report.
Note that the HA GDMS database is a live system and should be regularly checked in advance of detailed design and construction for any updates. The outcome of any such checks will be recorded in GIR for the scheme.
Site specific topographic information was obtained from http://www.HAGDMS.com.
2.2 Connect Plus (DBFO Area 5) The incumbent Geotechnical Advisor for the M25 DBFO (Design, Build, Finance and Operate), Connect Plus, has not been contacted with regards to geotechnical information along the scheme route covered by their contract (M4 Junctions 3 to 5 and associated Highways Agency link roads and slip roads).
Connect Plus should be consulted as the designs for required ground investigations are being finalised and throughout the design and construction phase. This will support identification, capture and realisation of additional opportunities afforded by potential integration with their forward programme of earthworks renewals and repairs. The outcome of tany such consultations with Connect Plus will be detailed in the GIR produced for the scheme. Effective consultation will also ensure that the M4 J 3 – 12 MMALR team remain fully informed of any new defects or repairs that may impact on that scheme and that Connect Plus remain fully appraised of the M4 MMALR teams activities.
2.3 EnterpriseMouchel (Area 3 Managing Agent) The incumbent Geotechnical Advisor for the Managing Agent Contractor (MAC), EnterpriseMouchel, was contacted (telephone conversation on 22nd April 2013) with regards to geotechnical information along the scheme route covered by their Area 3 contract (M4 Junctions 5 to 12 and associated Highways Agency link roads and slip roads). It was noted that recent earthworks inspections for the scheme (November 2012 – March 2013) have been uploaded to HA GDMS (refer to Section 2.1) and should be current. However, several defects were in the process of being repaired and they would be altered to Class 3A (repaired defect, refer to HD41/03) in the near future. The Geotechnical Asset Management Plan (GAMP) covering the scheme route would be available soon, following approval by the Highways Agency, and should detail the forward 5 year plan of earthworks renewals and repairs.
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It was also noted during discussion that the existing MAC arrangement would cease operation in November 2013. A new Asset Support Contract (ASC) would commence the network maintenance in Area 3 thereafter. It has recently been announced by the Highways Agency that EnterpriseMouchel will run the ASC contract in Area 3 until 2018. The network maintenance provider for this section of the scheme route should be consulted as the designs for required ground investigations are being finalised and throughout the design and construction phase. This will support identification, capture and realisation of additional opportunities afforded by potential integration with their forward programme of earthworks renewals and repairs. The results of any such consultation with EnterpriseMouchel will be detailed in the GIR for the scheme. Effective consultation will also ensure that the M4 J 3 – 12 MMALR team remain fully informed of any new defects or repairs that may impact on that scheme and that the network maintenance provider remains fully appraised of the M4 MMALR teams activities.
2.4 Utility Service Providers The results of enquires made by Mouchel (the employers services adviser for the Scheme), with utility service providers regarding the presence of their equipment and plant in the locality of the Scheme have been summarised in Drawings M4MM-MUH-ML-ZZ-DR-GE-200031 to 200060 which are included in Appendix C of this report.
2.5 British Geological Survey Desk study geological information has been obtained from the following 1:50,000 BGS Map Sheets, associated memoirs and guidance documents:
1:50,000 Sheet 270 South London (Solid and Drift Edition, 1981) 1:50,000 Sheet 269 Windsor (Solid and Drift Edition, 1999) 1:50,000 Sheet 268 Reading (Solid and Drift Edition, 2000) Ellison, R A & Williamson, I T, 1999. Geology of the Windsor and Bracknell district – a brief explanation of the geological map. Sheet Explanation of the British Geological Survey. 1:50,000 Sheet 269 Windsor (England and Wales). Mathers, S J & Smith, N J P, 2000. Geology of the Reading district – a brief explanation of the geological map. Sheet Explanation of the British Geological Survey. 1:50,000 Sheet 268 Reading (England and Wales). Hight, D W; Ellison, R A & Page, D P, 2004. Engineering in the Lambeth Group. CIRIA C583.
Since the publication of the above referred to geological maps and memoirs, the chrono- stratigraphic terminology for some of the strata has changed. The geological maps and memoirs have therefore been crossed referenced with the BGS’s online Lexicon of named rock units at http://www.bgs.ac.uk/lexicon to ensure the usage of current terminology. As detailed in Chapter 1 of this report the BGS has made archive borehole logs available on its website (http://www.bgs.ac.uk/data/boreholescans/). A search of this website has been undertaken for the study area and relevant borehole records have been obtained.
Details of the geology underlying the Scheme is contained in chapter 4 of this report.
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2.6 Records of Mine and Mineral Deposits A search of the BGS ‘Mining Plans Portal’ website (http://www.bgs.ac.uk/nocomico/) has been undertaken to determine the existence of any non coal mines in the study area.
The Mining Instability in Great Britain Volume 1/ii, which was prepared by Arup in December 1991, provides information on mining risk in the UK.
Historical and current Ordnance Survey (OS) mapping has also been reviewed to locate any relevant mines or quarries in the study area.
BGS Mineral Resources mapping has been reviewed to identify mineral resources and areas of active or inactive extractions. Three maps cover the area of the Scheme:
Berkshire (McEvoy et al., 2003) Buckinghamshire (Benham et al., 2003a) Hertfordshire and NW London Boroughs (Benham et al., 2003b).
A review of the solid geology of the study area (by consideration of published BGS maps) shows that coal bearing strata is not present in this area and therefore coal mining will not have taken place. As such, a Coal Authority report has not been obtained for the Scheme.
2.7 Landmark Information Group Given that the proposed Scheme comprises works at shallow depth within the confines of the existing motorway corridor, it was not considered necessary to procure a Scheme specific Envirocheck report. Mouchel (the employers adviser for the Scheme) however has made a number of Scheme specific historical Ordnance Survey (OS) plans they procured from the Landmark Information Group available to review as part of this PSSR. Mouchel also procured the following Envirocheck reports:
Envirocheck report ref. 26013375-1-1 which covers the Scheme between Junction 3 to Junction 4 (published 05/08/2008). Envirocheck report ref. 26013377-1-1 which covers the Scheme between Junction 4 to Junction 5 (published 05/08/2008). Envirocheck report ref. 20689418-1-1 which covers the Scheme from Junctions 5 to Wood Lane Overbridge (located in between Junctions 5 and 6 at Scheme Chainage 27200) (published 07/12/2006). Envirocheck report ref. 20689417-1-1 which covers the Scheme from Wood Lane Overbridge (located in between Junctions 5 and 6 at Scheme Chainage 27200) to Junction 8/9 (published 07/12/2006).
A number of historical reports available on HA GDMS also contain historical OS maps procured from the Landmark Information Group which cover discrete sections of the Scheme. Where the data coverage of the OS plans provided by Mouchel is sparse, these maps have been reviewed to provide additional information. Historical OS plans were obtained from the following HA GDMS reports:
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HA GDMS Report 17740 – contains Envirocheck report ref. 153425-3-1 which covers the Scheme between Junction 4 and Junction 5 (published 06/08/2002). HA GDMS Report 17740 - contains Envirocheck report ref. 153425-3-1 which covers the Scheme between Junction 5 and Datchet Road (published on 06/08/2002). HA GDMS Report 20120 – contains Envirocheck report ref. 5432271-1-2 which covers the Scheme between Junction 8/9 and Junction 10 (published 02/08/2004). HA GDMS Report 20563 – contains Envirocheck report ref. 9883951-4-2 which covers the Scheme between Junction 10 and Junction 12 (published 24/03/2005).
The findings of the historical review are discussed in detail in Section 4.2 of this report. Copies of the plans obtained are enclosed in electronic format on disc in Appendix D.
2.8 The Environment Agency Information on hydrology, hydrogeology and flood risk was obtained from the Environment Agency’s (EA) ‘Whats in your back yard?’ website (http://www.environment-agency.gov.uk/wiyby). The flood maps consulted show that along majority of the Scheme there are no flood defences located in the vicinity of the M4 corridor. This is with the exception of the stretch between Chainage 24400 and 27000 (where flood defences are located less than 100 meters to the south of the M4) and at Chainage 30300 where River Thames passes beneath the M4. Flood defences are also located approximately 500m to the south of the M4 between Chainage 27000 and 30300.
The published flood risk maps calculate the flood risk from 1 in 100 year and 1 in 1000 year flood events. These maps show that a number of areas along the Scheme are at risk from flooding arising from various watercourses that dissect the Scheme. These areas discussed further in Section 4.3 of this report
Ordnance Survey (OS) maps indicate that several watercourses and reservoirs are present in the immediate vicinity of the Scheme, some of which are named and others which are unnamed other than “drains”. The named watercourses are listed in Section 4.3 of this report and where these lie in close proximity to the Scheme their locations are shown on the Drawings in Chapter 8.
Hydrology and flood risk is discussed further in Section 4.3 whilst Section 4.7 covers hydrogeology.
2.9 Archaeological Information Archaeological information relevant to this scheme and within 250m of the carriageway has been extracted from the Technical Appraisal Report (TAR) produced by Mouchel in 2011 (Mouchel, 2011b). The TAR also contains a number of figures detailing the locations of archaeological sites which have also been reviewed.
One Scheduled Ancient Monument, a moated site at Chippenham Court, has been identified 200m north of the eastbound carriageway boundary between Junctions 6 and 7 (between Chainages 26100 and 29000).
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Forty two listed buildings have been identified within 250m of the M4 carriageway, fourteen of these are considered to be sensitive to potential development within the carriageway. These include a group of nine listed buildings which are associated with the former Cranford House estate, the Grade I listed church of St Peter and St Paul, a Grade II listed milestone, Grade II Ditton Farmhouse and associated Grade II barn and Grade II Riding Court Farmhouse.
Within the Technical Appraisal Report, two Registered Parks and Gardens dating to the post medieval period have been identified as being Grade II Registered; Ditton Park which abuts the northern boundary of the carriageway west of J5 and Grade II Herschel Park which is located approximately 50m north of the carriageway boundary east of J6.
There are seven Conservation Areas located within the study area, namely Upton, Cranford Park, Harlington Village, Datchet Village, Holyport, Huntercombe and Sindlesham. They were all designated Conservation Areas due to their historical and artistic merits and all contain a number of listed buildings.
A number of upstanding undesignated heritage assets cross the M4 carriageway. These include the Slough station to Windsor/Eton branch railway and Reading West to Theale Railway Line. Other undesignated assets which are within the highway boundary but are not upstanding monuments or do not survive, include; Billingbear Park Deer Park, a Roman Road, a number of prehistoric finds, a moated site and medieval settlement, Cranford medieval settlement and the possible site of Cresswells Manor. Anmers Farm to Pingewood settlement and Mill Dam to Brick Kiln Copse Cropmark features were also identified through aerial photograph interpretation.
No World Heritage Sites have been identified within the 250m buffer zone.
As the Scheme is confined to the existing highway boundary, there is unlikely to be any effect on archaeological features caused by the proposed works. Further details on relevant archaeological sites and consultation with an archaeological consultant however will be required during the ground investigation and detailed design stages of the project.
2.10 Aerial Photographs (historical and recent) Historical aerial photographs were not formally reviewed as part of this study though modern digital aerial photographs were sourced from http://www.HAGDMS.com and www.bing.com/maps. Historical aerial imagery that is available to view on the Google Earth website (http://earth.google.co.uk) via a commercial licence was also consulted. The aerial photographs only highlighted one recent change to the Scheme in comparison to the OS maps that being significant development of Junction 11.
2.11 MAGIC & Natural England
2.11.1 MAGIC
MAGIC (Multi-Agency Geographic Information for the Countryside) is a partnership project of bodies who have responsibilities for rural policy-making and management involving DEFRA (Department for Environment, Food and Rural Affairs), English Heritage, Natural England, the Environment Agency, Forestry Commission, Communities and Local Government and the Marine Management Organisation. Its website (www.magic.gov.uk) provides a web-based interactive map summarising the available information on UK designated Sites of Special Scientific Interest (SSSI),
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Highways Agency — M4 J3-12 MM-ALR internationally important sites (including World Heritage Sites and Ramsar sites) and biodiversity sites (including National and Local Nature Reserves). The interactive map shows that along the Scheme there are few special zones located adjacent to or within the close vicinity of the M4. These special zones include Nitrate Vulnerable Zones and Local and National Nature Reserves which are detailed in Section 4.13 of this report. 2.11.2 Natural England
A further source of environmental information is the Natural England website (www.natureonthemap.naturalengland.org.uk), which in similarity to MAGIC contains information on UK designated Sites of Special Scientific Interest (SSSI), internationally important sites (including Special Areas of Conservation), biodiversity sites, and Local Geological Sites (formally known as Regionally Important Geological Sites or RIGS). Details of findings are included in Section 4.13 of this report.
2.12 Other data sources Digital versions of 1:50,000 Ordnance Survey (OS) maps were sourced from www.bing.com/maps and images from Google’s Street View were consulted in relation to the location, description and topography along the length of the Scheme.
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3 FIELD STUDIES No field studies have been undertaken as part of this study.
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4 SITE DESCRIPTION
4.1 Location, Description and Topography The location and extent of the Scheme is summarised in Section 1.3 of this report which also details how the Scheme is broken down into four sub sections for ease of reporting purposes. The location, description and topography of each sub section are detailed below. 4.1.1 Section 1
Junction 3 to Junction 5 (Scheme Chainage 10000 to 20150)
Table 4.1 below provides a description of the site and it’s topography within Section 1 of the scheme. The topography is generally flat with a consistent elevation of around 25m above ordnance datum (AOD).
Table 4.1: Section 1 site description and topography
Junction Chainage Description and topography reference East of 10000 to Scheme Chainage commences approximately 800m east of Junction 3 10800 Junction 3 but limited works are proposed until chainage 10800 (ie Junction 3). Junction 3 10800 Junction 3 of the M4 intersects with the A312. Junction 3 is a grade separated junction, with the main carriageway and the slip roads constructed on embankments. The junction is surrounded by open land. Junction 3 to 10800 to Immediately west of Junction 3, the main carriageway and the slip Junction 4 13900 roads cross the River Crane at Chainage 11000. Between J3 and J4, the M4 is a dual three lane motorway. The M4 runs in a westerly direction along this section and the earthworks are at grade. The area to the north is predominantly urban (residential) whilst the area to the south is predominantly open land, with Cranford Countryside Park lying to the south of the M4 between Chainages 11300 and 12000. A possible gravel pit exists to the south of the M4 between Chainages 12700 and 13900. The M4 crosses over a railway tunnel at Chainage 13750. Junction 4 13900 Junction 4 connects the M4 with the A408 and is a grade separated interchange. The slip roads are generally constructed on embankment, as is the main carriageway whilst the roundabout is formed at grade. Junction 4 to 13900 to Between Junction 4 and Junction 4b, the M4 is a dual four lane Junction 4b 16600 motorway running approximately east to west. Predominantly urban (residential) areas bound the M4 to the north whilst open land generally bounds the M4 to the south. At Chainage 14600, the eastbound and westbound carriageways diverge until Chainage 15600 where they re-converge again. The westbound carriageway is formed in cutting on the left hand side and a bund front on the right hand side. The eastbound carriageway has a bund front and a bund back to the left hand side. The M4 passes north of Saxon Lake between Chainage 15500 and 15900. The M4 crosses over the
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Table 4.1: Section 1 site description and topography
Junction Chainage Description and topography reference River Colne at Chainage 16 000 and Wraysbury River at Chainage 16150. Junction 4b 16600 Junction 4B of the M4 connects the M4 with Junction 15 of the M25 motorway. The main carriageway through the junction is dual three lane motorway. The junction is predominantly constructed on embankment and at grade sections of earthworks. There are several short sections of cutting and one stretch of bund front and bund back on the M4 eastbound off slip to the M25 clockwise.
Junction 4b to 16600 to Between Junction 4b and Junction 5, the M4 is dual four lane Junction 5 20150 running approximately east to west. The carriageway is constructed on embankment up to Chainage 18400, beyond this point the motorway is constructed at grade up to Junction 5. Immediately after Junction 4b, Old Slade lake is present to the south of the M4 between Chainages 17200 and 17600 followed by the Old Slade Lane Sewage Works. A gravel pit is also located to the south of the M4 between Chainages 18500 and 19200. The area to the north of the motorway is dominated by a golf course between Chainages 18500 and 19200. Between Chainages 19200 and 20150 located at Junction 5, the land either side of the motorway is predominantly urban. Junction 5 20150 Junction 5 is a grade separated interchange with the A4. The main carriageway is formed on embankment along with the westbound on slip and the eastbound off slip. The westbound off slip, the eastbound on slip and the roundabout have been constructed at grade. The vicinity around Junction 5 is surrounded by predominantly urban developments.
4.1.2 Section 2
Junction 5 to Junction 8/9 (Scheme Chainage 20150 to 34100)
Table 4.2 below provides a description of the site and it’s topography within Section 2 of the scheme. The topography is generally flat with a consistent elevation of around 25m above ordnance datum (AOD). Section 2 lies within the Thames Valley.
Table 4.2: Section 2 site description and topography
Junction Chainage Description and topography reference Junction 5 20150 Junction 5 is a grade separated interchange with the A4. The main carriageway is formed on embankment along with the westbound on slip and the eastbound off slip. The westbound off slip, the eastbound on slip and the roundabout have been constructed at grade. The vicinity around Junction 5 is surrounded by predominantly urban developments.
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Table 4.2: Section 2 site description and topography
Junction Chainage Description and topography reference Junction 5 to 20150 to Between Junctions 5 and 6 the M4 is a dual three lane motorway Junction 6 26100 running in a south westerly direction to Chainage 21900. After this point the motorway runs in a north westerly direction to Junction 6. The motorway is predominantly constructed at grade up to Chainage 24600. West of this point the M4 is constructed on embankment up to Junction 6. The Queen Mother Reservoir is present to the south of the M4 between Chainages 20300 and 21700 whilst open land is present to the north of the motorway. Either side of the motorway is surrounded by open between Chainages 21700 and 24600. Between Chainages 24600 and 26100, the M4 is bounded by Slough to the north and open land and the Jubilee River to the south. The M4 crosses a railway line at Chainage 25550. Junction 6 26100 Junction 6 of the motorway intersects with the A355 as a grade separated junction. The main carriageway, the westbound off slip and the eastbound on slip are formed on embankment whilst the westbound on slip and the eastbound off slip are formed on at grade earthworks. Junction 6 is bounded to the north by Slough and to the south by open land and the Jubilee River. Junction 6 to 26100 to Between Junction 6 and Junction 7 the M4 is a dual three lane Junction 7 29000 carriageway running in a north westerly direction. This section of the M4 is constructed at grade. The Scheme is predominantly bound to the north by an urban (residential) area and to the south by open land and the Jubilee River. Between Chainages 27400 and 28400, the Thames Water Management Works is immediately south of the M4 corridor. Junction 7 29000 Junction 7 of the M4 is a grade separated interchange connecting the M4 and the A4. The junction is formed on embankments and at grade earthworks and is predominantly surrounded by open land. Junction 7 to 29000 to Between Junction 7 and Junction 8/9 the M4 is dual three lane Junction 8/9 34100 carriageway generally running in a south westerly direction. Between Chainages 29000 and 30300 the M4 is constructed at grade. Between Chainages 30300 and 31250 the M4 is on at grade and embankment earthworks and surrounded predominantly by open land. The M4 crosses the Jubilee River at Chainage 30300 whilst at Chainage 31250 the M4 crosses the River Thames. Bray Lake is present to the south of the M4 between Chainage 31600 and 32100. Between Chainage 31250 and Junction 8/9 the M4 is on a combination of at grade earthworks and embankments. The M4 is bound by urban areas between Chainages 34400 and 33300 and then open land up to Junction 8/9. Junction 8/9 34100 Junction 8/9 is a grade separated junction which connects the M4 with the A404 (M) and the A308 (M). The main carriageway runs through the junction at grade with the slip roads and roundabout constructed on embankments. The junction is surrounded by open land.
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4.1.3 Section 3
Junction 8/9 to Junction 10 (Scheme Chainage 34100 to 46150)
Table 4.3 below provides a description of the site and it’s topography within Section 3 of the scheme. The elevation of the motorway is variable across Section 3 increasing from approximately 25m AOD at Chainage 34100 to approximately 55m AOD at approximately Chainage 44700. The elevation of the motorway decreases westwards to Chainage 46150, where it is approximately 45m AOD.
Table 4.3: Section 3 site description and topography
Junction Chainage Description and topography reference Junction 8/9 34100 Junction 8/9 is a grade separated junction which connects the M4 with the A404 (M) and the A308 (M). The main carriageway runs through the junction at grade with the slip roads and roundabout constructed on embankments. The junction is surrounded by open land. Junction 8/9 to 34100 to Between Chainages 34100 and 46150 the M4 is a dual three lane Junction 10 46150 motorway running south westerly through open land. Between Chainage 34100 and 37500 the motorway is constructed predominantly on at grade earthworks and embankment with one 500m length of cutting. The M4 is constructed at grade between Chainages 37500 and 41100. From Chainage 41100 to Junction 10 the motorway is formed on a combination of at grade, embankment and cutting. Junction 10 46150 Junction 10 connects the M4 with the A329 (M). The junction is formed on a combination of at grade, embankment, cutting and bund front earthworks and is surrounded by open land.
4.1.4 Section 4
Junction 10 to Junction 12 (Scheme Chainage 46150 to 62900)
Table 4.4 below provides a description of the site and it’s topography within Section 4 of the scheme. The elevation of the motorway is variable across Section 4 increasing from Chainage 46150, where the elevation is approximately 45m AOD, to an approximate maximum level of 65m AOD at Chainage 52700. The elevation decreases to approximately 40m AOD towards Chainage 54800 (Junction 11) before increasing again to approximately 50m AOD at Chainage 62100 (Junction 12).
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Table 4.4: Section 4 site description and topography
Junction Chainage Description and topography reference Junction 10 46150 Junction 10 connects the M4 with the A329 (M). The junction is formed on a combination of at grade, embankment, cutting and bund front earthworks and is surrounded by open land. Junction 10 to 46150 to The M4 is dual three lane motorway between Junction 10 Junction Junction 11 54800 11 running in a westerly direction. Between Chainage 46150 and 47150 the M4 is constructed on embankments which are surrounded by open land. At Chainage 47150 the M4 crosses a railway line. Between Chainage 47150 and 49050 the motorway runs in a westerly direction and is predominantly constructed on embankment. This section of the M4 is bound to the north by Winnersh and to the south by a combination of open land and urban areas of Wokingham. The M4 continues in a westerly direction between Chainage 49050 and 51800 through open land on embankment and at grade earthworks. West of Chainage 51800, the M4 trends south westerly on a combination of at grade, embankment and cutting earthworks, between Reading to the north and Shinfield to the south. Junction 11 54800 Junction 11 is a grade separated junction connecting the M4 with the A33. The main carriageway through the junction is at grade with the slip roads being constructed on embankment. The city of Reading is located to the north of the junction whilst open land is present to the south. Junction 11 to 54800 to Between Junction 11 Junction 12 the M4 is dual three lane Junction 12 62900 carriageway running in a north westerly direction. The M4 between Chainage 54800 and 57150 is constructed on a combination of at grade and embankment earthworks. This section of the motorway is bound to the north by urban areas and to the south by open land. From Chainage 57150 to Chainage 62100 the M4 is constructed on at grade earthworks and then embankments approaching Junction 12. Between Chainages 56800 and 61400 various water bodies are present to the north and south of the M4. These are thought to be historic gravel pits which have been flooded. Between Chainage 57950 and 58500 an active sand and gravel pit is located north of the M4. The M4 crosses the River Kennet a Chainage 61100 and then Holy Brook at Chainage 61250. At Chainage 61400 the M4 crosses a railway line. Approaching Junction 12 the M4 passes south of Reading and north of Theale. Junction 12 62100 Junction 12 is a grade separated junction which connects the M4 with the A4. The majority of the junction is constructed on embankment. Reading is located to the north of the junction and Theale to the south. West of 62100 to West of Junction 12 the M4 trends in a north westerly direction Junction 12 62900 through open land on embankments and at grade to the western extent of the Scheme at Chainage 62900.
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4.2 Historical Development Sections 4.2.1 to 4.2.4 below describe the historical development of each Section of the Scheme. 4.2.1 Section 1
Junction 3 to Junction 5 (Scheme Chainage 10000 to 20150)
Within Section 1 of the Scheme, some urbanisation/industrial use has occurred close to Junction 5 with a number of warehouses (including the Royal Mail Heathrow Worldwide Distribution Centre and Honda UK Headquarters) being located to the north east and south east of the junction. The land to the north of the M4 between Junctions 3 and 4b has undergone a substantial increase in urbanisation (at present part of West Drayton).
The majority of the area between Junctions 3 and 5 is underlain by floodplain deposits and has therefore been subjected to extensive gravel works in the past, the majority of which have been backfilled more recently with landfill materials. Areas corresponding to adopted and un-adopted green belt land are located south of the M4 between Junction 3 and 4b and east of Sutton between Junctions 4b and 5.
Table 4.5 (Appendix E) provides a summary of the historical development of the study area based on OS maps (1:10,560 and 1:10,000 scale) dated between 1881 and 1999. Map sources are detailed in Chapter 2. 4.2.2 Section 2
Junction 5 to Junction 8/9 (Scheme Chainage 20150 to 34100)
Urbanisation adjacent to Section 2 of Scheme has taken place over the years, particularly in the area north of the M4 and to the east of the Junction 8/9. The land to the south of Section 2 remains predominantly agricultural in nature. The motorway between Junctions 5 and 8/9 was originally constructed as the Slough/Maidenhead by-pass in the early 1960’s which comprised a dual, two lane carriageway. In the early 1970’s the road was subsequently widened to dual three way motorway standard.
Several watercourses cross the M4 corridor within Section 2 and two large reservoirs lie in close proximity to the M4 corridor. There are sewage works adjacent to the westbound carriageway of the motorway between Junctions 6 and 7.
Table 4.6 (Appendix E) provides a summary of the historical development of the study area based on OS maps (1:10,560 and 1:10,000 scale) dated between 1881 and 2003. Map sources are detailed in Chapter 2. 4.2.3 Section 3
Junction 8/9 to Junction 10 (Scheme Chainage 34100 to 46150)
There has not been any significant urbanisation of the area surrounding Section 3 of the Scheme. At the present day, Section 3 of the Scheme is flanked by agricultural land and substantial stretches of woodland.
Several waterbodies of various sizes are present alongside Section 3, some of which resulted from previous extraction works related to construction of the M4.
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No existing or closed landfill sites have been located within the route corridor of Section 3 of the Scheme. No records were found of any historical mining activities or gravel extraction works (other than related to the construction of the M4) along this section.
Table 4.7 (Appendix E) provides a summary of the historical development of the study area based on OS maps (1:10,560 and 1:10,000 scale) dated between 1881 and 1999. Map sources are detailed in Chapter 2. 4.2.4 Section 4
Junction 10 to Junction 12 (Scheme Chainage 46150 to 62900)
Section 4 of the Scheme was constructed in the early 1970's. It is located on gently undulating land with an elevation of between 25m and 50mAOD. Land adjacent to the motorway comprises predominantly of agricultural land with some areas of woodland. In several locations built up areas are present to the north of motorway between Junctions 10 and 11 whilst mineral extraction sites lie adjacent to motorway (to the north and south of the M4 corridor) between Junctions 11 and 12.
Over the years substantial quantities of flint, which would be suitable for construction purposes, was extracted from the flood plain gravels. Extraction of this material was concentrated in the area between Junctions 11 and 12. An extensive network of pits lie either side of the motorway, some of which are still in use to the present day whilst others have been either flooded or backfilled with landfill material. No mining activities have been recorded.
Table 4.8 (Appendix E) provides a summary of the historical development of the study area based on OS maps (1:10,560 and 1:10,000 scale) dated between 1883 and 1999. Map sources are detailed in Chapter 2.
4.3 Hydrology and flood risk The Scheme falls within the Thames River Basin District. The drainage is dominated by the River Thames in the west of the study area and the River Colne in the east of the area (HA GDMS report 3269). A summary of the major watercourses crossed by the M4 within the study area is given in Table 4.9 below.
Table 4.9: Summary of major watercourses within the study area
Watercourse Chainage Scheme sub section
River Crane 10975 Section 1
Frogsditch Culvert 12055 Section 1
River Colne 16000 Section 1
Wraysbury River 16150 Section 1
Bigley Ditch Culvert 16443 Section 1
Table 4.9: Summary of major watercourses within the study area
Watercourse Chainage Scheme sub section
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Colne Brook 17170 Section 1
Horton Brook 18400 Section 1
Datchet Common Brook 21540 Section 2
Salthill Stream 25700 Section 2 Chalvery Ditches 26850 Section 2
Roundmoor Ditch and Boveley Ditch 30000 Section 2
Jubilee River 30300 Section 2 River Thames 31250 Section 2
The Cut 32070 Section 2 The Bourne 33800 Section 2
The Cut 35570 Section 3
Emm Brook 46450 Section 4 River Loddon 50400 Section 4
Foudry Brook 56540 Section 4
Clayhill Brook 59830 Section 4
River Kennet 61140 Section 4
Holy Brook 61250 Section 4
Table 4.10 (Appendix F) contains details of all under and over structures as well as retaining walls where a structure number has been assigned by the Highways Agency. This includes culverts but may not be comprehensive particularly in respect of pipes with a diameter of less that 600mm. Site inspections should be undertaken prior to finalisation of all Managed Motorway ITS infrastructure and completion of associated ground investigations and potential piling operations to prevent damage to any recorded or unrecorded understructures.
Sections 4.3.1 to 4.3.4 below describe the hydrological conditions of each Section of the Scheme by reference to the hydrological and flood risk data obtained from the Environment Agency’s (EA) ‘Whats in your back yard?’ website (http://www.environment-agency.gov.uk/wiyby). 4.3.1 Section 1
Junction 3 to Junction 5 (Scheme Chainage 10000 to 20150)
Section 1 crosses seven named major watercourses as listed in Table 4.9. HA GDMS report 3269 describes that an area of poor drainage exists either side of the M4 carriageway between Chainages 16800 and 20150. The report outlines the findings of a National Rivers Authority (now EA) study which suggested the area of poor drainage may be due to the backing up of
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The flood risk map obtained for Section 1 from the EA website indicates that flooding from the River Crane from 1 in 100 year and 1 in 1000 year flood events would abut the westbound on slip and eastbound off slip carriageways around Junction 3. A 1 in 1000 year flood event would result in flooding from Frog’s Ditch between Chainage 11800 and 12100, adjacent to the westbound carriageway of the M4.
Extensive areas of flooding from 1 in 100 year and 1 in 1000 year flood events are indicated around Junction 4b of the M4 associated with the River Colne, Wraysbury River, Colne Brook and Horton Brook. The majority of the flooding risk areas associated with the above watercourses are shown to occur to the north and south of the M4 carriageway however some flooding is shown to occur within the M4 corridor. 4.3.2 Section 2
Junction 5 to Junction 8/9 (Scheme Chainage 20150 to 34100)
Section 2 of the Scheme crosses eight named major watercourses as summarised in Table 4.9. Between Chainage 24400 and 27100, the Jubilee River runs parallel to the M4, approximately 80m south of the westbound carriageway. Chalvery Ditches runs parallel to the M4, approximately 50m south of the westbound carriageway between Chainages 25700 and 26850 at which point the ditch passes beneath the M4. Between Chainages 31600 and 32400 several water bodies are located to the south of the M4. It is believed that these are possibly old gravel pits which have subsequently been flooded.
Throughout Section 2 of the Scheme, extensive areas at risk of flooding from 1 in 100 year and 1 in 1000 year flood events are indicated which are associated with the watercourses described above. Frequently the flooded areas include the motorway corridor itself. 4.3.3 Section 3
Junction 8/9 to Junction 10 (Scheme Chainage 34100 to 46150)
Throughout Section 3, the M4 only crosses one named major watercourse, The Cut, at Chainage 35570 (see Table 4.9). Numerous ditches/drains are present either side of the M4 corridor between Chainages 39000 and 41100.
Flooding from 1 in 100 year and 1 in 1000 year flood events associated with The Cut watercourse is indicated between Chainages 35500 and 35600. To the north of the eastbound carriageway between Chainages 38300 and 39000, a 1 in 1000 year flood risk area is indicated, associated with Twyford Brook. In the same area to the south of the carriageway flooding from a 1 in 100 year flood from The Cut is shown to abut the carriageway. From a 1 in 1000 year flood the area is shown to cross the carriageway. An area of flooding from a 1 in 1000 year flood event generated by a minor river is indicated to cross the M4 carriageway between Chainage 42900 and 43500.
4.3.4 Section 4
Junction 10 to Junction 12 (Scheme Chainage 46150 to 62900)
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Section 4 of the Scheme crosses six named major watercourses as listed in Table 4.9. The area to the south of the M4 is crossed by numerous drainage channels between Chainages 51100 and 53200. Numerous waterways and water bodies are shown either side of the M4 corridor between Chainages 56000 and 62100. These are likely to be related to past sand and gravel excavations in this area which have since been flooded.
Flooding from Emm Brook is indicated to abut the carriageway and cross the carriageway in places immediately west of Junction 10. The flood risk is from 1 in 100 year and 1 in 1000 year flood events, but only a 1 in 1000 year flood event would result in flood waters extending into the M4 corridor itself. Between Chainages 49000 and 51200, extensive flood areas are shown either side of the M4 corridor from a 1 in 100 year flood event and across the M4 corridor itself from a 1 in 1000 year flood event. This is associated with the River Loddon.
Extensive areas of flooding either side of the M4 (and across the M4 in frequent locations) are shown between Chainages 54800 and 62900 from both 1 in 100 year and 1 in 1000 year flood events. These are a result of the numerous drains and watercourses that exist along this stretch of Section 4.
4.4 Man-made Features Man-made features existing along the scheme that are likely to have an impact on any widening proposals include bridges, retaining walls, culverts and cross carriageway ducts (CCDs). Table 4.10 (Appendix F) contains details of all under and over structures as well as retaining walls where a structure number has been assigned by the Highways Agency. This includes culverts (see section 4.3) but may not be comprehensive particularly in respect of pipes with a diameter of less that 600mm. Site inspections should be undertaken prior to finalisation of all Managed Motorway ITS infrastructure and completion of associated ground investigations and potential piling operations to prevent damage to any recorded or unrecorded structures. 4.4.1 Overbridges
The structures on the M4 were built throughout the 1960’s and early 1970’s to accommodate a dual two-lane motorway. When the motorway was later widened to three-lanes the structures were generally not modified. Between Jct 4b to 8/9 this has resulted in the hard shoulder being discontinuous through a number of overbridges and 11 bridges (see Figure 1 in Chapter 8) will need to be demolished and replaced before the hard shoulder can be converted into a running lane.
The overbridge replacement options are heavily influenced by requirements for managing traffic over the side road, raising the side road for improved headroom and deeper construction depths and access for construction. The preferred option is generally a single span steel composite deck built on line with the side road closed to traffic, via a diversion, for the duration of the construction. To address the different constraints at each crossing other solutions have been identified that have significant benefits and will be considered further in the next stage, these include: 2 span bridges to reduce construction depths, re-use of existing sub structures and fully off line solutions.
A summary of the 11 overbridges that will require replacement is given in Table 4.11 below.
Table 4.11: Summary of overbridges requiring replacement
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Overbridge Chainage Location Scheme Summary of preferred solution sub section Old Slade 17559 Between Section 1 Old Slade Lane Bridge will be constructed during Lane junctions temporary road closure. Both traffic and services will be 4b and 5 diverted during the construction phase. The existing structure will be demolished and replaced on line by a new single span composite deck supported on full height abutments. Riding Court 22448 Between Section 2 Riding Court Bridge will be constructed during temporary junctions road closure. The substructure however will be retained in 5 and 6 order to be used as supports for the new bridge. The traffic will be diverted temporarily during the construction phase. The proposed superstructure consists of a new two span composite deck constructed on line. Recreation 23919 Between Section 2 Recreation Ground Bridge will be constructed during Ground junctions temporary road closure. The traffic will be diverted during 5 and 6 the construction phase. The existing structure will be demolished and replaced on line by a new single span composite deck supported on full height abutments. Datchet Road 24279 Between Section 2 Datchet Road Bridge will be constructed partially off line to junctions east. Two lanes of traffic will be maintained during 5 and 6 construction. Part of the new structure will be constructed off line to the east of the existing bridge. Traffic will then be diverted to the partially built structure allowing demolition of the existing bridge and completion of the new. The new structure will comprise a single span composite deck supported on full height abutments. Wood Lane 27314 Between Section 2 Wood Lane Bridge will be constructed fully off line to the junctions east of the existing structure. This will be achieved within 6 and 7 existing land boundaries. However, this will require higher retaining walls. The existing structure will be demolished following the completion of the proposed bridge. The proposed superstructure consists of a new single span composite deck supported on full height abutments. Oldway Lane 28436 Between Section 2 Oldway Lane Bridge will be constructed during temporary junctions road closure. The traffic will be diverted temporarily during 6 and 7 the construction phase. The existing structure will be demolished and replaced by a new single span composite deck supported on full height abutments. Huntercombe 29022 Between Section 2 Huntercombe Spur Bridge will be constructed on line while Spur junctions maintaining two lanes of traffic. Parts of the existing 6 and 7 structure will be demolished to allow for phase 1 of the new bridge to be constructed. Traffic and utilities will be diverted to the partially constructed bridge allowing the demolition of further elements of the existing structure and completion of the new build. The new structure will comprise a two span steel-concrete deck supported on existing abutments and piers. Huntercombe 29544 Between Section 2 Huntercombe Lane Bridge will be constructed during Lane junctions temporary road closure. The traffic will be diverted to Marsh 7 and 8/9 lane during the construction phase. The existing structure will be demolished and replaced by a new single span composite deck supported on full height abutments. Marsh Lane 30621 Between Section 2 Marsh Lane Bridge will be constructed during temporary junctions road closure. The traffic will be diverted to Huntercombe 7 and 8/9 lane during the construction phase. The existing structure will be demolished and replaced by a new single span composite deck supported on full height abutments. Table 4.11: Summary of overbridges requiring replacement
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Overbridge Chainage Location Scheme Summary of preferred solution sub section Monkey Island 31604 Between Section 2 Monkey Island Lane Bridge will be constructed off line to Lane junctions the west of the existing structure. Traffic and Utilities will be 7 and 8/9 kept on the old bridge until the new bridge is complete. After diversions the old bridge will be demolished. The proposed superstructure consists of a new single span composite deck supported on full height abutments. Ascot road 33342 Between Section 2 Ascot Road Bridge will be constructed partially off line to junctions the east of the existing structure. Traffic will be maintained 7 and 8/9 in both directions during the construction phase. Half of the new single span composite bridge will be constructed off line to the east and will be completed following the full demolition of the existing bridge. The proposed superstructure consists of a new single span composite deck supported on full height abutments.
4.4.2 Underbridges
A number of existing underbridges on the M4 were built to accommodate only a 2 lane and hard shoulder motorway. Some structural alterations are now needed before the existing hard shoulder can be converted into a running lane.
A total of 82 underbridges including culverts, underpasses and subways have been identified within the extents of the scheme (see Table 4.10 in Appendix F). A total of 8 structures have been identified as requiring widening and a further 6 have been identified as needing or likely to need minor works involving upgrades to the existing parapets or adjustments to existing Sistema barrier systems.
A summary of the preferred solution for each of the 14 affected bridges is summarised in Table 4.12 below.
Table 4.12: Summary of underbridges requiring structural alterations
Structure Name Chainage Location Description Work Proposals
Sipson Road 14364 West of 2.5m span RC pedestrian 3.6m south side asymmetric Subway Junction 4 subway widening Langley I/C East 20050 Junction 5 3 span post-tensioned integral 4.2m north and south symmetric box girder widening
Langley I/C 20105 Junction 5 2.9m span RC pedestrian 3.5m north and south symmetric Subway subway widening
Langley I/C West 20256 Junction 5 3 span post-tensioned 4.2m north and south symmetric integral box girder widening
Ashleys Arch 21539 West of 1.5m dia. concrete pipe North headwall and parapet Culvert Junction 5 upgrade Water Main 23761 Junction 5- 2.7m span RC box Widening by 2.8m north and Subway Junction 6 3.1m south
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Table 4.12: Summary of underbridges requiring structural alterations
Structure Name Chainage Location Description Work Proposals
Water and Gas 24322 Junction 5- 3.9m span RC box Widening by 2.1m north and Main Subway Junction 6 1.4m south Windsor Branch 25566 East of J6 5 span (equal 14-15m skew) 15.1m south side asymmetric Railway 33o skewed prestressed box widening beam infill deck slab Thames Bray 31240 West of 3 span (82.3m main, 11.6m 9m south side asymmetric Bridges Junction 8/9 side) profiled steel girder widening composite deck
A308 Windsor 32716 West of 3 span (23.3m main, 13.11m Parapets upgrade Road Junction 8/9 side) skew 35o. RC slab deck.
Southern Region 47142 West of Single 12.2m span, 31o Parapets upgrade to H4a Winnersh Junction 10 skewed prestressed beam and infill deck
Mortimer Line 57168 West of Single 11.5m span, Sistema barrier modification Railway Junction 11 prestressed beam and infill deck River Kennet 61156 East of Single 40.9m span, skew 25o. Central reserve deck/ edge Junction 12 Steel girder composite deck cantilever works Western Region 61448 East of Single 10.2m span, 45o Sistema barrier modification Theale Junction 12 skewed prestressed beam and infill deck
4.4.3 Cross carriageway ducts (CCDs)
The following table summarises the locations of cross carriageway ducts associated with motorway communications, lighting etc. that are present along the motorway. Unfortunately despite extensive enquiries it has not been possible to obtain records between Junctions 3 and 8 and thus Table 4.13 below includes only those CCDs between Junctions 8 and 12.
Table 4.13: Summary of cross carriageway ducts
Chainage Chainage Marker post Marker post Scheme sub section (carriageway A) (carriageway B) (carriageway A) (carriageway B) 34660 34663 45/6 +11A 45/6 + 11B Section 3
35097 35096 46/0 +24A 46/0 +24B Section 3
36519 36520 47/5 -33A 47/5 -33B Section 3
38139 38140 49/1 -12B 49/1 -12B Section 3
39236 39235 50/2 -17A 50/2 -17B Section 3
39636 39634 50/6 -22A 50/6 -22B Section 3
40520 40522 51/4 +72A 51/4 +72B Section 3
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41066 41065 52/0 +26A 52/0 +18B Section 3
41675 41676 52/6 +33A 52/6 +33B Section 3
42232 42229 53/2 -18A 53/1 +86B Section 3
43147 43147 54/1 -04A 54/1 -04B Section 3
43477 43479 54/4 +32A 54/4 +20B Section 3
44147 44147 55/1A 55/1B Section 3
45018 45020 55/9 +56A 55/9 +59B Section 3
45439 45441 56/4 +14A 56/4 +14B Section 3
45625 45625 56/6 -27A 56/6 -27B Section 3
45839 45839 56/7 +91M 56/7 +91M Section 3
45947 45962 56/9 +10A 56/9 +10B Section 3
46318 46317 57/3 -22A 57-3 -22B Section 4
46656 46656 57/6 -27L 57/6 -27L Section 4
46944 46939 57/9A 57/9B Section 4
47274 47273 58/2 +28A 58/2 +37B Section 4
48007 48008 58/9 +55A 58/9 +51B Section 4
48908 48917 59/8 +52A 59/8 +70B Section 4
50606 50606 61/5 +45A 61/5 +62B Section 4
52292 52295 63/2 +44A 63/2 +56B Section 4
53079 53079 64/0 +39A 64/0 +39B Section 4
53619 53618 64/5 +79A 64/5 +79B Section 4
54604 54604 65/5+60M 65/5+60M Section 4
54833 54833 65/7 +88A 65/7 +86B Section 4
55958 55958 66/9+23A 66/9 +23B Section 4
57095 57095 68/0 +50A 68/0 +50B Section 4
57452 57450 68/4 +03A 68/4 +03B Section 4
58037 58037 68/9 +90A 68/9 +90B Section 4
58450 58448 69/4 +01A 69/4 +01B Section 4
59207 59209 70/1 +60A 70/1 +61B Section 4
59726 59728 70/6 +83A 70/6 +83B Section 4
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60517 60518 71/4 +70A 71/4 +70B Section 4
61342 61342 72/3A 72/3 +01B Section 4
61511 61509 72/4 +71A 72/4 +63B Section 4
61860 61860 72/8 +20M 72/8 +20M Section 4
62141 62139 73/1A 73/1B Section 4
62389 62389 73/3 +30L 73/3 +30L Section 4
4.4.4 Other man-made features
In addition to the motorway itself and its associated infrastructure (including overbridges, underbridges, approach embankments, gantries, culverts), the M4 corridor contains a number of other man-made features in the form of flood defences, railway lines, tunnels, artificial bodies of water, pylons and adjacent parallel roads.
The EA’s ‘Whats in your back yard?’ website (http://www.environment-agency.gov.uk/wiyby) records flood defences adjacent to the eastbound carriageway of the motorway at Chainage 15100. Flood defences are recorded alongside the Jubilee River running parallel to the M4 motorway (approximately 100-150 meters to the south) between Chainages 24300 and 27000. The EA’s website also shows flood defences associated with the Jubilee River passing underneath the M4 at Chainage 30279 (Thames Flood Channel Underbridge). The Jubilee River is a hydraulic channel opened in 2002 and constructed as part of the EA’s Maidenhead, Eton and Windsor Flood Alleviation Scheme (MEWFAS).
Railway lines cross the motorway in four places along the Scheme, at Chainages 25574, 47147, 57164 and 61450. An underground railway tunnel linking Heathrow Airport to the mainline railway crosses the M4 at Chainage 13760. A further underground feature, St Peters Subway, is located at Chainage 12474.
A number of pylons adjacent to the motorway can be found at Chainage 18000, between Chainages 34300 and 24700, at Chainage 38640 and between Chainage 40400 and 41000. Details of the location of these and all other utility apparatus are shown on the drawings in Appendix C.
Several major and minor roads exist directly adjacent to the M4, running parallel to the carriageway, which may constrain expansion of the existing motorway. The adjacent Shepiston Lane approaches the M4 from the north between Chainage 12500 and 13100. A number of minor access roads related to an urban area are shown running parallel to the north side of the M4 between Chainage 14200 and 15700 around 100 meters from the carriageway. Between Chainage 20700 and 22200 the M4 is flanked to the north and south by two parallel running roads, Riding Court Road and Major’s Farm Road respectively. At Chainage 23400 to 23600 Datchet Road comes within the 100 meters to the west of the motorway. Thames House access road between Chainage 27350 and 27450 approaches from the south to within 50m of the M4. Also, between Chainage 47800 and 48150 London Road approaches the M4 from the north to within 50m of the carriageway. Sections of the M4 between Chainage 51200 and 53000 and Chainage 53700 and 54400 are contained to the north by Lower Earley Way and the B3270 respectively.
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The locations of other man-made features present along the Scheme, such as gravel pits, artificial ponds and reservoirs are discussed in Section 4.3 (Hydrology and flood risk) and 4.8 (Mining and quarrying) of this report. Housing areas and industrial developments neighbouring the M4 motorway are discussed in Section 4.2 (Historical Development) of this report.
4.5 Geomorphology The M4 trends approximately westerly directon through the study area over subdued topography. Elevations range from an approximate minimum elevation of 25mAOD between Junction 3 and Junction 8/9 to an approximate maximum elevation of 65mAOD between Junction 10 and Junction 11.
HA GDMS report 3269 provides a useful geomorphological summary of the study area on which the following paragraphs are based.
The area lies on the north western flank of the London Basin syncline within the broad expanse of the Middle Thames Valley, throughout which the evolution of the present day landscape (ie geomorphological history) dates from the waning of the Alpine orogeny.
The details of this landscape have been shaped by at least three periods of glaciation, during which the northern ice caps extended as far south as the Chiltern escarpment (ie to within 20 kilometers of the M4 corridor). These advancing and retreating ice caps produced isostatic movements, crystal warping and cycles of rapid deposition of glacial, periglacial and riverine sediments alternating with periods of rapid channel and sheet erosion of these sediments and even of bedrock. The arctic conditions near the ice sheets caused the transportation of finely divided rock waste by gale force winds, so that the superficial deposits of the area include material known as loess. Deposits of the wind-transported fine sand and silt are marked on geological maps as ‘brickearth’.
Between Juntions 8/9 and 4b the M4 Motorway runs in a general east/west direction along a depression first created by the ‘proto Thames’ when it was diverted from its original more northerly course (through the Vale of St Albans) by a barrier of ice of the northern glaciations. This caused the river to cut a new more south-easterly channel into ‘bedrock’ of Eocene (London Clay). It has since meandered many times, therefore widening and deepening this channel into the form of an open valley, and depositing a series of Terrace gravels at successively lower levels along its flanks.
Specifically Junction 4b (M4/M25) lies at the former confluence of the River Colne (Colnbrook) with the Thames in its post-glacial diverted valley. The Thames has subsequently migrated some 7km further south to its present location near Staines.
West of Junction 4b the M4 Motorway approaches the north bank of the present meandering course of the river until it crosses it at Bray approximately 1km east of Junction 8/9. Although not within sight of the Thames for any of this length (except for the crossing point) the route is entirely over the former courses of the river. Both the geology and the geomorphology of the alignment are therefore determined by the palaeogeography of the river and its tributaries. In geotechnical terms this means that the M4 runs over recent Alluvium of the river Colne (a Thames tributary) in the east and further west it runs over Terrace Gravels (Boyn Hill, Taplow and Flood-plain) of the River Thames, all of these having been deposited within former, now buried channels.
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4.6 Geology Sections 4.5.1 to 4.5.4 below describe the recorded geology of each Section of the Scheme based on the data collated from sources identified in Chapter 2. 4.6.1 Section 1
Junction 3 to Junction 5 (Scheme Chainage 10000 to 20150)
The strata that are anticipated to be encountered along Section 1 are summarised in Table 4.14 and are described in further detail in the sub-sections below.
Table 4.14: Summary of expected geology within Section 1
Formation Name Formation Age Lithology*
Made Ground Recent Made Ground associated with the motorway pavement construction and embankment fill, comprising of well compacted, locally obtained material, is expected to be encountered along the route. The thickness of this material depends on the height of the earthwork. Alluvium Flandrian Normally soft to firm, consolidated, compressible silty clay, but can contain layers of silt, sand, peat and basal gravel. A stronger, desiccated surface zone may be present. Langley Silt Member Devensian to Post-Anglian Varies from silt to clay, commonly yellow-brown and massively bedded. Shepperton Gravel Member Devensian Gravel with clay and sand. (River Terrace Deposits) Taplow Gravel Formation Wolstonian Sand and gravel, locally with lenses (River Terrace Deposits) of silt, clay or peat.
London Clay Formation Eocene Fine, sandy, silty clay/silty clay. Glauconitic at base.
* Information taken from BGS GeoIndex (2013) except Made Ground
4.6.1.1 Superficial Deposits
a. Made Ground
The 1:50 000 BGS Map Sheet 269 Windsor (Solid and drift Edition, 1999) does not generally record areas of Made Ground along the motorway alignment. Made Ground however will be present associated with the motorway pavement construction and any embankment fill. The embankments are thought to have been constructed of well compacted, locally won material, such as River Terrace Deposits, London Clay and deposits of the Lambeth Group (Mouchel, 2011b). The thickness of this material will depend on the height of the earthwork and the nature of the material at formation.
At Junction 4B, between Chainages 16400 and 17400, the motorway is recorded to cross an area of infilled ground, possibly relating the interchange with the M25 motorway and associated slip/link
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b. Alluvium
Alluvium deposits encountered within Section 1 of the Scheme are coincident with the locations of the River Crane, River Colne, Wraysbury River, Colne Brook and to the east of Horton Brook. Between Chainages 15700 and 16500 there are extensive deposits of alluvium north of the M4 corridor and to a lesser extent to the south of the M4 associated with the River Colne, Wraysbury River and Colne Brook. The 1:50000 BGS Map Sheet 269 Windsor (Solid and drift Edition, 1999) and the associated memoir (Ellison and Williamson, 1999) do not record any details on the composition of the alluvium. The memoir (Ellison and Williamson, 1999) however does note that in abandoned channels thin deposits of peat up to 1m thick have accumulated within the alluvium. Further information from the BGS GeoIndex (2013) records the alluvium in this area as normally soft to firm, consolidated, compressible silty clay. It notes that layers of silt, sand, peat and basal gravel also occur.
c. Langley Silt
Between Chainages 10000 to 10700 and 11500 to 15500, the 1:50000 BGS Map Sheet 269 Windsor (Solid and drift Edition, 1999) records the outcropping superficial deposits as Langley Silt. The map notes that this material is typically sandy clay and silt. The memoir (Ellison and Williamson, 1999) indicates that the Langley Silt blankets most of the River terrace Deposits in the area and comprises brown silt or “brickearth”, ranging from a few centimetres to over 2m thick. They also note that the material is generally thought to be windblown, possibly reworked during river flooding, with some colluvial components. The map sheet records several areas where the brickearth has been worked out alongside the motorway alignment, exposing the lower River Terrace Deposits (see Shepperton Gravel Member below). These areas appear to have then been infilled.
d. Shepperton Gravel Member
The Shepperton Gravel Member (the first terrace of the River Terrace Deposits of the River Thames and its tributaries) is recorded to outcrop at Chainages 16400 to 17300 and 18300 to 19000 on the 1:50000 BGS Map Sheet 269 Windsor (Solid and drift Edition, 1999). The exposure at Chainages 16400 to 17300 may be related to construction of the interchange with the M25 motorway and the removal of the recent alluvial deposits. The map also shows several areas of exposure south of the M4 motorway between Chainages 16400 to 18900, possibly due to gravel extraction and removal of the recent alluvium. The memoir (Ellison and Williamson, 1999) indicates that the deposit is predominantly comprised of angular flint. Further information from the BGS GeoIndex (2013) records the material as gravel with sand and clay.
e. Taplow Gravel
The Taplow Gravel (the third terrace of the River Terrace Deposits of the River Thames and its tributaries) is recorded on the 1:50000 BGS Map Sheet 269 Windsor (Solid and drift Edition, 1999) as outcropping between Chainages 10700 to 10900, 11100 to 11500 and 18900 to 20150. The map notes the River Terrace Deposits are mainly comprised of gravels. The memoir covering the sheet area (Ellison and Williamson, 1999) records the Taplow Gravel as the third post-diversionary
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Thames, Blackwater and Loddon terrace (post diversionary is following the diversion of the River Thames to its present valley approximately 0.5 million years ago). The memoir indicates that the Taplow Gravel is predominantly made up of angular flint. Further information from the BGS GeoIndex (2013) suggests that the material is generally sand and gravel, with local lenses of silt, clay and peat.
4.6.1.2 Solid Geology
The solid geology underlying Section 1 of the Scheme, as recorded on the 1:50 000 BGS Map Sheet 269 Windsor (Solid and drift Edition, 1999), is the London Clay Formation of the Thames Group. The memoir covering the sheet area (Ellison and Williamson, 1999) notes that the London Clay Formation and the Harwich Formation together comprise the Thames Group. The Harwich Formation comprises the sandy beds at the base of the London Clay Formation which were previously referred to as the “London Clay Basement Bed” (Ellison and Williamson, 1999). The London Clay Formation is described as dark grey clay with subordinate silt and fine sand (Ellison and Williamson, 1999). Ellison and Williamson (1999) also note that beds with calcareous cementstone and pyrite occur, with the pyrite producing selenite crystals in the weathered zone of the London Clay Formation through reaction with acidic groundwater. The BGS Map Sheet 269 Windsor (Solid and drift Edition, 1999) records the London Clay as being between 87m and 111m thick. The London Clay Formation is shown to rest unconformably on strata belonging to the Lambeth Group.
A description of the London Clay included in the Technical Appraisal Report (Mouchel, 2011b) records the material as comprising stiff or very stiff blue-grey silty clay with local beds, partings or lenses of fine sand. The report also notes that weathering near the ground surface may result in the London Clay becoming firm or firm to stiff and mottled blue-grey and brown. Information contained within HA GDMS Report 20433, which has been extracted from the geological memoirs, suggests that the London Clay Formation may also contain subordinate thin glauconitic sands and pebble beds.
Bedrock is recorded to be at (or close to) ground level at Junction 3 (Chainage 11000) and between Chainages 18400 and 19000.
4.6.1.3 Geological Structure
The 1:50 000 BGS Map Sheet 269 Windsor (Solid and drift Edition, 1999) does not record any faults within Section 1 of the Scheme. The map does not provide any information on the dip of any the strata. Ellison and Williamson (1999) however note that knowledge on structures in concealed strata within the district is poor. They also note that exposed strata (on the map sheet) the northern limb of the London Basin, a synclinorium with a roughly east-west axis. These strata are recorded to dip at <1° towards the south-east. The memoir (Ellison and Williamson, 1999) also states that a regional set of north-west trending faults are present with several mapped in the Windsor district.
In addition, the geological structure of this section has been reviewed using the BGS dataset within the HA GDMS website. The dataset does not record any faults, seams or mass movement deposits within this section of the study area. 4.6.2 Section 2
Junction 5 to Junction 8/9 (Scheme Chainage 20150 to 34100)
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The strata that are anticipated to be encountered along Section 2 are summarised in Table 4.15 and are described in further detail in the sub-sections below.
Table 4.15: Summary of expected geology within Section 2
Formation Name Formation Age Lithology*
Made Ground Recent Made Ground comprising of well compacted, locally obtained material, associated with the motorway pavement construction and embankment fill, is likely to be found along the route of the scheme. This material has variable thickness depending on the type of earthwork present. Alluvium Flandrian Normally soft to firm, consolidated, compressible silty clay, but can contain layers of silt, sand, peat and basal gravel. A stronger, desiccated surface zone may be present. Older Alluvium Flandrian Normally soft to firm, consolidated, compressible silty clay, but can contain layers of silt, sand, peat and a basal gravel. A stronger, desiccated surface zone may be present. Shepperton Gravel Member (River Devensian Gravel with clay and sand. Terrace Deposits) Kempton Park Gravel Formation Devensian Sand and gravel, locally with lenses (River Terrace Deposits) of silt, clay or peat.
Taplow Gravel Formation (River Wolstonian Sand and gravel, locally with lenses Terrace Deposits) of silt, clay or peat.
London Clay Formation Eocene Fine, sandy, silty clay/silty clay. Glauconitic at base.
Lambeth Group Paleocene Variable: the component formations are Upnor Formation, Reading Formation and Woolwich Formation. * Information taken from BGS GeoIndex (2013) except Made Ground
4.6.2.1 Superficial Deposits
a. Made Ground
The 1:50 000 BGS Map Sheet 269 Windsor (Solid and drift Edition, 1999) does not record areas of Made Ground directly beneath the motorway footprint with the exception of a section between Chainages 33500 to 34100 (Junction 8/9). As noted in Section 4.6.1.3, Made Ground should be anticipated within the majority of the motorway corridor, associated with the motorway pavement construction and any earthworks. Embankments through this Section are thought to have been constructed of well compacted, locally won material, such as River Terrace Deposits, London Clay and deposits of the Lambeth Group (Mouchel, 2011b).
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The 1:50 000 BGS Map Sheet 269 Windsor (Solid and drift Edition, 1999) records areas of infilled ground, disturbed ground, Made Ground and worked ground adjacent to the westbound and eastbound carriageways.
b. Alluvium
Within this section, deposits of alluvium (recent) are recorded associated with Datchet Common Brook, Salt Hill Stream, Chalvey Ditches, the River Thames, The Cut and The Bourne. The 1:50 000 BGS Map Sheet 269 Windsor (Solid and drift Edition, 1999) and the associated memoir (Ellison and Williamson, 1999) do not record any details on the composition of the alluvium. However, the memoir does note that in abandoned channels thin deposits of peat up to 1m thick have accumulated within the alluvium. The BGS GeoIndex (2013) records the material as normally soft to firm, consolidated, compressible silty clay, but can contain layers of silt, sand, peat and basal gravel.
c. Older Alluvium (Alluvium 1)
The 1:50 000 BGS Map Sheet 269 Windsor (Solid and drift Edition, 1999) records older alluvium in Section 2 of the Scheme between Chainages 27400 to 30300 and 31200 to 32000. The older alluvium is a similar material to alluvium (recent) but found in abandoned floodplains, slightly elevated above present flooding levels (Ellison and Williamson, 1999). The BGS GeoIndex (2013) describes this material as normally soft to firm, consolidated, compressible silty clay, with layers of silt, sand, peat and basal gravel.
d. Shepperton Gravel Member (River Terrace Deposits)
The 1:50 000 BGS Map Sheet 269 Windsor (Solid and drift Edition, 1999) records deposits of Shepperton Gravel between Chainages 20300 to 21500, 21600 to 23900, 24800 to 25700, 27100 to 27400 and 30300 to 31200. The Shepperton Gravel is the first terrace of the post-diversionary Thames, Blackwater and Loddon River Terrace Deposits. It predominantly comprises angular flint gravel (Ellison and Williamson, 1999). In addition, the material is described as gravel with clay and sand by the BGS GeoIndex (2013).
e. Kempton Park Gravel Formation (River Terrace Deposits)
Between Chainages 32900 and 33700 the 1:50 000 BGS Map Sheet 269 Windsor (Solid and drift Edition, 1999) records deposits of Kempton Park Gravel. This the second post-diversionary river terrace deposit of the River Thames and its tributaries. Again, it is predominantly recorded to comprise angular flint gravel (Ellison and Williamson, 1999).
f. Taplow Gravel Formation (River Terrace Deposits)
The 1:50 000 BGS Map Sheet 269 Windsor (Solid and drift Edition, 1999) records Taplow Gravel between Chainages 20150 and 20300. The Taplow Gravel is the third post-diversionary River Terrace Deposit of the River Thames and its tributaries. The memoir (Ellison and Williamson, 1999) indicates that the Taplow Gravel is predominantly made up of angular flint.
4.6.2.2 Solid Geology
a. London Clay Formation
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The London Clay Formation is as described in Section 4.6.1.2.
b. The Lambeth Group
Deposits of the Lambeth Group are recorded on the 1:50 000 BGS Map Sheet 269 Windsor (Solid and drift Edition, 1999) between Chainages 24000 to 27100 and between Chainages 28500 and 34100 (end of Section 2). The Lambeth Group comprises the Upnor Formation and the Reading Formation. The authors of the memoir (Ellison and Williamson, 1999) note that in the Windsor district these deposits are usually undifferentiated on borehole logs, with the strata being colloquially referred to as “Reading Beds”. The Lambeth Group is noted to be between typically 22m and 28m thick in the area covered by the map sheet but may reduce in thickness to the west.
The Upnor Formation, formerly known as the “Reading Formation Bottom Bed”, predominantly comprises medium grained, variably pebble, glauconitic sand and is up to 6m thick (Ellison and Williamson, 1999). The deposit contains variable amounts of glauconite grains and sporadic beds of flint pebbles (Hight et al, 2004).
The Reading Formation is predominantly comprised of colour mottled clays with subordinate silt and fine to medium grained sand (Ellison and Williamson, 1999). The colour of the material is variable and includes pale brown, pale grey-blue, dark brown, pale green, red-brown and crimson depending on the oxidation state of the material (Hight et al, 2004). The clay contains numerous fissures resulting in a blocky texture and thinly laminated beds of brown silt and sand are also present, which may constitute up to 50% of the formation (Hight et al, 2004). The distribution of the sand beds is variable and unpredictable (HA GDMS report 20433). Within the study area thin, black, carbonaceous clays are recorded locally in the middle of the Reading Formation sequence (Hight et al, 2004).
4.6.2.3 Geological Structure
The 1:50 000 BGS Map Sheet 269 Windsor (Solid and drift Edition, 1981) does not record any faults or dip details of the bedrock for this section of the scheme. The regional geological structure is as described in Section 4.6.1.3.
To the south of the M4 motorway between Chainages 22800 and 27000, an anticlinal feature, known as the Windsor anticline, is shown. This presents outcrops and subcrops of the Upper Chalk (see section 4.6.4.2 below) and Lambeth Group (see section 4.6.2.2 above) deposits at the near surface, through localised folding and/or faulting.
Hight et al (2004) notes that the Lambeth Group has been gently folded on a regional scale, with a dip of generally less than 1°.
In addition, the geological structure of this section has been reviewed using the BGS dataset within HA GDMS website. The dataset does not record any faults, seams or mass movement deposits within this section of the study area.
4.6.3 Section 3
Junction 8/9 to Junction 10 (Scheme Chainage 34100 to 46150)
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The strata that are anticipated to be encountered along Section 3 are summarised in Table 4.16 and are described in further detail in the sub-sections below.
Table 4.16: Summary of expected geology within Section 3
Formation Name Formation Age Lithology*
Made Ground Recent Made Ground likely to be encountered along the route of the scheme is well compacted, sourced locally, has variable thickness and is associated with motorway pavement construction and embankment fill. Alluvium Flandrian Normally soft to firm, consolidated, compressible silty clay, but can contain layers of silt, sand, peat and basal gravel. A stronger, desiccated surface zone may be present. Older Alluvium Flandrian Normally soft to firm, consolidated, compressible silty clay, but can contain layers of silt, sand, peat and a basal gravel. A stronger, desiccated surface zone may be present. River Terrace Deposits 4 Devensian Sand and gravel, locally with lenses of silt, clay or peat.
River Terrace Deposits 5 Devensian Sand and gravel, locally with lenses of silt, clay or peat.
Head Quaternary Polymict deposit: comprises gravel, sand and clay depending on upslope source and distance from source. Poorly sorted and poorly stratified deposits formed mostly by solifluction and/or hillwash and soil creep. Essentially comprises sand and gravel, locally with lenses of silt, clay or peat and organic material. London Clay Formation Eocene Fine, sandy, silty clay/silty clay. Glauconitic at base.
Lambeth Group Paleocene Variable: the component formations are Upnor Formation, Reading Formation and Woolwich Formation. * Information taken from BGS GeoIndex (2013) except Made Ground
4.2.3.1 Superficial Deposits
a. Made Ground
The 1:50 000 BGS Map Sheet 269 Windsor (Solid and drift Edition, 1999) records an area of Made Ground beneath the motorway between Chainages 34100 and 34500 at Junction 8/9. The 1:50 000 BGS Map Sheet 268 Reading (Solid and drift Edition, 2000) records areas of Made Ground, none of which is located beneath the M4 corridor. Made Ground associated with the motorway
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Highways Agency — M4 J3-12 MM-ALR construction however is expected along Section 3. Where embankments have been constructed they are thought to be made up of locally won material.
b. Alluvium
An area of recent alluvium is also recorded around Junction 8/9 at Chainage 34100 associated with The Cut watercourse. The 1:50 000 BGS Map Sheet 269 Windsor (Solid and drift Edition, 1999) notes the alluvium to generally comprise sand, silt and clay. The BGS GeoIndex (2013) provides further detail on the alluvium describing it as normally soft to firm, consolidated, compressible silty clay, with layers of silt, sand, peat and basal gravel.
c. Older Alluvium
On the 1:50 000 BGS Map Sheet 269 Windsor (Solid and drift Edition, 1999) a deposit of Older Alluvium is recorded between Chainages 38800 and 39000. This is associated with Twyford Brook that used to flow east to west across the M4 alignment but subsequent to the motorway construction has been cut off with water now flowing to the north down a new stream, The Cut. The memoir (Ellison and Williamson, 1999) covering the section of the works does not record any details on the composition of the alluvium but it does note that in abandoned channels thin deposits of peat up to 1m thick have accumulated within the alluvium.
d. River Terrace Deposits
The 1:50 000 BGS Map Sheets covering the Section 3 of the Scheme records the superficial material to comprise isolated bodies of River Terrace Deposits between Chainages 41500 to 41700, 41900 to 42200, 44400 to 44600 and 44700 to 44900. These are recorded as the fourth (Lynch Hill Gravel) and fifth (Boyn Hill Gravel) post-diversionary River Terrace Deposits of the River Thames and its tributaries. The map also records the Plateau Gravel as being current- bedded alternations of gravel and sand representing high and ancient terraces.
e. Head Deposits
Head deposits are recorded between Chainages 34700 to 34800 and 36100 to 36300 on the 1:50 000 BGS Map Sheet 269 Windsor (Solid and drift Edition, 1999). The memoir (Ellison and Williamson, 1999) states that Head deposits have formed from periglacial and downwash activity since the Devensian. The thickness is recorded as being between a few centimetres to over 2m. On lower slopes the Head generally comprises sand and clay with variable amounts of gravel, but in the south of the district the Head comprises a high proportion of gravel and occurs in solifluction lobes. Hight et al (2004) reports that any slopes formed within the Reading Formation of the Lambeth Group (see section 4.6.2.2) are likely to be mantled by 1-3m of Head deposits which contains shear surfaces. Where Head deposits overlay clays of the Reading Formation, the clay beneath the Head is likely to be brecciated and softer than the clay at depth.
4.6.3.2 Solid Geology
a. London Clay Formation
The Lambeth Group is as described in Section 4.6.2.2 whilst the London Clay Formation is as described in Section 4.6.1.2.
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b. Lambeth Group
The 1:50 000 BGS Map Sheet 269 Windsor (Solid and drift Edition, 1999) and the 1:50 000 BGS Map Sheet 268 Reading (Solid and drift Edition, 2000) cover Section 3 of the Scheme. The BGS map sheets record that from Chainages 34100 to 35300 and 35400 to 36600, the solid geology comprises strata of the Lambeth Group. Between Chainages 35300 to 35400 and 36600 to 46150 (Junction 10) the solid geology is recorded as the London Clay Formation of the Thames Group.
Superficial deposits are generally absent beneath the scheme between Chainages 34100 and 46150 indicating that bedrock is at, or very close to, ground level. Hight et al (2004) notes that due to the general absence of superficial deposits, the bedrock deposits may have been subject to periglacial activity. As the Lambeth Group is thinly bedded and contains water-bearing sands, it is highly susceptible to disturbances caused by ground ice and cryoturbation.
4.6.3.3 Geological Structure
The 1:50 000 BGS Map Sheet 269 Windsor (Solid and drift Edition, 1999) and the 1:50 000 BGS Map Sheet 268 Reading (Solid and drift Edition, 2000) record two normal faults running approximately north-south at Chainages 35600 (downthrow indicated to the west) and 36200 (downthrow indicated to the east). . At Chainage 36750 another normal fault (downthrow indicated to the west) is shown running north-south, approximately 100m south of the M4 motorway. The memoir (Ellison and Williamson, 1999) notes that the strata lie on the northern limb of the London Basin, with a dip in the order of <1° towards the south-east. The memoir also noted that a regional set of north-west trending faults exist in the Windsor district.
The memoir ‘Geology of the Reading district. A brief explanation of geological sheet 268’ published in 2000 (Mathers and Smith, 2000), provides more detail on the geological structure, noting that in the north of the district the strata dip south eastwards at 1-2°. It records that towards the south of the district, along the axis of the London Basin syncline, the strata are almost horizontal. The memoir records that the Palaeogene strata are cut by two near vertical faults, aligned north-south, with throws up to 20m east and west (see drawing M4MM-MUH-ML-ZZ-DR-GE-200016). The London Basin is also down tilted to the east, which is thought to have occurred during the Quaternary, at approximately 1m per kilometer (Mathers and Smith, 2000). 4.6.4 Section 4
Junction 10 to Junction 12 (Scheme Chainage 46150 to 62900)
The strata that are anticipated to be encountered along Section 4 are summarised in Table 4.17 and are described in further detail in the sub-sections below.
Table 4.17: Summary of expected geology within Section 4
Formation Name Formation Age Lithology*
Made Ground Recent The Made Ground along Section 4 of the Scheme is well compacted, obtained from local sources, has variable thickness and relates mostly to motorway pavement construction and embankment fill. Alluvium Flandrian Normally soft to firm, consolidated, compressible silty clay, but can
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Table 4.17: Summary of expected geology within Section 4
Formation Name Formation Age Lithology*
contain layers of silt, sand, peat and basal gravel. A stronger, desiccated surface zone may be present. Brickearth Quaternary Varies from silt to clay, usually yellow-brown and massive.
Langley Silt Member Devensian Varies from silt to clay, commonly yellow-brown and massively bedded. Beenham Grange Gravel Member Devensian Gravel, variably sandy and clayey. (River Terrace Deposit)
River Terrace Deposits 1 Devensian Sand and gravel, locally with lenses of silt, clay or peat.
River Terrace Deposits 2 Devensian Sand and gravel, locally with lenses of silt, clay or peat.
River Terrace Deposits 3 Devensian No lithological description included.
River Terrace Deposits 4 Devensian Sand and gravel, locally with lenses of silt, clay or peat.
River Terrace Deposits 5 Devensian Sand and gravel, locally with lenses of silt, clay or peat.
London Clay Formation Eocene Fine, sandy, silty clay/silty clay. Glauconitic at base. Lambeth Group Paleocene Variable: the component formations are Upnor Formation, Reading Formation and Woolwich Formation.
Upper Chalk - Seaford Chalk Campanian (Upper Cretaceous) No lithological description included. Formation and Newhaven Chalk Formation (Undifferentiated) * Information taken from BGS GeoIndex (2013) and (Mathers and Smith, 2000), except Made Ground
4.6.4.1 Superficial Deposits
a. Made Ground
Made Ground, associated with the motorway construction, is expected along Section 4 of the study area. Where embankments have been constructed they are thought to be formed of well compacted, locally won material, such as River Terrace Deposits, London Clay and deposits of the Lambeth Group (Mouchel, 2011b).
The 1:50 000 BGS Map Sheet 268 Reading (Solid and drift Edition, 2000) records an area of worked ground south of the M4 between Chainages 49000 and 49500. Between Chainages 56000 and 60900 extensive areas recorded as Made Ground, worked ground and infilled ground are shown to the north and south of the M4. These are likely to be associated with sand and gravel extraction. An area of worked ground is recorded south of the M4 around Chainage 61600.
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b. Alluvium
Alluvium is recorded within this section of the study area on the 1:50 000 BGS Map Sheet 268 Reading (Solid and drift Edition, 2000). An area of alluvium exists associated with Emm Brook watercourse immediately west of Junction 10. Alluvium is recorded between Chainages 49400 and 50400 around the location of the River Loddon and other smaller un-named watercourses. Alluvium associated with the Loddon and Blackwater valleys described in the memoir (Mathers and Smith, 2000) is described as silty clay, less than 2m thick. A further section of recorded alluvium exists between Chainage 60900 up to the east of Junction 12. This includes the location of the River Kennet, Holy Brook and several smaller un-named watercourses/drains. The memoir (Mathers and Smith, 2000) describes alluvium of the Kennet Valley as interbedded silty clay, shell marl, peat, thin gravel seams and reworked tufa.
c. Brickearth
Brickearth deposits are recorded on the 1:50 000 BGS Map Sheet 268 Reading (Solid and drift Edition, 2000) between Chainages 51100 and 51900. The map describes the brickearth as comprising of clayey and sandy silt. The memoir (Mathers and Smith, 2000) describes the brickearth as brown silty clay and around 2.0m thick. These deposits are thought to be partially alluvial in origin and partially a result of downslope movement of the London Clay (Mathers and Smith, 2000).
d. Langley Silt Member
The Langley Silt Member is recorded at Junction 12 (Chainage 62100), on the 1:50 000 BGS Map Sheet 268 Reading (Solid and drift Edition, 2000). The map describes the material as clayey and sandy silt. The memoir (Mathers and Smith, 2000) describes the Langley Silt as sandy and clayey silt deposits, up to 1.5m thick in the Kennet Valley area. The deposits are thought to represent an older Devensian floodplain alluvium with a possible loessic content (Mathers and Smith, 2000).
e. Beenham Grange Gravel Member (River Terrace Deposit)
The Beenham Grange Gravel Member is recorded between Chainages 56500 and 60900 on the 1:50 000 BGS Map Sheet 268 Reading (Solid and drift Edition, 2000). The map describes the material as sand and gravel River Terrace Deposits. The memoir (Mathers and Smith, 2000) records this material as being a low level terrace deposit of the Kennet Valley, occurring 1 to 3m above the floodplain. The material is described as variably sandy and silty flint gravels containing rare silt, clay and peat lenses (Mathers and Smith, 2000). The deposit is generally up to 5m thick (Mathers and Smith, 2000). f. River Terrance Deposits
Within Section 4 of the Scheme, the 1:50 000 BGS Map Sheet 268 Reading (Solid and drift Edition, 2000) records first terrace deposits between Chainages 56400 to 56500, second terrace deposits between Chainages 46400 to 46700, 50400 to 51100 and 55400 to 56400, third terrace deposits between Chainages 48900 to 49000, fourth terrace deposits between Chainages 46800 to 47800 and 48100 to 48900 and fifth terrace deposits between Chainages 52900 to 53200.
The map describes all the River Terrace Deposits as comprising of sand and gravel. Due to their location, these terrace deposits are considered to be associated with the Loddon-Blackwater valley sequence of terraces described in the memoir (Mathers and Smith, 2000). They are all described
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4.6.4.2 Solid Geology
a. London Clay Formation
The 1:50 000 BGS Map Sheet 268 Reading (Solid and drift Edition, 2000) shows that between Chainages 46150 to 56600 and 59100 to 60300, the solid geology is recorded as the London Clay Formation. The BGS map sheet shows the London Clay to be part of the Thames Group, along with the Harwich Formation. The memoir (Mathers and Smith, 2000) states that it has not proved practical to map these units separately.
The memoir (Mathers and Smith, 2000) describes the Harwich Formation as comprising a basal flint pebble bed overlain by highly glauconitic shelly sands and clayey silts. The formation is intensely burrowed and is locally cemented resulting in calcareous sandstone (Mathers and Smith, 2000). The 1:50 000 BGS Map Sheet 268 Reading (Solid and drift Edition, 2000) records the Harwich Formation as being between 3 and 6m thick.
The memoir (Mathers and Smith, 2000) describes the London Clay as comprising blue-grey, silty clays and clayey silts with subordinate thin glauconitic sands and pebble beds. The map describes the London Clay as clay, variably silty with beds of sand, silt and flint pebble seams. It is noted to be between 55 and 100m thick.
b. Lambeth Group
The 1:50 000 BGS Map Sheet 268 Reading (Solid and drift Edition, 2000) records the solid geology as the Lambeth Group (formerly the Woolwich and Reading Beds) between Chainages 56600 to 59100 and 60300 to 61300. The BGS map shows the Lambeth Group to comprise the Reading Formation and the Upnor Formation. The memoir (Mathers and Smith, 2000) states that the Lambeth Group is between 19 and 28m thick but usually between 21 and 25m thick.
The 1:50 000 BGS Map Sheet 268 Reading (Solid and drift Edition, 2000) describes the Upnor Formation as sand and clay which is highly glauconitic with flint nodules at the base. It is recorded as being between 1 and 6m thick. The memoir (Mathers and Smith, 2000) describes the Upnor Formation as being green, blue and grey in colour. The Upnor Formation also commonly contains fossiliferous material (Mathers and Smith, 2000). The memoir notes that although 6m of the Upnor Formation have been recorded in the extreme south-west of the region, it is usually less than 1m thick (Mathers and Smith, 2000).
The 1:50 000 BGS Map Sheet 268 Reading (Solid and drift Edition, 2000) describes the Reading Formation as colour mottled clay with sand beds, between 18 and 27m thick. The memoir (Mathers and Smith, 2000) states that the Reading Formation is commonly 20m thick. It describes the Reading Formation as being characteristically red and grey in colour but also purple, brown and orange (Mathers and Smith, 2000). Beds of sand commonly 2m thick but up to 7m thick occur at all levels within the formation; their distribution being variable and unpredictable (Mathers and Smith, 2000). Blocks of silicified tree trunks, beds rich in leaf remains and lignitic horizons have been observed (Mathers and Smith, 2000).
c. Upper Chalk
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The 1:50 000 BGS Map Sheet 268 Reading (Solid and drift Edition, 2000) records the solid geology as the Upper Chalk between Chainages 61300 and 62900. The BGS map shows the Upper Chalk to comprise the Upper Chalk and the Lewes Nodular Chalk Member. The Upper Chalk is described on the BGS map as comprising white, nodular and soft with flint seams which are between 90 and 130m thick. The Lewes Nodular Chalk Member is described on the BGS map as comprising white, very hard and nodular with flint seams.
The BGS GeoIndex (2013) refers to the Upper Chalk as the Seaford Chalk Formation and Newhaven Chalk Formation (undifferentiated). The memoir ‘Geology of the Reading district. A brief explanation of geological sheet 268’ published in 2000 (Mathers and Smith, 2000) indicates that faunal and fossiliferous evidence has lead to the Upper Chalk being subdivided into the Seaford Chalk Formation and the Newhaven Chalk Formation.
Between Chainages 46150 to 46300, 46700 to 46800, 47800 to 48100, 49000 to 49400, 51900 to 52900 and 53200 to 55400 there are no recorded superficial deposits, indicating that bedrock is at, or close to, ground level.
4.6.4.3 Geological Structure
The 1:50 000 BGS Map Sheet 268 Reading (Solid and drift Edition, 2000) does not record any faults through Section 4 of the Scheme.
The memoir (Mathers and Smith, 2000) notes that in the north of the region, the surface strata dip south eastwards at 1 to 2° but towards the south of the district the strata are almost horizontal along the axis of the London Basin syncline. The Palaeogene strata are gently folded and cut by near-vertical faults aligned northwards with throws up to 20m (Mathers and Smith, 2000). The London Basin is also down tilted to the east, which is thought to have occurred during the Quaternary, at approximately 1m per kilometre (Mathers and Smith, 2000).
In addition, a review of the BGS dataset within the HA GDMS website shows that there are no recorded faults, seams or deposits of mass movements within this section.
4.7 Hydrogeology The Environment Agency (EA) has produced a series of maps showing aquifer designations for both superficial deposits and bedrock which have been made available on the EA’s ‘Whats in your back yard?’ website (http://www.environment-agency.gov.uk/wiyby). The EA use the following aquifer designations which are consistent with the Water Framework Directive. Principal aquifer – these are layers of rock or drift deposits that have high intergranular and/or fracture permeability - meaning they usually provide a high level of water storage. They may support water supply and/or river base flow on a strategic scale. Secondary A aquifer - permeable layers capable of supporting water supplies at a local rather than strategic scale, and in some cases forming an important source of base flow to rivers. Secondary B aquifer - predominantly lower permeability layers which may store and yield limited amounts of groundwater due to localised features such as fissures, thin permeable horizons and weathering.
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Unproductive strata - These are rock layers or drift deposits with low permeability that have negligible significance for water supply or river base flow.
The aquifer designations of the superficial deposits encountered across the Scheme are summarised in Table 4.18. These have been inferred from correlations between the superficial deposit aquifer designation maps on the EA website and BGS geological mapping.
Table 4.18: Summary of aquifer designations of superficial deposits
Superficial deposits EA Aquifer Designation Scheme Section
Taplow Gravel Formation Principal aquifer Section 1 Section 2 Langley Silt Member Unproductive strata Section 1
Alluvium Secondary A aquifer Section 1 Section 2 Section 3 Section 4 Shepperton Gravel Member Principal aquifer Section 1 Section 2 Kempton Park Gravel Formation Principal aquifer Section 2
Head Secondary (Undifferentiated) Section 3 aquifer River Terrace Deposits Secondary A aquifer Section 3 Section 4 Brickearth Secondary B aquifer Section 4
Beenham Grange Gravel Member Secondary A aquifer Section 4
The aquifer designations of the bedrock encountered in the study area are summarised in Table 4.19. These have been inferred from correlations between BGS mapping and the bedrock aquifer designation maps on the EA website.
Table 4.19: Summary of aquifer designations of bedrock
Bedrock EA Aquifer Designation Scheme Section
London Clay Formation Unproductive strata Section 1 Section 2 Section 3 Section 4 Lambeth Group Secondary A aquifer Section 2 Section 3 Section 4 Upper Chalk - Seaford Chalk Principal aquifer Section 4 Formation and Newhaven Chalk Formation (undifferentiated)
Source Protection Zones (SPZ), which exist within the scheme extents, have been identified using the EA ‘Whats in your back yard?’ website (http://www.environment-agency.gov.uk/wiyby). Three categories of SPZ were identified along the Scheme namely zone 1 (inner protection zone), zone 2 (outer protection zone) and zone 3 (source catchment protection zone). The EA defines the SPZ’s as the following:
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Zone 1 (inner protection zone) - Defined as the 50 day travel time from any point below the water table to the source. This zone has a minimum radius of 50 metres. Zone 2 (outer protection zone) - Defined by a 400 day travel time from a point below the water table. This zone has a minimum radius of 250 or 500 metres around the source, depending on the size of the abstraction. Zone 3 (source catchment protection zone) - Defined as the area around a source within which all groundwater recharge is presumed to be discharged at the source. In confined aquifers, the source catchment may be displaced some distance from the source.
Relevant SPZ’s that were identified are summarised in Sections 4.7.1 to 4.7.4 of this report which describe the hydrogeology of each of the four Scheme Sections. The vulnerability of groundwater to contamination within the study area has also been reviewed. The findings for each of the Scheme Sections are summarised in the same sections.
It is noted that groundwater levels in the London area are rising due to reduced water abstraction (Hight, D W; Ellison, R A & Page, D P. 2004). The Lambeth Group, and in particular the Upnor Formation, is in hydrogeological continuity with the underlying Thanet Sand and Chalk (Hight, D W; Ellison, R A & Page, D P. 2004). As a result of this groundwater rise, there may be an increase in pore water pressure and swelling of the clay may result in a reduction in shear strength (Hight, D W; Ellison, R A & Page, D P. 2004).
Ciria C583 (2004) suggests that oxidised pyrite in sands in the Lambeth Group may lead to very acidic waters causing deterioration of buried structures. As the water table lowered due to high abstraction rates, the pyrite in the sands was oxidised (Hight, D W; Ellison, R A & Page, D P. 2004). As the water table rises again the oxidised pyrite will give rise to potentially corrosive acidic groundwater (Hight, D W; Ellison, R A & Page, D P. 2004). 4.7.1 Section 1
Junction 3 to Junction 5 (Scheme Chainage 10000 to 20150)
Within Section 1 of the Scheme, a SPZ zone 2 exists between Chainages 17600 and 18600. Within this area, a SPZ zone 1 is recorded between Chainages 17700 and 18100. A SPZ zone 3 is located between Chainages 19800 and 20150 (Junction 5) which extends into Scheme section 2.
Where aquifers are present in Section 1 (Table 4.17), groundwater vulnerability maps show that the majority are classified as major aquifers of high vulnerability. There are also some sections that are designated as major aquifers of intermediate vulnerability. 4.7.2 Section 2
Junction 5 to Junction 8/9 (Scheme Chainage 20150 to 34100)
SPZ zone 3 continues from Scheme section 1 between Chainages 20150 (Junction 5) and 21200. Between Chainages 21200 and 27800, a SPZ zone 2 is recorded and within this area an SPZ zone 1 is located between Chainages 23100 and 24600. A further SPZ zone 3 is located between Chainages 29800 and 34100 (Junction 8/9) incorporating a SPZ zone 2 between Chainages 30000 and 32500 and a SPZ zone 1 between Chainages 31000 and 31900.
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The majority of the aquifers in Section 2 are classified as major aquifers of intermediate vulnerability. There are some areas classified as major aquifers of high vulnerability and immediately before Junction 8/9 there is an area classified as a minor aquifer of intermediate vulnerability. 4.7.3 Section 3
Junction 8/9 to Junction 10 (Scheme Chainage 34100 to 46150)
A SPZ zone 3 is recorded between Chainages 34100 and 43500 incorporating a SPZ zone 2 between Chainages 38100 and 40300 and a SPZ zone 1 between Chainages 38800 and 39700. A further SPZ zone 3 is recorded between Chainages 44300 and 46150 (Junction 10); with a SPZ zone 2 recorded between Chainages 45400 and 46150.
The majority of this section is not underlain by strata classified as an aquifer. Where sections of recorded aquifers do exist beneath the M4 corridor they are classified as minor aquifers of low vulnerability and minor aquifers of high vulnerability. 4.7.4 Section 4
Junction 10 to Junction 12 (Scheme Chainage 46150 to 62900)
The SPZ zone 2 described in Section 4.7.3 continues between Chainages 46150 (Junction 10) and 46900. Westwards from Chainage 53800 (located just east of Junction 11) up to the Schemes western extent at Chainage 62900 , the entire scheme falls within a SPZ. A SPZ zone 3 is recorded between Chainages 53800 and 62900 incorporating a SPZ zone 2 between Chainages 54000 and 60500 and again between Chainages 62000 and 62900. SPZ zone 1’s exists between Chainages 54700 and 56700 and again between 62200 and 62900.
Where aquifers are designated along this section of the M4 they are classified as minor aquifers of high vulnerability and close to Junction 12 there is an area classified as a major aquifer of high vulnerability.
4.8 Mining and Quarrying The following coal mining, mining and quarrying data was obtained from a number of sources which include the ‘Geology of the Windsor and Bracknell district. A brief explanation of the geological map Sheet 269 Windsor’ memoir published in 2000 and ‘Geology of the Reading district. A brief explanation of geological sheet 268’ memoir published in 1999. These memoirs report that study area has had a history of sand and gravel extraction from river terrace deposits. Clay has also been extracted, commonly from the deposits of the Reading Formation and also from the London Clay Formation for brickmaking, reservoir and landfill linings.
The review of Mining Instability in Great Britain Volume 1/ii, prepared by Arup, published in December 1991, has been summarised in the Technical Appraisal Report (Mouchel, 2011b). The document confirms that the Reading area (between Chainages 34100 and 62900) has an extensive history of chalk mining and brickmaking using the Lambeth Group deposits.
The BGS ‘Mining Plans Portal’ website (http://www.bgs.ac.uk/nocomico/) does not record any coal, deneholes, evaporites, ironstone, metalliferous or rock mines within the study area.
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A review of the solid geology of the study area (by consideration of published BGS maps) shows that coal bearing strata is not present in this area and therefore coal mining will not have taken place. As such, a Coal Authority report has not been obtained for the Scheme.
Sections 4.8.1 to 4.8.4 below describe the coal mining, mining and quarrying activities of each Section of the Scheme.
4.8.1 Section 1
Junction 3 to Junction 5 (Scheme Chainage 10000 to 20150)
A review of historical mapping available for Section 1 of the Scheme is summarised in Section 4.2 of this report. The mapping suggests that the area surrounding the M4 has been subjected to extensive gravel works in the past, the majority of which have been backfilled more recently with landfill material. A clay pit is also recorded north east of Junction 4 on the historical maps, which at a later data is recorded as a gravel works.
Section 1 of the Scheme is covered by BGS Mineral Resources mapping for Berkshire (McEvoy et al., 2003), Buckinghamshire (Benham et al., 2003a) and Hertfordshire and NW London Boroughs (Benham et al., 2003b).
The maps record mineral resources from sub-alluvial and river terrace deposits both adjacent to and beneath the footprint of the M4 corridor. Areas recorded as ‘Surface Planning Permission (valid and expired)’ and ‘Mineral Workings Inactive, worked-out and/or restored site’ are recorded both adjacent to and beneath the footprint of the M4 corridor. Active mineral workings in the vicinity of this section of the M4 (within 1km) are recorded at: South of Junction 4: Wallgarden Farm and Harmondsworth Lane (South) Chainage 18500: Sutton Lane (1) approximately 800m south of the M4
These workings predominantly relate to sand and gravel extraction with occasional common clay and shale and crushed rock workings. 4.8.2 Section 2
Junction 5 to Junction 8/9 (Scheme Chainage 20150 to 34100)
A review of historical mapping available for Section 2 of the Scheme is summarised in Section 4.2 of this report. The mapping suggests that the area surrounding the M4 has been a source of sand and gravel in the past with some extraction likely to be on going.
Section 2 of the Scheme is covered by BGS Mineral Resources mapping for Berkshire (McEvoy et al., 2003) and Buckinghamshire (Benham et al., 2003a). The maps record mineral resources from river terrace and sub-alluvial deposits. East of Junction 8/9, mineral resources from the Lambeth Group are recorded. Occasional areas of ‘Surface Planning Permission (valid and expired)’ and ‘Mineral Workings Inactive, worked-out and/or restored sites’ are shown beneath and adjacent to the M4 corridor. These relate to sand and gravel resources. Two active sand and gravel workings are shown to the south of the M4 (within 1km of the motorway) these are recorded as: Chainage 31000: Eton
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Chainage 32000: Bray Pit (Monkey Island)
4.8.3 Section 3
Junction 8/9 to Junction 10 (Scheme Chainage 34100 to 46150)
A review of historical mapping available for Section 3 of the Scheme is summarised in Section 4.2 of this report. The mapping suggests that there were no historical mining activities or gravel extraction works along this section of the M4. Several borrow areas/re-graded areas are recorded in this section which are thought to relate directly to the M4 construction. They are located at: Chainage 36600 to 37200 Chainage 39700 to 39900 At Junction 10 – Winnersh Interchange
Section 3 of the Scheme is covered by BGS Mineral Resources mapping for Berkshire (McEvoy et al., 2003). The map records mineral resources from river terrace, sub-alluvial and Lambeth Group deposits. Occasional areas recorded as ‘Surface Planning Permission (valid and expired)’ and ‘Mineral Workings Inactive, worked-out and/or restored sites’ are recorded adjacent to and under the M4 corridor. 4.8.4 Section 4
Junction 10 to Junction 12 (Scheme Chainage 46150 to 62900)
A review of historical mapping available for Section 4 of the Scheme is given in Section 4.2 of this report. The historical mapping shows that flint (gravel) has been extracted over an extensive area north and south of the M4, between Chainages 46150 and 62100. The mapping indicates that some of the sand and gravel pits are still in use, some have been flooded and others have been backfilled with landfill material.
The ‘Geology of the Windsor and Bracknell district. A brief explanation of the geological map Sheet 269 Windsor’ memoir published in 2000 note that chalk has been excavated in this area for marling adjacent loamy land, for agricultural lime and as a source of flint.
Section 4 of the Scheme is covered by BGS Mineral Resources mapping for Berkshire (McEvoy et al., 2003). The map records mineral resources from river terrace and sub-alluvial deposits. At Junction 12 (Chainage 62100) mineral resources from the White Chalk Subgroup are recorded. There are occasional areas recorded as ‘Surface Planning Permission (valid and expired)’ and ‘Mineral Workings Inactive, worked-out and/or restored sites’ between Chainages 46150 (Junction 10) and 54800 (Junction 11) adjacent to and beneath the M4 corridor. These areas become extensive north, south and beneath the M4 corridor between Chainages 54800 (Junction 11) and 62100 (Junction 12). They represent sand and gravel extraction from the river terrace deposits.
4.9 Landfills, geo-environmental Issues and possible contamination Issues The study of the resources available on the EA’s ‘Whats in your back yard?’ website (http://www.environment-agency.gov.uk/wiyby a review of the available HA GDMS reports have revealed several sources of possible contamination present along the M4 between Junctions 3 and 12.
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A metal recycling site (vehicle dismantler) called ‘Car Spares of West Drayton Ltd’ is located approximately 100m to the north of the motorway at Chainage 16150. The recycling site is located adjacent to the Wraysbury River which flows southerly towards the M4 and passes beneath it. The presence of the metal recycling site not only poses a potential threat of ground and groundwater contamination in the close vicinity of the site but migration of the pollutants along the river towards the M4.
At Chainage 17850, the Iver South Sewage Treatment works are present approximately 150m to the south of the motorway. A household waste amenity site is located approximately 100m north of the motorway at Chainage 25550.
In between Chainages 27400 and 28400, the area south and adjacent to the M4 is used for fuel and power production, it also contains the Slough Sewage Treatment Works. HA GDMS’s Report 12715 describes this area as comprising ‘sludge beds and sludge lagoons (with estimated depths of 5m), likely contaminated with heavy metals and other chemical and microbiological pollutants’. The sludge lagoons are contained by a high bound and located extremely close to the M4 motorway and the Oldway Lane Overbridge.
A review of HA GDMS report 12715 (in particular the ‘Areas Requiring Special Geotechnical Consideration’ section) revealed the possible presence of a radiation hazard associated with Radium 226 to the north of the motorway between Chainages 21100 and 21900 associated with the Calor Gas Site and Ditton Park. This area has previously been used by Ministry of Defence. Large compasses and instrument dials were painted with luminous paint during the 1940's and 1950's which contained Radium 226. Radioactivity has since been discovered in the northern part of the site. The southern part of the site however has not been surveyed for the presence of radioactivity.
Based on records obtained from the EA’s ‘Whats in your back yard?’ website (http://www.environment-agency.gov.uk/wiyby a review of the available HA GDMS reports, a number of authorised and historical landfill sites are present adjacent to the M4 along the Scheme. The locations of the landfills in relation to the Scheme’s Chainage and the available information on waste type received are presented in the Table 4.19 in Appendix G.
In addition to the above information listed in Table 4.19, HA GDMS report 12715 refers to British Drilling Association Guidance Notes for the Safe Drilling of Landfills and Contaminated Land (1992) which has its own colour-coded landfill classification system. According the British Drilling Association Guidance, the following landfill sites are designated as shown.
a. Yellow sites
Sites containing waste food, vegetable matter, floor sweeping’s, household waste, animal carcasses, sewage sludge, trees, bushes, garden waste, leather, rubber and latex, tyre, epoxy resin, electrical fittings, soaps, cosmetics, non-toxic metal and organic compounds tar, pitch, bitumen, solidified wastes, dye stuffs, fuel ash, silica dust etc.
Priors Way Aysgarth Park The Myrke
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a. Red sites
Sites containingtoxic metal and organic compounds, pharmaceutical and veterinary wastes, phenols, medical products, solvents, beryllium, micro-organisms, asbestos, thiocyanates, cyanides, etc. hydrocarbons, peroxides, chlorates, flammable and explosive materials, materials that are particularly corrosive or carcinogenic, etc.
Herschel Park Slough Sewage Treatment Works Larbourne Farm Upton Court Park Ditton Park
The Scheme area comprises a motorway corridor and therefore fuel and oil spillage incidents might have taken place on both the carriageways and the road verges. Any excavation into these areas may encounter contamination in the form of polycyclic aromatic hydrocarbons (PAHs) and aliphatic hydrocarbons.
4.10 Natural Cavities HA GDMS report 20563 addressed the risk associated with Natural Cavities between junctions 10 and 12 within the section of the scheme where the Upper Chalk is recorded. That report noted that:
‘….. there are no recorded natural cavities located within 1 km of the route. The nearest recorded natural cavity is 2.1 km away at NGR 46800 172200. Immediately southeast of Junction 12 the route passes over the edge of the Lambeth Group outcrop. lt is possible that solution features may be present there, and in the near vicinity the tertiary cover to the chalk is absent or thin.’
The source reference for this is:
PBA/DEFRA Natural cavity and non coal mining cavity database searches for route of M4, Junctions 10-12, Highways Comms Scheme Phase 6. Letter ref 1 0083/493/CNE/AD/JF, dated 20 April 2005.
4.11 Unexploded ordnance As the Scheme is associated with upgrades of existing post war infrastructure (ie construction of the M4 and M25) it is assumed that the risk associated with the potential presence of unexploded ordnance has been appropriately mitigated as part of those works. However, works are proposed as part of this Scheme are to be undertaken outside of the footprint of the M4 appropriate safety checks should be made prior to works commencing.
4.12 Earthworks Instability The HA GDMS website (http://www.HAGDMS.com) contains records of defects recorded during the principal inspections of the earthworks. A review of the HA GDMS database found a large number of instability related defects across the whole Scheme, a total of 140 No. defects were recorded in Section 1, 68 No. defects in Section 2, 66 No. defects in Section 3 and 113 No. defects
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The instability related defects are summarised within Tables H.1 to H.4 presented in Appendix H and are shown on Drawings M4MM-MUH-ML-ZZ-DR-GE-200001 to 200030.
4.13 Environmental Considerations Available information obtained from EA, MAGIC and Natural England websites indicate a number of Local and National Natural Reserves and one SSSI along the route of the Scheme. These are discussed in detail below.
The Scheme also cuts through a number of Priority Habitats (mainly Ancient and Semi-Natural Woodlands) which occur primarily between Junctions 8/9 and 10. This may cause an issue regarding possible disturbance of nesting birds and other local species living in trees. The Scheme is crossed by several watercourses therefore special consideration should be given to possibility of surface and groundwater contamination during the construction stage
A review of data held by MAGIC (Multi-Agency Geographic Information for the Countryside) and Natural England identified the following environmental considerations.
a. Nitrate Vulnerable Zones
Nitrate Vulnerable Zones were shown to be located at the following localities: Either side of the motorway between Chainages 13900 and 16700 (i.e. between Junctions 4 and 4B). To the south of the M4 between Chainages 27300 and 29000 (Junction 7). Between Chainage 29000 (Junction7) and 39700. Between Chainages 55900 and Ch60900. The entire area incorporating Junction 10.
b. Nature reserves
Local Nature Reserves were recorded on the MAGIC website at the following locations:
Pearman’s Copse-adjacent to the eastbound carriageway of the M4 between Chainages 52000 and 53000.
Ockwells Park-an area located approximately 100 meters north west of Junction 8/9 (Chainage 34100).
The Queen’s Mother Reservoir located to the south west of Junction 5 is mapped as “Important Bird Area”.
The Natural England website records the presence of Cranford Countryside Park (County Park) to the south west of Junction 3.
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The Queen’s Mother Reservoir located to the south west of Junction 5 is mapped as “Important Bird Area”.
The Natural England website records the presence of Cranford Countryside Park (County Park) to the south west of Junction 3.
c. Habitat Inventories
The MAGIC interactive map shows that two different types of Habitat Inventories are present along the Scheme, Lowland Raised Bogs and Floodplain Grazing Marsh.
Lowland Raised Bogs were reported in the following locations: Either side of the motorway between chainages 10900 (Junction 3) and 25000. To the north of the eastbound carriageway between chainages 34300 and Ch36500. Either side of the M4 between chainages 40400 and Ch56000.
Areas of Floodplain Grazing Marsh were mapped in the following localities: Between chainages 26200 and 28750. Between chainages 33350 and 34800. Between chainages 50000 and 50700. Between chainages 61150 and 61250.
d. Biodiversity Action Plan Priority Habitats
Biodiversity Action Plan Priority Habitats were identified at the following locations along the Scheme: North west of Junction 3. To the south of the M4 between chainages 18000 and 18800 (in between Junction 4B and 5). Between chainages 21500 and 21800 (between Junctions 5 and 6). Between chainages 24300 and 24900 (between Junctions 5 and 6). Between Junctions 8/9 and Junction 10. Between chainages 47200 and 47400 (between Junctions 10 and 11). Between chainages 49800 and 50200 (between Junctions 10 and 11). Between chainages 55800 and 56000 (between Junctions 11 and 12).
e. Sites of Special Scientific Interest (SSSIs)
No UK designated Sites of Special Scientific Interest (SSSI) or internationally important sites (including World Heritage Sites and Ramsar sites) were identified along or within the vicinity of the Scheme from data held by MAGIC. However, environmental maps available on the Natural England website revealed that a SSSI (Great Thrift Wood) is present approximately 300m north of the eastbound carriageway of the M4 between chainages 35150 and 36100. This designation is due to the tree species and flora present in the wood.
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5 GROUND CONDITIONS
5.1 Introduction This section of the PSSR presents a preliminary assessment of the expected ground conditions along the entire route of the Scheme. Sections 5.2 to 5.5 below present the expected ground conditions and likely engineering properties of strata encountered for each of the Scheme sub- sections based on the available data presented in Sections 2 to 4 of this report and relevant historical reports available from HA GDMS.
5.2 Section 1 Junction 3 to Junction 5 (Scheme Chainage 10000 to 20150)
The following strata are anticipated to be encountered along Section 1:
Made Ground (embankment fill) Alluvium Langley Silt Member River Terrace Deposits (two types)
- Taplow Gravel Formation - Shepperton Gravel Member London Clay
The following sub-sections describe each of these strata in more detail and present their likely engineering properties which have been obtained from historical HA GDMS reports. 5.2.1 Made Ground
As discussed in the Technical Appraisal Report (Mouchel, 2011b), Made Ground associated with the motorway pavement construction and embankment fill, comprising of well compacted, locally obtained material, is expected to be encountered along the route. The thickness of this material depends on the height of the earthwork.
Engineering properties for the Made Ground between Junction 3 and Junction 5 were obtained from the following HA GDMS report:
HA GDMS report 17740 – covering Junctions 4 to 5
The available data includes various different types of Made Ground therefore the range of values is quite broad. These values are presented in Table 5.1 in Section 5.2.5 in Appendix I. 5.2.2 Superficial deposits
a. Alluvium
The alluvium is generally described as soft to stiff clay with variable proportions of silt, sand and gravel. Engineering properties for the alluvium between Junction 3 and Junction 5 were obtained from the following HA GDMS report:
HA GDMS report 17740 – covering Junctions 4 to 5
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HA GDMS Report 17740 describes the engineering properties of alluvium associated with various river channels in the area. The results in HA GDMS report 17740 were drawn from three samples of alluvium which were compared to results presented in a Geotechnical Desk Study Report for the M25, prepared by LG Mouchel & Partners (1991). These values are presented in Table 5.1 in Section 5.2.5 below.
b. Langley Silt Member
The Langley Silt Member is characterised as a silt and clay, commonly yellow-brown and massively bedded material. Engineering properties for the Langley Silt Member in Section 1 were obtained from the following HA GDMS report:
HA GDMS report 17740 – covering Junctions 4 to 5
It is noted that HA GDMS report 17740 compares scheme specific historical data with historical data produced by LG Mouchel & Partners (1991) for M25 motorway further to this. The report states that the engineering parameters derived by LG Mouchel & Partners (1991) generally fall within the ranges presented in HA GDMS report 17740. These values are presented in Table 5.1 in Section 5.2.5 below.
c. River Terrace Deposits
The River Terrace Deposits are variable but are generally described as medium dense to very dense sandy gravel with varying proportions of clay and silt. The River Terrace Deposits comprise several sub-formations namely the Taplow Gravel Formation, the Kempton Park Gravel Formation, the Shepperton Gravel Member and Plateau Gravel.
The sub-formations of the River Terrace Deposits have not been specified individually in the historical reports available on HA GDMS, as such all engineering parameters are referred to as River Terrace Deposits. This may be one of the reasons why values of undrained shear strength (cu) range quite significantly, from 9 to 127 kPa.
Engineering properties for the River Terrace Deposits in Section 1 were obtained from the following HA GDMS reports:
HA GDMS report 3269 – covering Junctions 4b to 5 HA GDMS report 17740 – covering Junctions 4 to 5
These values are presented in Table 5.1 in Section 5.2.5 below. 5.2.3 Bedrock
The London Clay is described in the Technical Appraisal Report (Mouchel, 2011b) as stiff or very stiff blue-grey silty clay with local beds, partings or lenses of fine sand.
Engineering properties for the London Clay in Section 1 were obtained from the following HA GDMS reports:
HA GDMS report 3269 – covering Junctions 4b to 5 HA GDMS report 17740 – covering Junctions 4 to 5
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These values are presented in Table 5.1 in Appendix I. The ranges for the undrained shear strength (cu) are similar, ranging between a minimum of 40kPa and a maximum of 220kPa. Recorded ‘N’ values from the Standard Penetration Tests (SPT) were very similar between the three HA GDMS reports, ranging from 6 to 75. The range of liquid limits from HA GDMS report 17740 indicates the deposit to have a high to very high plasticity. 5.2.4 Groundwater
Groundwater information has been obtained from the following HA GDMS reports: HA GDMS report 3269 – covering Junctions 4b to 5 HA GDMS report 17740 – covering Junctions 4 to 5
HA GDMS report 3269 notes that gravel deposits overly the majority of the site and are likely to be highly permeable and susceptible to contamination. The report also states that groundwater levels are generally expected to be within a metre or two of the (natural) ground surface in the valley bottoms. On valley sides the water levels in the gravels are to be expected at an increased depth due to the free draining nature of the gravels. The report also advises care on interpreting groundwater levels due to the localised effects of abstraction wells and also any effects of buried channels of alluvium within the gravels.
HA GDMS report 17740 reports that between Junction 4 and Junction 5 much of the route is across outcrop of the river terrace deposits and ground water levels can be expected to be high (less than 2m below ground level). Higher groundwater levels are also anticipated in the floor of the Colne Valley (River Colne – Chainage 16000). The report also notes that groundwater levels in the river terrace deposits are likely to be hydrostatic. 5.2.5 Summary of predicted engineering properties
Table 5.1 (Appendix I) summarises the predicted engineering properties of the strata found along Section 1 of the Scheme. The data was extracted from relevant HA GDMS reports, as listed in the sub-sections above, and includes values obtained from laboratory tests, correlations and/or recommended design values. The table presents the minimum and maximum test results and where available information on average or median values and the number of tested specimens have also been included.
5.3 Section 2 Junction 5 to Junction 8/9 (Scheme Chainage 20150 to 34100)
The following strata are anticipated to be encountered along Section 2:
Made Ground (embankment fill) Alluvium Langley Silt Member River Terrace Deposits (three types) o Taplow Gravel Formation o Kempton Park Gravel Formation o Shepperton Gravel Member London Clay Lambeth Group
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The following sub-sections describe each of these strata in more detail and present their likely engineering properties which have been obtained from historical HA GDMS reports. 5.3.1 Made Ground
Made Ground comprising of well compacted, locally obtained material, associated with the motorway pavement construction and embankment fill, is likely to be found along the route of the scheme (Mouchel, 2011b). This material has variable thickness depending on the type of earthwork present.
Engineering properties for the Made Ground between Junction 5 and Junction 8/9 were obtained from the following HA GDMS reports:
HA GDMS report 17320 – covering Junction 8/9 HA GDMS report 17740 – covering Junctions 5 to 6 HA GDMS report 19243 – covering Junction 5 HA GDMS report 24298 – covering Junction 6 HA GDMS report 24471 – covering Junction 5
Ranges of the obtained data vary significantly. This may result from the variation in Made Ground present along Section 2 of the Scheme. These values are presented in Table 5.2 in Section 5.3.5 in Appendix I. 5.3.2 Superficial deposits
a. Langley Silt Member
The Langley Silt Member is characterised in the Technical Appraisal Report (Mouchel, 2011b) as typically consisting of brown to orange silt, with minor fractions of clay and fine sand. It is expected to be variable in composition.
Engineering properties for the Langley Silt Member in Section 2 were obtained from the following HA GDMS reports:
HA GDMS report 17740 – covering Junctions 5 to 6
HA GDMS report 17740 compares scheme specific historical data with historical data produced by LG Mouchel & Partners (1991) for M25 motorway further to this. The report states that the engineering parameters derived by LG Mouchel & Partners (1991) generally fall within the ranges independently derived and presented in HA GDMS report 17740. These values are presented in Table 5.2 in Appendix I.
b. Alluvium
Section 2 of the Scheme contains two types of alluvium from the Flandrian and Quaternary geological periods which are likely to have similar characteristics. The alluvium is described in the Technical Appraisal Report (Mouchel, 2011b) as normally consolidated, very soft to soft, silty clay containing angular to sub-rounded fine to medium gravel or clayey silt of variable, often organic composition. A desiccated near surface zone may be present which tends to be typically firm to stiff in consistency.
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Engineering properties for the Alluvium in Section 2 were obtained from the following HA GDMS reports:
HA GDMS report 17740 – covering Junctions 5 to 6 HA GDMS report 20433 – covering Junction 8/9 to 10 HA GDMS report 24298 – covering Junction 6
These reports show a range of undrained shear strength values between 20 and 120kPa. Based on HA GDMS reports 20433 and 24298, the estimated values of cohesion (c’) and angle of shearing resistance (ɸ’) are 1kPa and 24 degrees, respectively. The information regarding moisture content, liquid limit, plastic limit, plasticity index, liquidity index, coefficient of volume compressibility (mv ) and coefficient of consolidation(cv) were found only in HA GDMS report 17740. This report also compares scheme specific historical data to historical information obtained in LG Mouchel & Partners (1991) report for M25 motorway. These values are presented in Table 5.2 in Appendix I.
c. River Terrace Deposits
The River Terrace Deposits are variable in nature and include a number of sub-formations, namely the Taplow Gravel Formation, the Kempton Park Gravel Formation, the Shepperton Gravel Member and Plateau Gravel. The general characteristics of River Terrace Deposits are medium dense to very dense sandy gravel with varying proportions of clay and silt. The historical reports available on HA GDMS do not specify particular sub-formations therefore all engineering parameters are referred to as River Terrace Deposits.
Engineering properties for the River Terrace Deposits in Section 2 were obtained from the following HA GDMS reports:
HA GDMS report 3269 – covering Junctions 5 to 8/9 HA GDMS report 17740 – covering Junctions 5 to 6 HA GDMS report 19243 – covering Junction 5 HA GDMS report 20433 – covering Junction 8/9 to 10 HA GDMS report 24298 – covering Junction 6 HA GDMS report 24471 – covering Junction 5
Such generalisation of the River Terrace Deposits may be among the reasons why values of SPT N value range significantly across the historical reports ranging from 9 to 127 blows. The range of angle of shearing resistance (ɸ’) also shows variation ranging between 32 and 39 degrees. All the other parameters appear to have similar ranges. These values are presented in Table 5.2 in Appendix I. 5.3.3 Bedrock
a. London Clay
The London Clay in Section 2 of the Scheme is described by the Technical Appraisal Report (Mouchel, 2011b) as stiff or very stiff blue-grey silty clay with local beds, partings or lenses of fine sand, weathering to firm or firm to stiff and mottled blue-grey and brown.
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Engineering properties for the London Clay in Section 2 were obtained from the following HA GDMS reports:
HA GDMS report 3269 – covering Junctions 5 to 8/9 HA GDMS report 17740 – covering Junctions 5 to 6 HA GDMS report 19243 – covering Junction 5 HA GDMS report 20433 – covering Junction 8/9 to 10
All four HA GDMS reports show a similar broad range of cu values stretching between 49 to nearly 250 kPa. The ‘N’ values from standard penetration tests (SPT’s) range significantly between 6 and 75 blows. Moisture content and plasticity index values seem to fall into similar ranges of 7 to 43% and 15 to 51%, respectively. All the HA GDMS reports suggest that the upper bound value of ɸ’ is no more than 23 degrees. HA GDMS reports 17740 and 19243 present comparable lower ranges for the liquid limit of the material at 58% and 65%, respectively. Considerable differences are also noted when comparing values of coefficient of volume compressibility (mv) found in these two reports. The lower limit for mv in HA GDMS report 19243 is 0.058m2/MN, whereas HA GDMS report 17740 suggests a value of 0.15 m2/MN. These values are presented in Table 5.2 in Appendix I.
b. Lambeth Group
Information on the Lambeth Group, comprising the Upnor Formation and the Reading Formation, in Section 2 has been obtained from the following HA GDMS Reports:
HA GDMS report 3269 – covering Junctions 5 to 8/9 HA GDMS report 17320 – covering Junction 8/9 HA GDMS report 20433 – covering Junction 8/9 to 10
The presented results are assumed to be associated with the Reading Formation only. This is recorded to comprise mottled silty clay and clay as the dominant lithology but in areas beds of silt and sand may constitute up to 50% of the formation (Hight, D W; Ellison, R A & Page, D P. 2004). For this reason and based on the historical information available the Reading Formation is subdivided into clay and sand units for the purposes of assessing the engineering characteristics. The results of historical investigations suggest a large range of undrained shear strength (cu) values for the clay unit; ranging from 39 to 350kPa. SPT ‘N’ values for the clay and sand units also vary considerably, ranging between 5 and 110 (clay units) and 20 to 150 (sand units). The angle of shearing resistance (ɸ’) for the clay units seems to be consistent across the reports ranging between 24 and 26 degrees. The only available value of ɸ’ for the sand unit is 32 degrees found in HA GDMS report 20433. These values are presented in Table 5.2 in Appendix I. 5.3.4 Groundwater
Groundwater information has been obtained from the following HA GDMS reports: HA GDMS report 3269 – covering Junctions 5 to 8/9 HA GDMS report 17740 – covering Junctions 5 to Chainage 23500
HA GDMS report 3269 notes that gravel deposits overly the majority of the site and are likely to be highly permeable and susceptible to contamination. The report also states that groundwater levels are generally expected to be within a metre or two of the (natural) ground surface in the valley
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Highways Agency — M4 J3-12 MM-ALR bottoms. On valley sides the water levels in the gravels are to be expected at an increased depth due to the free draining nature of the gravels. The report also advises care on interpreting groundwater levels due to the localised effects of abstraction wells and also any effects of buried channels of alluvium within the gravels.
HA GDMS report 17740 reports that between Junction 5 and MP34/5 (Chainage 23500) much of the route is across outcrop of the river terrace deposits and ground water levels can be expected to be high (less than 2m below ground level). The report also notes that groundwater levels in the river terrace deposits are likely to be hydrostatic but between Junction 5 and MP34/5 (Chainage 23500) the London Clay is relatively thin and underlain by the Lambeth Group which have higher permeabilities, therefore groundwater pressures may be higher than hydrostatic. The report advises care when excavating in the London Clay in this area (Junction 5 to Chainage 23500) to avoid piping failure in the base of excavations due to high seepage forces. 5.3.5 Summary of predicted engineering properties
Table 5.2 (Appendix I) summarises the predicted engineering properties of the strata found along Section 2 of the Scheme. The data was extracted from relevant HA GDMS reports, as listed in the sub-sections above, and includes values obtained from laboratory tests, correlations and/or recommended design values. The table presents the minimum and maximum test results and where available information on average or median values and the number of tested specimens have also been included
5.4 Section 3 Junction 8/9 to Junction 10 (Scheme Chainage 34100 to 46150)
The following strata are anticipated to be encountered along Section 3:
Made Ground (embankment fill) Alluvium River Terrace Deposits Head Deposits London Clay Lambeth Group
The following sub-sections describe each of these strata in more detail and present their likely engineering properties which have been obtained from historical HA GDMS reports. 5.4.1 Made Ground
The Technical Appraisal Report (Mouchel, 2011b) states that the Made Ground likely to be encountered along the route of the scheme is well compacted, sourced locally, has variable thickness and is associated with motorway pavement construction and embankment fill.
Engineering properties for the Made Ground between Junction 8/9 and Junction 10 were obtained from the following HA GDMS reports:
HA GDMS report 1506 – covering Junctions 8/9 to 10 HA GDMS report 1512 – covering Junction 10
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HA GDMS report 3269 – covering Junctions 8/9 HA GDMS report 17320 – covering Junction 8/9 HA GDMS report 20960 – covering Junction 10
The HA GDMS reports show significant variations in parameter ranges. This may result from variations in the Made Ground used along Section 3 of the Scheme. These values are presented in Table 5.3 in Appendix I. 5.4.2 Superficial deposits
a. Alluvium
Alluvium present in Section 3 of the Scheme is described by the Technical Appraisal Report (Mouchel, 2011b) as normally consolidated, soft to very soft, silty clay containing angular to sub- rounded fine to medium gravel or clayey silt of variable, often organic composition. A zone of desiccated, firm to stiff material near the surface may also occur.
Engineering properties for the alluvium in Section 3 were obtained from the following HA GDMS reports: HA GDMS report 1506 – covering Junctions 8/9 to 10 HA GDMS report 20433 – covering Junction 8/9 to 10
The two HA GDMS reports show a similar range of undrained shear strength values (cu). Other parameters are only recorded in HA GDMS report 20433. These values are presented in Table 5.3 in Appendix I.
b. River Terrace Deposits
Section 3 of the Scheme comprises River Terrace Deposits 4 and River Terrace Deposits 5. Both deposits are of Quaternary age containing dense to very dense sandy gravel, locally with lenses of silt, clay or peat.
Engineering properties for the River Terrace Deposits in Section 3 were obtained from the following HA GDMS reports:
HA GDMS report 1506 – covering Junctions 8/9 to 10 HA GDMS report 3269 – covering Junctions 8/9 HA GDMS report 20433 – covering Junction 8/9 to 10 HA GDMS report 26747 – covering Junction 10
The historical reports available on HA GDMS do not distinguish between River Terrace Deposits 4 and River Terrace Deposits 5 therefore all engineering parameters are referred to as River Terrace Deposits. The HA GDMS reports show that the standard penetration test (SPT) ‘N’ values are of similar ranges, ranging between 10 and 55 blows (HA GDMS report 1506), 6 and 40 blows (HA GDMS report 26747) and 10 and 30 blows (HA GDMS report 3269). The values for the angle of internal friction (ɸ’) vary from 30 to 36 degrees. Only one report, HA GDMS report 26747, provides information on the liquid limit (LL) and the plasticity index (PI) of the River Terrace Deposits. These values are presented in Table 5.3 in Appendix I.
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c. Head
The head deposits occurring along Section 3 of the Scheme are described by the BGS GeoIndex website (http://www.bgs.ac.uk/geoindex/) as polymict deposits comprising gravel, sand and clay, with peat and organic material in places. The composition is dependent on the upslope source and the distance from the source. Generally, the material is poorly sorted and poorly stratified formed mostly by solifluction and/or hillwash and soil creep (BGS GeoIndex, 2013). As reported by CIRIA C583 ‘Engineering in the Lambeth Group’ (2004) any slopes formed within the Reading Formation of the Lambeth Group are likely to be mantled by 1-3m of Head deposits containing shear surfaces. In places where clays of the Reading Formation are overlain by Head deposits, the clay beneath the Head is likely to be brecciated and softer than the clay beneath (Hight, D W; Ellison, R A & Page, D P. 2004).
HA GDMS report 1512 is the only historical HA GDMS report which describes the occurrence of head deposits along Section 3 of the Scheme and it’s anticipated origins. No specific engineering properties however are provided within the report. 5.4.3 Bedrock
a. London Clay
The London Clay in Section 3 of the Scheme is described by the Technical Appraisal Report (Mouchel, 2011b) as stiff or very stiff blue-grey silty clay with local beds, partings or lenses of fine sand, weathering to firm or firm to stiff and mottled blue-grey and brown.
Engineering properties for the London Clay in Section 3 were obtained from the following HA GDMS reports:
HA GDMS report 1506 – covering Junctions 8/9 to 10 HA GDMS report 1512 – covering Junction 10 HA GDMS report 3269 – covering Junctions 8/9 HA GDMS report 20433 – covering Junction 8/9 to 10 HA GDMS report 20960 – covering Junction 10 HA GDMS report 26747 – covering Junction 10
HA GDMS reports 26747, 1512 and 20960 make a distinction between parameters of weathered and un-weathered London Clay.
Across the historical HA GDMS reports, the obtained values for undrained shear strength for weathered and un-weathered London Clay reported similar ranges, varying between 20 and 370kPa. The range of standard penetration test (SPT) ‘N’ values is also very broad, ranging from 4 to 60 blows for weathered and un-weathered London Clay. The lower limit of range for the angle of internal friction (ɸ’) for weathered and un-weathered London Clay was reported at 20 degrees. The upper limit of the range however is greater in un-weathered London Clay at 30 degrees compared to 23 degrees in weathered deposits. HA GDMS report 1512 reports a lower limit for plasticity index (PI) of just 1% for both weathered and unweathered London Clay, compared to values of 15% and 24% presented in the other HA GDMS reports. The values for remaining parameters fall into similar ranges. These values are presented in Table 5.3 in Appendix I.
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b. Lambeth Group
Engineering properties for the Lambeth Group, comprising the Upnor Formation and the Reading Formation, in Section 3 were obtained from the following HA GDMS reports:
HA GDMS report 1506 – covering Junctions 8/9 to 10 HA GDMS report 3269 – covering Junctions 8/9 HA GDMS report 17320 – covering Junction 8/9 HA GDMS report 20433 – covering Junction 8/9 to 10
The results presented in Table 5.3 (Appendix I) are assumed to be associated with the Reading Formation only. This is recorded to comprise mottled silty clay and clay as the dominant lithology but in areas beds of silt and sand may constitute up to 50% of the formation (Hight, D W; Ellison, R A & Page, D P. 2004). For this reason and based on the historical information available the Reading Formation is subdivided into clay and sand units for the purposes of assessing the engineering characteristics. The results suggest a large range of undrained shear strength values for the clay and sand units ranging between 35 to 350kPa. The standard penetration tests (SPT) ‘N’ values for the clay unit and sand unit also show a wide range of results ranging between 5 and 110 blows and 13 to 150 blows respectively. The angle of shearing resistance (ɸ’) for the clay unit seems to be consistent across the HA GDMS reports ranging between 24 and 26 degrees. The only available value of ɸ’ for the sand unit is 32 degrees found in HA GDMS report 20433. These values are presented in Table 5.3 in Appendix I. 5.4.4 Groundwater
Groundwater information has been obtained from the following HA GDMS reports: HA GDMS report 1506 – covering Junctions 8/9 to 10 HA GDMS report 20433 – covering Junction 8/9 to 10
HA GDMS report 1506 states that the groundwater levels within the River Terrace Deposits are generally a metre or two from the surface. It notes that the groundwater levels in the London Clay and Reading Beds is likely to be variable. A hydrostatic condition is expected with piezometric pressures close to ground surface where the underlying chalk is not used for abstraction. Where the chalk is used for abstraction the piezometric levels towards the base of the London Clay and within the Reading Beds may be reduced. The report also notes that where the Upper Chalk is overlain by the London Clay (between Junctions 8/9 to 10) the water levels reflect the peizometric surface in the Chalk. Works at or near to ground level would not be affected by the Chalk piezometric levels. It indicates that according to work by the BGS (1:100,000 hydrogeological map) the piezometric surface is at approximately +30mAOD. The reports notes that data from a ground investigation by Soil Mechanics in 1992 (refer to HA GDMS report 18152) indicates the piezometric surface to be +24mAOD by Junction 8/9.
HA GDMS report 20433 presents groundwater levels based on monitoring results in HA GDMS 18152. It records groundwater levels of 24.24mAOD in BH220 (Chainage 34500) and 27.4mAODin BH16 (Chainage 36300). 5.4.5 Summary of predicted engineering properties
Table 5.2 (Appendix I) summarises the predicted engineering properties of the strata found along Section 3 of the Scheme. The data was extracted from relevant HA GDMS reports, as listed in the
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Highways Agency — M4 J3-12 MM-ALR sub-sections above, and includes values obtained from laboratory tests, correlations and/or recommended design values. The table presents the minimum and maximum test results and where available information on average or median values and the number of tested specimens have also been included
5.4.6 Section 4
Junction 10 to Junction 12 (Scheme Chainage 46150 to 62900)
The following strata are anticipated to be encountered along Section 4:
Made Ground (embankment fill) Alluvium Brickearth Beenham Grange Gravel Member River Terrace Deposits – five different types recorded Langley Silt Member London Clay Lambeth Group
The following sub-sections describe each of these strata in more detail and present their likely engineering properties which have been obtained from historical HA GDMS reports. 5.4.7 Made Ground
The Technical Appraisal Report (Mouchel, 2011b) describes the Made Ground along Section 4 of the Scheme as being well compacted, obtained from local sources, having variable thickness and related mostly to motorway pavement construction and embankment fill.
Engineering properties for the Made Ground in Section 4 were obtained from the following HA GDMS reports:
HA GDMS report 1506 – covering Junction 10 HA GDMS report 1512 – covering Junction 10 HA GDMS report 17780 – covering Junction 11 HA GDMS report 19429 – covering Junction 12 HA GDMS report 20153 – covering Junctions 10 to 11 HA GDMS report 20563 – covering Junctions 10 to 12 HA GDMS report 20960 – covering Junction 10
The reports show significant variations in parameters ranges which may result from of the variation in Made Ground used along Section 4 of the scheme. These values are presented in Table 5.4 in Appendix I. 5.4.8 Superficial deposits
a. Alluvium
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The Technical Appraisal Report (Mouchel, 2011b) describes the alluvium as normally consolidated, soft to very soft, silty clay containing angular to sub-rounded fine to medium gravel clayey silt of variable, often organic composition. The report also suggests a zone of desiccated material of firm to stiff consistency may occur near the surface.
Engineering properties for the alluvium in Section 4 were obtained from the following HA GDMS reports:
HA GDMS report 1506 – covering Junction 10 HA GDMS report 17780 – covering Junction 11 HA GDMS report 19429 – covering Junction 12 HA GDMS report 20399 – covering Junctions 10 to 12 HA GDMS report 20153 – covering Junctions 10 to 11
All five HA GDMS reports suggest the lower limit of undrained shear strength to be around 20kPa. Two of the HA GDMS reports however suggest the upper bound value of undrained shear strength to be considerably higher, around 150kPa (HA GDMS reports 1506 and 20153) compared to the value of 60kPa quoted in HA GDMS reports 17780, 20399 and 19429). The angle of shearing resistance (ɸ’) ranges between 20 and 28 degrees across four HA GDMS reports. HA GDMS report 20153 provides a considerable amount of information on anticipated values of Atterberg Limits. These values are presented in Table 5.4 in Appendix I.
b. Brickearth
The Technical Appraisal Report (Mouchel, 2011b) describes the Brickearth as typically consisting of brown to orange silt, with a minor fraction of fine sand, variable in composition. Information on the engineering properties of the Brickearth deposit was only available in HA GDMS report 1560. These values are presented in Table 5.4 in Appendix I.
c. Beenham Grange Gravel Member
This material is described by the BGS GeoIndex website (http://www.bgs.ac.uk/geoindex/) as gravel, with variable amounts of sand and clay. A review of the historical HA GDMS reports did not reveal any information regarding the engineering properties of the Beenham Grange Gravel Member.
d. River Terrace Deposits
A review of the HA GDMS dataset identifies five different types of River Terrace Deposits occurring in Section 4 of the Scheme. The characteristics of each type are very similar, described by the BGS GeoIndex website (http://www.bgs.ac.uk/geoindex/) as sand and gravel, locally with lenses of silt, clay or peat.
The HA GDMS database provides very limited amount of information regarding the engineering properties of the River Terrace Deposits. Engineering properties for the River Terrace Deposits in Section 4 were obtained from the following HA GDMS reports:
HA GDMS report 1506 – covering Junction 10
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HA GDMS report 17780 – covering Junction 11 HA GDMS report 20399 – covering Junctions 10 to 12
The above HA GDMS reports suggest a range for the undrained shear strength of between 10 and 55kPa, a value for the angle of internal friction (ɸ’) of around 36 degrees, a unit weight of 21kN/m3 and a cohesion of 0kPa. These values are presented in Table 5.4 in Appendix I. 5.4.9 Bedrock
a. London Clay
The London Clay occurring in Section 4 of the Scheme is described by the Technical Appraisal Report (Mouchel, 2011b) as very stiff blue-grey silty clay with local beds, partings or lenses of fine sand, weathering to firm and firm to stiff, mottled blue-grey and brown.
Engineering properties for the London Clay in Section 4 were obtained from the following HA GDMS reports:
HA GDMS report 1506 – covering Junction 10 HA GDMS report 1512 – covering Junction 10 HA GDMS report 17780 – covering Junction 11 HA GDMS report 20153 – covering Junctions 10 to 11 HA GDMS report 20399 – covering Junctions 10 to 12 HA GDMS report 20563 – covering Junctions 10 to 12 HA GDMS report 20960 – covering Junction 10
Values of undrained shear strength range significantly for weathered and un-weathered London Clay ranging between 20 and 370kPa. Similarly, ‘N’ values from standard penetration tests (SPT) show a broad array, ranging between 4 and 50 blows for weathered and un-weathered material. The angle of shearing resistance (ɸ’) varies from 20 to 30 degrees. The lower bound value for the plasticity index (PI) included in HA GDMS report 1512 is equal to 1%, which is very low compared to the remaining data which suggest this value is between 15% and 29%. All other parameters fall within similar ranges. These values are presented in Table 5.4 in Appendix I.
b. Lambeth Group
Engineering properties for the Lambeth Group, comprising the Upnor and the Reading Formation, in Section 4 were obtained from the following HA GDMS reports:
HA GDMS report 1506 – covering Junction 10 HA GDMS report 20399 – covering Junctions 10 to 12 HA GDMS report 20563 – covering Junctions 10 to 12
The results presented in Table 5.4 (Appendix I) are assumed to be associated with the Reading Formation only. This is recorded to comprise mottled silty clay and clay as the dominant lithology but in areas beds of silt and sand may constitute up to 50% of the formation (Hight, D W; Ellison, R A & Page, D P. 2004). For this reason and based on the historical information available the Reading Formation is subdivided into clay and sand units for the purposes of assessing the
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Highways Agency — M4 J3-12 MM-ALR engineering characteristics. Undrained shear strength for both the sand and clay units have a broad range, ranging between 35 to 350kPa. Values of ‘N’ from the standard penetration test (SPT) also vary significantly, ranging between 5 and 50 blows for the clay unit and 13 and 50 blows for the sand unit (HA GDMS report 20563). Information on the angle of shearing resistance (ɸ’) for clay and sand units have been found only in HA GDMS report 20399 and are reported as 24 and 32 degrees respectively. These values are presented in Table 5.4 in Appendix I.
c. Seaford Chalk Formation
The Seaford Chalk Formation, formerly known as the Upper Chalk Formation, comprises soft white or greyish-white, microcrystalline limestone containing flints (HA GDMS report 20399). On BGS Sheet 268, the Upper Chalk is described as white, nodular and soft with seams of flint (BGS, 2000). It is recorded as being between 90m and 130m thick (BGS, 2000).
Engineering properties for the Seaford Chalk Formation in Section 4 were obtained from the following HA GDMS reports:
HA GDMS report 1506 – covering Junction 10 HA GDMS report 19429 – covering Junction 12
HA GDMS report 1506 shows that the undrained shear strength (cu) of the chalk increases with depth from 15kPa at 7.5m to more than 35kPa at 15m. HA GDMS report 19429 provides information on the angle of internal friction (ɸ’), unit weight and cohesion (c’), which are estimated to be 40 degrees, 20kN/m3 and 10kPa, respectively. These values are presented in Table 5.4 in Appendix I. 5.4.10 Groundwater
Groundwater information has been obtained from the following HA GDMS reports: HA GDMS report 1506 – covering Junction 10 HA GDMS report 20399 – covering Junctions 10 to 12 HA GDMS report 20563 – covering Junction 10 to 12
HA GDMS report 1506 states that the groundwater levels within the River Terrace Deposits are generally a metre or two from the surface. It notes that the groundwater levels in the London Clay and Reading Beds is likely to be variable. A hydrostatic condition is expected with piezometric pressures close to ground surface where the underlying chalk is not used for abstraction. Where the chalk is used for abstraction the peizometric levels towards the base of the London Clay and within the Reading Beds may be reduced. The report also notes that where the Upper Chalk is overlain by the London Clay (between Junctions 8/9 to 10) the water levels reflect the piezometric surface in the chalk. Works at or near to ground level would not be affected by the chalk piezometric levels. It indicates that according to work by the BGS (1:100,000 hydrogeological map) the piezometric surface is at approximately +30mAOD.
HA GDMS report 20399 notes that limited information regarding groundwater conditions was obtained from the existing ground investigation data. The report presents groundwater depths for several strata but also notes that the groundwater conditions are based on readings and observations made in the late 1960's (pre-construction investigation) and the early 1990's (post- construction investigation) and may not reflect the current condition.
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HA GDMS report 20563 presents groundwater levels between Junction 10 and 12, based on information contained on borehole logs in HA GDMS report 18152. It records groundwater levels at between +34mAOD and +53mAOD within the near surface Made Ground, Alluvium and Quaternary River Deposits. 5.4.11 Summary of predicted engineering properties
Table 5.2 (Appendix I) summarises the predicted engineering properties of the strata found along Section 4 of the Scheme. The data was extracted from relevant HA GDMS reports, as listed in the sub-sections above, and includes values obtained from laboratory tests, correlations and/or recommended design values. The table presents the minimum and maximum test results and where available information on average or median values and the number of tested specimens have also been included
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6 PRELIMINARY ENGINEERING ASSESSMENT
6.1 Introduction This section of the PSSR will present a preliminary consideration of the proposed design implications along the scheme route. Table 6.1 in Appendix J presents a summary of geotechnical constraints with respect to proposed structures (sign/signal gantries), Emergency Refuge Areas (ERAs)’s and retaining walls. Overbridges and underbridges are considered individually in section 6.10 below.
6.2 Location
6.2.1 Section 1
Junction 3 to Junction 5 (Scheme Chainage 10000 to 20150)
For this section of M4 MM scheme the existing motorway is generally at-grade (earthworks <2.5m deep/high) with the exception of the main motorway junction interchanges where the mainline is generally carried on embankment over the interchanges. A section of shallow cut (<4m deep) runs alongside the westbound carriageway between Chainage 14600 to 15600. The mainline is also carried on low height (<3m high) embankment between Junction 4b and chainage 18200. The semi-urban nature of the motorway may limit the potential for cost effective land take, if required. 6.2.2 Section 2
Junction 5 to Junction 8/9 (Scheme chainage 20150 to 34100)
This section of the scheme commences at grade from Junction 5 until chainage 24700 from where the mainline carriageway is carried on embankment (<5m). West of Junction 6 the motorway returns to being at-grade until Junction 7. Between Junction 7 and Junction 8 the motorway is carried by intermittent sections of embankment (<5m) and at-grade earthworks. 6.2.3 Section 3
Junction 8/9 to Junction 10 (Scheme chainage 34100 to 46150)
This section of the scheme is predominantly at grade with locally discrete sections of cutting (~3m) and embankment (<5m). The motorway is generally set within a rural environment. 6.2.4 Section 4
Junction 10 to Junction 12 (Scheme chainage 46150 to 62900)
West of Junction 10 the motorway is carried on embankment until chainage 51000. From here heading west the motorway continues in a semi-rural setting with localised sections of cutting and at-grade earthworks until Junction 11. Between Junction 11 and Junction 12 the motorway is generally at-grade with the exception of embankments at locations where it crosses a railway line (chainage 57200) and then on the approach to the Junction 12 interchange. The motorway is significantly constrained by open water filled gravel pits between Reading Motorway Service Area and Theale Interchange (Junction 12).
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6.3 Soils
6.3.1 Made Ground
It is anticipated that Made Ground, in the form of road construction materials, will be encountered underneath the carriageway and within the immediate vicinity of the existing carriageway and any link/slip roads. Due to the age of the existing motorway earthworks it can be assumed that engineered fill will have been used to construct the earthworks throughout the scheme route in a generally controlled manner. The engineered fill is expected to consist of locally won material (from nearby cuttings or borrow areas) and generally be well compacted. The potential exists for areas of poorly compacted or softened material to be encountered by the scheme works. The stability of existing earthworks is discussed in Section 6.14.
Considering the nature of the local fill source materials it is possible that some of the Made Ground forming the embankments will be derived from the London Clay Formation that can exhibit high pH and sulphate content values. It is suggested that testing of samples of existing embankment fill (and Made Ground) should be undertaken in order to assess its chemical aggressiveness and to inform the specification of buried concrete to be used on the scheme. Furthermore, non- engineered fill may also be encountered along the scheme route as a result of the infilling of worked out clay (brick manufacture) and gravel pits.
There are extensive sections of the scheme where the clay and gravel pits are recorded to have been infilled with landfill materials. These abut the motorway boundary and may encroach on the motorway verges. The significant areas of landfill are generally located between Junctions 3 and Junction 5, around Datchet Road Overbridge (Chainage 23300 to 25000), to the east of Junction 8/9 and to the east of Junction 12. These sites are detailed in Table 4.19 in Appendix G and shown on drawings M4MM-MUH-ML-ZZ-DR-GE-200001 to 200030. The depths of the landfill deposits are unknown but it is likely that the clay / sand and gravel will have been worked out in its entirety. The landfill material is likely to be poorly compacted and potentially degradable. Both these factors contribute towards variable founding conditions and the potential for large settlements under self weight over long durations. The issues of leachate and methane production associated with landfill deposits should also be considered when working in these areas. 6.3.2 Alluvium (and old Alluvium)
The deposit is expected to comprise variable amounts of clay, silt, sand and gravel. Layers of peat and organic clay/silt are also anticipated. Areas of alluvium, as mapped at a regional 1:50,000 scale by the BGS, are presented on drawings M4MM-MUH-ML-ZZ-DR-GE-200001 to 200030.
Ellison and Williamson (1999) record the potential ground constraints for the Alluvium as the potential presence of compressible strata, the variable foundation conditions and the risk of flooding.
Due to the relatively low height of the earthworks the alluvium may be encountered at the anticipated founding level of the proposed structures. Therefore, consideration may need to be given to the proposed foundation solution, as the ground conditions may preclude the use of shallow foundations in these areas.
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In any potential areas of widening (for ERA’s) measures may be necessary to limit post- construction settlements. Consideration should also be given to the side slopes in areas of widening over alluvium as shallower slopes will be required to mitigate against potential slope instability. 6.3.3 Langley Silt member (Brickearth)
Deposits of Brickearth are recorded to blanket the majority of the River Terrace Deposits across this section of the scheme. They may be up to 2m thick and given the relatively low height of the earthworks the material may be encountered at the anticipated founding level of any proposed structures . Northmore, Bell and Culshaw (1996) note that whilst Brickearth can provide a suitable foundations for civil engineering structures and should behave normally in short-term excavations, it has been shown to be metastable. They report that under pressures in excess of 200kPa collapse of the soil may occur, upon remoulding the metastable brickearth loses this characteristic and therefore can be used for embankment construction. Therefore, the deposits may not be considered as a suitable bearing stratum for any proposed shallow foundations, noting the pressures outlined above.
Tomlinson (2001) notes that Brickearth deposits are also liable to collapse of their structure on wetting which may occur as a result of flooding or broken water mains. He also notes that excavations should be protected from erosion by flowing water. Ellison and Williamson (1999) record the potential ground constraints for the Langley Silt as its metastable nature when wet. 6.3.4 River Terrace Deposits
The River Terrace Deposits generally mantle the solid deposits through this section of the scheme. They are coarse grained deposits that were laid down during period of high sea levels compared to present. They consist of medium dense to very dense sandy flint gravels with occasional clay layers. They have been extensively worked in the scheme area for their use as a concrete aggregate.
Ellison and Williamson (1999) record the potential ground constraints for the River Terrace Deposits as a high water table, the possibility of undocumented and filled former pits and a potential for variable ground conditions. 6.3.5 Thames Group (London Clay Formation)
The London Clay Formation is the predominant “bedrock” deposit along the scheme from its onset at Junction 3 to Datchett Road overbridge (chainage 24279) and then again from Junction 8/9 (approx.) to Pingewood (chainage 56700). The deposit thickness generally thins towards the west, away from the centre of the London Syncline. HA GDMS Report 3629 (Acer Consulting Ltd., 1992) noted that “site investigation boreholes had proved up to 30m of grey stiff fissured silty or sandy clay with scattered shell debris”.
Ellison and Williamson (1999) note that the potential ground constraints for the London Clay are a potential for ground heave, high concentrations of sulphate in the ground and groundwater and; the potential for a perched water table in the Claygate Member (top part of London Clay). Paleogene deposits (including London Clay Formation and Lambeth Group) are noted by Ellison and Williamson (1999) to contain pyrite when unweathered. In near surface deposits of these materials the weathering process oxidises the pyrite to give sulphate ions in solution. In London Clay this weathering process also reacts with calcium carbonate to produce selenite crystals, leading to
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The Lambeth Group is generally present as the “bedrock” material between Datchett Road (chainage 24279)Overbridge and Junction 8/9 (approx.). It is also present from Pingewood (chainage 56700) to the Theale Interchange (Junction 12).
Ellison and Williamson (1999) note that the potential ground constraints for the Lambeth Group (Reading Formation) are the variable foundation conditions, a potential for ground heave, a perched water table and sink holes close to the contact with the Upper Chalk. Hight et al (2004) reports an extensive list of significant engineering hazards with regard to the Lambeth Group. Based on the current scheme proposals the hazards posing the highest risk have been refined to:
Variable lithology including weak rock, gravel, sand, silt and low to high plasticity clays varying vertically and laterally through the deposits. Water bearing sand filled channels that are laterally impersistent Hard bands, pebble beds and calcretions presenting obstructions to boring, piling and excavations. Pebble beds abrasive due to flint gravels. Presence of glauconite and pyrite. Weathering of the Lambeth Group can release these products that are aggressive to materials. Presence of smectite within clay material and resultant large volumetric changes (shrink/swell) due to changes in moisture content. Chalk solution features. These affect the Lambeth Group materials through subsidence or collapse into voids created by dissolution of the underlying chalk.
HA GDMS Report 20563 (Mouchelparkman, 2005) states that as the area has an extensive history of brick making from the 16th century it is likely that further unrecorded mineral extraction may have occurred in the vicinity of the M4 between Junction 10 to Junction 12. 6.3.7 Upper Chalk
The Upper Chalk is recorded to outcrop directly beneath the scheme route from the River Kennet (chainage 61156) to Theale Interchange (Junction 12). Further to the east it underlies the entire site lying uncomformably beneath the tertiary deposits of the Lambeth Group and Thames Group (London Clay Formation).
Ellison and Williamson (1999) note that the potential ground constraints for the Upper Chalk are slightly elevated natural radon emissions, the requirements for groundwater protection, possibility of undocumented and filled former pits and dissolution cavities and sink holes. Lord, Clayton & Mortimore (2002) note that dissolution features pose the biggest risk to foundations (and earthworks) in chalk. They also consider that when selecting foundations in chalk, consideration should always first be given to the use of shallow foundations, either bearing directly on the chalk itself or on superficial deposits overlying the chalk (not withstanding the presence of dissolution features).
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HA GDMS Report 20563 (Mouchelparkman, 2005) notes that there are no recorded natural cavities with the Upper Chalk within 1km of Junction 10-12 of the M4 motorway. The nearest natural cavity is noted to be 2.1km to the north. The report states that immediately to the southeast of Junction 12 (Theale Interchange) the route passes over the Lambeth Group outcrop and it is possible that solution features may be present there and in the near vicinity where the tertiary cover to the Upper Chalk is thin or absent.
6.4 Groundwater As noted in Section 6.3.4 there will be several groundwater constraints within this section of the scheme. High groundwater tables are anticipated within the River Terrace Deposits (perched on top of tertiary deposits). Given the relatively high permeability of the terrace deposits, these are likely to be hydraulic continuity with the local river systems (River Thames etc.). Consideration should be given to the potential for contamination from particular construction methodologies and where practicable these should be mitigated through the design.
It would also be prudent to consider the effect of any ongoing or planned groundwater pumping activities, or the cessation of pumping activities, on the groundwater levels used in design. There are also expected to be localised perched water tables (and springs) in granular layers of the London Clay Formation and the Lambeth Group. These should be taken into consideration when planning excavations and selecting piling methodologies.
Further detailed site specific groundwater monitoring should be undertaken as part of any proposed ground investigations. The proposed groundwater monitoring duration should consider the potential for seasonal variations.
The planned scheme works should also consider any impact from the various source protection zones located throughout the scheme route (refer to Section 4.7). This is likely to impact on the proposed ground investigation and construction methodologies as well as potentially the scheme drainage works.
6.5 Cuttings If proposed ERAs and gantries are to be located within the existing cuttings it is likely that local widening of the existing verge will be required. As there is likely to be little or no scope for additional land-take (to maintain the cut slopes at the present gradients) the cutting slopes will become locally steepened and/or will likely to require some form of strengthening/retaining to maintain overall stability of the earthwork. Strengthening of the cutting is likely to comprise soil nailing or similar, whilst a retaining solution may consider options such as a sheet piled cantilever wall or a gabion wall. The preferred solution will depend on the ground and groundwater conditions at the proposed location and the geometry of the existing and proposed verge width, earthwork slope and motorway boundary.
Excavated cut material generated during the earthworks operations may be suitable for re-use as fill elsewhere on the scheme, provided it meets the appropriate acceptability criteria. It is anticipated that there may be a proportion of topsoil and unsuitable material that cannot be reused on site and will require removal and appropriate disposal.
Excavations into existing earthworks slopes will need to ensure that the stability of the existing earthworks is not compromised, especially in the short term (during construction).
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6.6 Embankments If proposed ERA’s and gantries are located in the areas of existing embankments it is likely that the embankments will require widening to accommodate the extra verge width. There is likely to be minimal scope for additional land-take along the scheme route limiting the embankment base widths. Therefore, widening of the crest will create steep side slopes in combination with a strengthening or retaining solution. These are likely to take the form of sheet piled retaining walls, localised widening with a strengthened earthwork (reinforced earth) or a gravity retaining wall such as gabion or crib walling.
Due to the nature of the widening and the proposed vehicle loading the widening will comprise an appropriate engineering fill material. This material may be sourced from local cut material generated on the scheme (where appropriate materials exisit) or from an off-site source. On embankments, the loadings exerted from the proposed gantry foundations has the potential to compromise the stability of the existing slopes in either the permanent works case or during construction. This may be particularly of concern where existing earthworks instability has been recorded or where earthworks renewals/repairs have previously been implemented. Excavations into existing earthworks slopes will need specific consideration and precautions to ensure that the stability of the existing earthworks is not compromised, especially in the short term (during construction). For the construction of Thames Bray Bridge and possibly others a solution has been recommended which includes piecemeal installation with the use of 200 tonne cranes situated at carriageway level on the widened embankment. Bearing capacity and stability assessments will be required for this critical short term temporary case.
6.7 At-grade Due to the nature of at-grade sections generally requiring narrower verges the potential exists for the motorway land boundary to be in close proximity to the edge of the hard shoulder. This boundary proximity is likely to constrain the possibility of widening to accommodate ERAs and gantry bases without additional land take. The extent of any temporary working areas and access should also be considered as these may have a larger footprint than the final proposed structures.
6.8 Pavement Foundations Only limited localised widening of the highway pavement, requiring full depth construction, is anticipated. This is expected to be limited to the areas where existing bridges are to be widened or replaced, as the existing hard shoulder is discontinuous.
Elsewhere, where the motorway carriageway and hard shoulder already exist, an assessment of the pavement and subgrade strength may be required prior to any re-surfacing/re-conditioning. This will be dependent the life expectancy of the existing surfacing. The existing hardshoulders have a surface course of either thin asphalt or HRA with stone chippings. It is assumed that the conversion of hardshoulders to running lanes will require a new surface course to provide adequate texture. New road surface proposals will be low noise. Areas of existing Thin Surface Course will remain if pavement life and skid resistance are adequate.
The subgrade strength in any areas of the newly constructed ERA’s is dependent on the material into which the cut is formed or on the compacted material forming the embankment. Appropriate testing of the subgrade at formation level in cutting areas and acceptability testing/compaction control in embankment areas will be required.
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6.9 Drainage It is likely that the existing motorway drainage along the scheme route for the hard shoulder will require some re-design and reconstruction to accommodate the addition of the ERA’s, the localised widening and the hardening of the central reserve.
It is anticipated that additional drainage capacity will come from the oversizing of pipes and construction of enlarged manholes to provide retention and maintain existing outfall (discharge) rates.
6.10 Replacement/widening of restrictive Overbridges and Underbridges
6.10.1 Introduction
There are 11 overbridges and 14 underbridges that require either demolition and replacement or significant works. A documented optimisation process has been used to derive the most appropriate structural design for each bridge taking account of as-built drawings (where available), land availability, buildability, cost, diversion routes and local authority requirements. This has resulted in significant usage of retaining walls, use of partial off-line designs to enable traffic flow during construction and potential re-usage of some sub-structures.
All overbridges and 4 underbridges require significant geotechnical consideration and these are listed in Table 6.2 below. Other underbridges and subway extensions in particular will require consideration of the potential for excessive differential settlement between the old and new sections which will require mitigation by design.
Table 6.2: Summary of bridges requiring significant geotechnical consideration
Structure Name HA Structure Marker Post Scheme chainage Ref: (approx.) Old Slade Lane Overbridge 948 28/5 17559 Langley Interchange East Underbridge 952 30/9 20050 Langley Iinterchange West Underbridge 953 30/10 20256 Riding Court Overbridge 960 33/3 22448 Recreation Ground Overbridge 962 34/8 23919 Datchet Road Overbridge 963 35/2 24279 Windsor Railway Underbridge 966 36/5 25566 Wood Lane Overbridge 971 38/2 27314 Oldway Lane Overbridge 972 39/3 28436 Huntercombe Spur Overbridge 973 39/9 29022 Huntercombe Lane Overbridge 974 40/4 29544 Marsh Lane Overbridge 977 41/5 30621 Thames Bray Underbridge 978 42/1 31240 Monkey Island Lane Overbridge 979 42/5 31604
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Table 6.2: Summary of bridges requiring significant geotechnical consideration
Structure Name HA Structure Marker Post Scheme chainage Ref: (approx.) Ascot Road Overbridge 986 44/2 33342
The overbridges will be modified through a combination of online and offline replacement involving staged demolition and re-construction. Underbridges will be widened to accommodate the ALR alignment. Further information on the proposals is presented in the Extended Options Phase Report (URS, 2013). The geotechnical information at pertinent bridges, for the Extended Options Phase Report was produced by URS, as part of the M4 MMALR Alliance design team. This information is included as Sections 0 to 6.10.15 of this report.
For ease of reference note that alluvium (inc old alluvium), faults and historic ponds/watercourses are shown on drawings M4MM-MUH-ML-ZZ-DR-GE-200001 to 200030 together with:
identified historical ground investigations (see Section 1.4, Appendix A and Appendix B) landfill sites (see Section 4.9 and Appendix G locations of earthworks defects (see Section 4.12 and Appendix H)
Details of previous geotechnical studies and the historic excavation hole nomenclature used for this report are included in section 1.4. Appendix B contains copies of all relevant historical ground investigation logs.
6.10.2 Old Slade Lane Overbridge (948)
6.10.2.1 Existing and proposed structures The existing overbridge is a 3-span structure with a total span of 67.4m and a central span of 36.3m. The piers and abutments have combined spread foundations at 20.05mAOD.
The preferred solution is to demolish the existing structure and to construct a new single-span composite bridge (36.1m clear span) on-line during temporary road closure. The new bridge will have spread foundations and about the same level as the existing foundations.
Alternatively, if full closure, demolition and construction on-line are not practicable due to traffic requirements or the need to carry utilities, the single-span composite bridge could be constructed fully off-line to the east.
6.10.2.2 Ground conditions
The geological survey map of area shows superficial alluvial deposits of clay, silt, sand and gravel overlying London Clay.
The geological interpretation drawing, marked as Figure 2.10 (ref. RT-DTP0201-206-02) in HA GDMS Report 3268 shows several boreholes in the vicinity of the existing bridge. The closest for which records are available were put down by Harrison & Company in 1992. Borehole 102:19 was about 30m west of the south abutment and Borehole 103:19 was about 20m west of the north abutment.
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River terrace deposits of sandy gravel (part of the Shepperton Gravel Member) were encountered at a level of 21.5mAOD in both boreholes, beneath more than 2m of made ground (2.2m in 102:19 and 2.9m in 103:19). The sandy gravel was 5.5m thick on the south side (102:19) and 3.9m thick on the north side (103:19), and was underlain by London Clay, which was recovered as fissured silty clay with fine sand partings and was proved to a maximum depth of 12.0m.
Water strikes occurred in both boreholes and ‘standing levels’ were recorded at 2.8m and 2.5m depth, close to the top of the sandy gravel. However, these are unlikely to be reliable indicators of equilibrium groundwater level as water was added to assist boring.
SPT tests were carried out in both the sandy gravel and the London Clay. Those in the sandy gravel gave a range of N-values from 26 to 50, indicating a medium-dense to very dense state. N- values in the London Clay ranged from 21 to 46, which suggest a stiff to very stiff condition with an average undrained shear strength of probably about 150kPa in the upper 5m of this formation.
The geological interpretation drawing in HA GDMS Report 3268 also shows summary logs of further boreholes taken to a depth of 30m or more in the general area of Old Slade Lane bridge. In several of them, the Woolwich and Reading Beds (now considered part of the Lambeth Group formation) are shown as having been encountered beneath the London Clay, although there is great variation in levels which may indicate uncertainty in classification.
For preliminary purposes, ground conditions may be assumed as follows. Engineered fill is likely to have replaced the upper layers of topsoil and made ground over most of the area. Distinction between the London Clay and Lambeth Group formations will have no influence on design and construction of the new bridge.
Top level Base level Thickness (m) Soil Strata Formation (mAOD) (mAOD) 24.0 21.5 2.5 Replacement fill Topsoil / made ground 21.5 17.0 4.5 Dense gravel and sand Shepperton Gravel 17.0 (0.0) (17.0) Stiff/very stiff fissured clay London Clay Mainly mottled clays with silt (0.0) - - Lambeth Group1 and sand layers
Notes: 1. Formerly known as the Woolwich and Reading Beds formation in this area
6.10.2.3 Alternative foundation types
Spread foundations are proposed for the preferred option of a new single-span bridge built on-line. They would be equally suitable for the alternative off-line bridge construction.
Bored piles founded in the stiff to very stiff London Clay would also be feasible.
For the alternative off-line solution, retaining walls will be required on the east side to keep the new construction within existing land boundaries. Reinforced soil walls at the toe of the existing slope to the east are proposed as the most suitable solution. Foundation conditions will need to be confirmed by further ground investigation.
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6.10.2.4 Bearing pressures and settlements
The bearing pressures and settlements mentioned below are broad estimates based on very limited information. They are not intended as precise, conservative or limiting values and should not be taken out of this context.
A net allowable bearing pressure of at least 250kPa would be appropriate for spread foundations in the Gravel. The results of SPT tests in boreholes 20m to 30m west of the existing bridge would suggest a dense state and hence a bearing pressure well in excess of this figure, but this would need to be confirmed by further ground investigation at the abutments.
For the preferred on-line option, the new spread foundations will be in a similar location to the existing foundations and at about the same level, approximately 20.0mAOD. The underlying strata (about 3.0m of Gravel over London Clay) will have effectively been preloaded and this will reduce settlement, to probably less than 10mm.
This does not apply to the alternative off-line location, where settlements of about 25mm would be expected, including a small long-term component due to consolidation of the London Clay.
An alternative foundation of 10m long piles of 900mm nominal diameter would provide an allowable working load of about 1,000kN and pile group settlement would be less than 10mm.
6.10.2.5 Potential geotechnical problems and construction difficulties
For the preferred on-line bridge replacement, the proposed level of the new spread foundations is 20.05mAOD, the same as the existing foundations. In the two boreholes west of the bridge, ‘standing levels’ were recorded at 20.9mAOD and 21.9mAOD. Based on this information, groundwater would thus need to be lowered by 1m to 2m to enable foundations to be constructed in the dry. Unless this were done with great care, groundwater flow could cause settlement of the M4 pavements.
The borehole water levels may not be reliable as water was added to assist boring. Nevertheless, before deciding on the most suitable method of foundation construction, it will essential to determine groundwater levels at the bridge during further ground investigation.
Made ground was also revealed in these boreholes to a maximum depth of 2.9m (21.5mAOD). This consisted mainly of granular material but also contained various foreign matter, wood and clay. If similar material had been present at the site of the existing bridge, this would almost certainly have been excavated and replaced with engineered fill.
It is possible that made ground or other unsuitable soils might still be present immediately east of the existing bridge in the area of the alternative off-line construction, especially near the toes of the embankment slopes where reinforced soil walls are required. If so, these materials would need to be excavated and replaced with granular fill or the wall foundation levels lowered. The abutment foundations are less likely to be affected as they would be at a lower level, but again this would need to be confirmed by ground investigation.
6.10.3 Langley Interchange East & West Underbridges (952 & 953)
6.10.3.1 Existing and proposed structures
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The existing underbridges at Langley Interchange are 3-span structures with similar overall spans (about 41.1m) and deck widths (about 25.9m). They both have spread foundations. Founding levels are not precisely known but are believed to be approximately 22.0mAOD at the East bridge (972) and 21.0mAOD at the West bridge (973).
The preferred new bridges option is to widen the existing bridges on both sides by 5.7m to give a new overall width of 37.3m. It is proposed to support the new bridge extensions on pile foundations.
A group of four piles will be installed at each intermediate pier, with pile cap soffit levels about 1.5m above existing foundation levels. This will enable the pile caps to straddle the existing spread foundations. A further pair of piles will be installed at a high level to support an integral abutment.
The approach embankments will need to be widened on both sides. If the existing slope angles are maintained, the increase in embankment width will be similar to that of the bridge deck. If there is insufficient space to enable this to be achieved, either the slopes will need to be steepened or retaining walls incorporated in the design.
6.10.3.2 Ground conditions
Very little information is available on ground conditions at these structures. The geological survey map of area shows superficial deposits of sand and gravel of the Taplow Gravel formation overlying London Clay.
A borehole (BH285:044) included in HA GDMS report 17283, was undertaken from the top of the existing M4 embankment and is shown in Figure A29 of the report. The figure is of a drawing marked ‘M25 Heston & Godstone Control Offices, Additional CCTV Camera Sites’. The location plan is unclear. However, a camera platform exists on the north side of the embankment about 30m east of the east abutment of Bridge 972 (Langley Interchange East) and it seems very probable that the borehole was close to this position.
From a ground level of 30.4mAOD, the borehole log shows 7.0m of made ground overlying 3.0m of very dense clayey sand and gravel, in which the borehole terminated.
The upper 3.7m of made ground is marked ‘fill’ and the remainder is described variously as organic silty clay, clayey sandy gravel and gravelly clay. Two SPT tests in the sand and gravel gave values of 56 and 106, which supports its description as very dense. The level of the underlying London Clay formation was not determined.
A water strike occurred at 7.0m depth, at the top of the sand and gravel, and the water level rose by approximately 1.0m to 24.4mAOD. It is presumed that this was the equilibrium level at the time the borehole was drilled.
No further information is available in the immediate area of the interchange structures. However, a series of boreholes were carried out 300m to 800m further to the east, including the area of Sutton Lane overbridge. A summary of the data obtained is presented as a longitudinal section in the drawing DTP 0903/BSU/R/T/007 Rev A dated May 1992, in HA GDMS report 3268.
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The nearest borehole, BH122:019, was about 300m from the Langley Interchange East structure. Boreholes BH119:019 and BH120:019 were located a further 300m east, at the north and south abutments of Sutton Lane overbridge, and Boreholes BH116:019 and BH117:019 were another 200m east, at the site of an existing gantry.
Ground levels varied between 26.0mAOD and 24.0mAOD. Abbreviated strata descriptions are shown simply as embankment fill, river terrace deposits or London Clay. Except at the gantry boreholes, river terrace deposits were encountered beneath a varying thickness of embankment fill. These are presumed to be the Taplow Gravel deposits shown on the geological survey map.
There was considerable variation in thickness of the Gravel, from 1.7m to 6.0m. The underlying London Clay was encountered at levels ranging from 21.3mAOD to 17.6mAOD and was proved to a maximum depth of 14.4m (6.0mAOD) without penetration.
For preliminary purposes, ground conditions at Langley Interchange Bridges 972 and 973 may be assumed as follows.
Top level Base level Thickness (m) Soil Strata Formation (mAOD) (mAOD) 23.5 20.0 3.5 Dense sand and gravel Taplow Gravel 20.0 (6.0) - Stiff/very stiff clay(?) London Clay
In view of the very limited data at the bridge sites, the assumed thickness of the Gravel and the assumed top level of the London Clay must be regarded as tentative.
6.10.3.3 Alternative foundation types
To limit differential settlement between the new and existing foundations, it is proposed to use bored piles to support the widened bridge decks. The new piles at the intermediate piers will have pile cap soffit levels of about 23.5mAOD for the East bridge and 22.5mAOD for the West bridge. The upper part of the pile shaft will be in the dense Gravel but the piles will derive resistance mainly from the underlying London Clay.
The upper section of the abutment piles will be in embankment fill (both the existing fill and the new fill to be placed to widen the embankments) but will similarly derive their resistance mainly from the London Clay.
Spread foundations would be a feasible alternative for the extended deck, solely from the viewpoint of bearing capacity. However, total and differential settlement would be significantly greater than for piles and they are not therefore recommended.
Retaining walls might be required for the widened embankments in some areas. Reinforced soil walls at the embankment toe would probably be founded in the Taplow Gravel, if necessary after removal of any near-surface soft alluvium or other unsuitable soils.
6.10.3.4 Bearing pressures and settlements
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The bearing pressures and settlements mentioned below are broad estimates based on very limited information. They are not intended as precise, conservative or limiting values and should not be taken out of this context.
Estimation of pile capacity must be regarded as tentative in the absence of information on the thickness of the Gravel and the properties of the underlying London Clay. Based on information from elsewhere in the Thames Valley, it is assumed that the London Clay is in a generally stiff or very stiff condition at these bridge sites but possibly only firm near its upper surface.
For a nominal pile diameter of 750mm, piles for the intermediate piers would be expected to provide net allowable working loads of about 1,000kN and 1,300kN for pile lengths of 15m and 20m, respectively. Depending upon the properties of the London Clay, pile group settlement is not expected to exceed 10mm. About half of this may occur as post-construction settlement due to consolidation of these predominantly clay soils.
The abutment piles will be in areas where new fill will be placed to widen the existing embankments. Subsequent settlement of the fill material and of any underlying soft soils has the potential to induce negative skin friction on the pile shaft and this may need to be taken into account in detailed design. For preliminary purposes, it is assumed that net shaft resistance through the embankment will be neutral. For the same toe levels, net allowable working loads for the abutment piles will therefore be the same as those given above for the intermediate pier piles.
Further information from ground investigation is essential for detailed pile design.
A net allowable bearing pressure of about 250kPa would be appropriate for spread foundations in the Taplow Gravel. However, total settlement would be greater than for piles and, depending upon the thickness of Gravel, a significant post-construction settlement could occur due to consolidation of the underlying London Clay. For this reason, spread foundations are not considered a suitable alternative to bored piles for these bridges.
6.10.3.5 Potential geotechnical problems and construction difficulties
The increased loads from the bridge extensions, transferred to the ground through the new piles, will cause stress increases in the strata beneath the existing spread foundations and this may cause a small settlement. The effect will be greatest in the foundations nearest to the new piling and will tend to moderate differential settlement between new and existing parts of the structure. Potential ground movements will need to be examined at the detailed design stage.
Where there is sufficient space to widen the embankments at their current slope angles, this will be the most suitable solution. If soft alluvium or other unsuitable soils are present in areas of proposed widening, they will need to removed and replaced with suitable material before the additional fill is placed. Where the available space is not sufficient, the new side slopes will need to be steeper than the existing slopes or retaining walls will be required either at the shoulder or toe of the embankments.
Slope steepening could be achieved simply by using fill material of superior properties, such as well-graded granular fill, or by introducing slope reinforcement.
Various types of retaining wall could be used. For a small retained height (about 1m) an L-shaped concrete wall at the top of slope would be suitable. For greater heights, a reinforced soil wall at the
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There are no reliable groundwater data in the areas of these structures. Groundwater level will probably be close to the top of the Taplow Gravel but this is unlikely to be a problem for foundation construction.
6.10.4 Riding Court Overbridge (960)
6.10.4.3 Existing and proposed structures
The existing overbridge is a 4-span structure, square to the M4 Motorway, with a total span of 46.9m. The abutments and piers have spread foundations at about 17.2mAOD.
The preferred solution is to construct a new two-span composite deck on-line on the existing substructure during temporary road closure and traffic diversion. The existing verge piers will be demolished and the abutments and centre pier modified to accept the new deck. New retaining walls will be constructed in front of the abutments on both sides to enable the carriageways to be widened.
If it is decided not to utilise the existing substructure, a new single-span composite bridge (36.5m clear span) would be constructed on-line, founded on new spread foundations.
Alternatively, if full closure, demolition and construction on-line are not practicable due to traffic requirements, a single-span composite bridge could be constructed partly off-line to the west while maintaining one lane of traffic. For all options, the approach embankments will need to be raised to cater for the higher deck level of the new bridge and this will require embankment widening. Where necessary, retaining walls will be constructed to keep the new works within existing land boundaries.
6.10.3.4 Ground conditions
The geological survey map of area shows superficial sand and gravel deposits of the Shepperton Gravel Member overlying London Clay.
The only available borehole information is from BGS records (SU97NE313F:173), which show a series of water-well boreholes drilled in 1960 by C.Isler & Co Ltd about 100m north of the bridge site at Riding Court Farm. The six borehole logs, marked A to F, all have the same grid co- ordinates which would suggest, prima facie, that they were put down within about 10m of each other, but that may not have been the case.
Strata depths are given but as there is no indication of ground level at the borehole positions, strata levels cannot be determined. The soils encountered were logged simply as topsoil, ballast, sand and blue clay. The ballast and sand layers are almost certainly part of the Shepperton Gravel and the blue clay is the London Clay. However, the material described as topsoil almost certainly includes other deposits. The average thickness of ‘topsoil’ in the six boreholes (A-F) was 1.3m, which is most unlikely to be the case unless this material was stockpiled. Since the drillers were primarily interested in the
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The thickness of the underlying Gravel has a wide range, from 4.6m to 8.7m (average 6.0m), which also suggests that the boreholes were not as close to each other as their grid co-ordinates imply. The London Clay was proved to a depth of 0.6m in each hole.
All boreholes are reported to have ‘touched water’, at 0.6m to 1.8m depth, generally at the top of the Gravel but in some cases in the overlying soils.
Original ground level at the bridge site was about 18.5mAOD to 19.0mAOD, falling gently to the north. The terrain is fairly flat in this area. In the absence of any other information on ground conditions, it is tentatively assumed that the strata encountered in the six water-well boreholes some 100m to the north may be indicative of the strata at the bridge site.
There is no information on the thickness of the London Clay. At the Recreation Ground overbridge, about 1.5km to the west, this formation is shown to extend down to at least 8mAOD. In any case, the underlying Lambeth Group strata are generally as competent as the London Clay as far as pile foundations are concerned
For preliminary purposes, ground conditions may therefore be assumed as follows.
Top level Base level Thickness (m) Soil Strata Formation (mAOD) (mAOD) 19.0 17.5 1.5 Soft clay(?) Alluvium
17.5 11.5 6.0 Gravel and sand Shepperton Gravel1
11.5 (?) - Stiff/very stiff(?) clay London Clay
Notes: 1. Referred to in previous records as Flood Plain Gravel
6.10.3.5 Alternative foundation types
The preferred solution is to demolish the verge piers and support a new composite deck on the existing abutments and centre pier which, according to the pre-construction drawings, have spread foundations at 17.2mAOD.
Tentative correlation with the strata reported in the water-well boreholes some 100m north of the structure suggests that the foundations were constructed in the upper levels of the Shepperton Gravel after excavating through 1m or 2m of soft alluvium. The abutments and piers are likely to be founded on a reasonable thickness of gravel and sand (between 4m and 8m) overlying London Clay.
For the single-span composite bridge option, built either on-line or partly off-line to the west, spread foundations in the Gravel would clearly also be suitable. Alternatively, piles founded in the London Clay could be used.
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The approach embankments will need to be raised by about 0.9m. For the preferred on-line solution, this can be achieved most simply by widening the embankments or constructing L-shaped concrete walls at the top of the slope. For the alternative single-span partly off-line solution, retaining walls might be needed on the west side, in which case reinforced soil walls founded in the Gravel would be the best option.
6.10.3.6 Bearing pressures and settlements
The bearing pressures and settlements mentioned below are broad estimates based on very limited information. They are not intended as precise, conservative or limiting values and should not be taken out of this context.
No test results are available for the gravels and sands. However, data from elsewhere in the Thames Valley suggest that these river terrace deposits, though variable, are in a generally medium-dense to dense state, and a net allowable bearing pressure of at least 250kPa would be appropriate for spread foundations.
It is estimated that the net bearing pressure on the existing centre pier foundation is about 180kPa. The average pressure on the abutment foundations is much less, but the foundations will have been designed to take account of the overturning moment due to out-of-balance earth forces.
The increase in loading on the centre pier for the proposed two-span composite deck option is estimated to be about 10% and that for abutments is about 36%. These increases are considered to be well within the capacity of the existing foundations. There will be a small additional settlement at the abutments, probably less than 10mm. Additional settlement at the centre pier will be negligible.
If bored piles were used for an alternative single-span option, 900mm nominal diameter would provide an allowable working load of about 1,000kN for a pile length of about 12m and pile group settlement would be less than 10mm.
6.10.3.7 Potential geotechnical problems and construction difficulties
There are no known geotechnical problems for the preferred option of using the existing substructure to support a new two-span bridge deck built on-line.
For the alternative single-span options, on-line or partly off-line to the west, the abutment would be supported on new spread foundations constructed in the zone between the existing verge piers and abutments. The provisional founding levels of 18.2mAOD (south) and 17.5mAOD (north) are above existing foundation levels. It is probable, but by no means certain, that any soft ground present in these areas would have been removed during the original construction. Further ground investigation will be essential to determine the extent of any remaining soft ground and, if necessary, foundations would be lowered or constructed on engineered fill after its removal.
There is no information on groundwater level at this bridge. It may be close to the proposed foundation levels of the alternative single-span bridge options but further investigation is required. Soft ground may be present in the area west of the existing structure where the alternative part off- line solution would be constructed and could affect the widened embankments and/or any reinforced soil walls. Any such soils will need to be removed and replaced with suitable fill or otherwise taken into account in the detailed design.
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The preferred on-line two-span bridge option requires new retaining walls in front of the south and north abutments at distances of approximately 5.0m and 4.5m, respectively. The corresponding retained heights will be about 2.8m (south) and 4.8m (north), and there will be 1(v):2(h) soil slopes on top. Steel sheet piling (or another form of embedded wall) is proposed.
To avoid unacceptable lateral displacement and/or excessive embedment of heavy section sheet piles, it is anticipated that the walls may need to be restrained by permanent ground anchors at capping beam level. The ground anchors would be drilled at a suitable angle through the gaps in the abutment substructure to miss the abutment foundations. Resistance would be derived from fixed anchor lengths grouted into the Gravel layer beneath the approach embankments.
6.10.5 Recreation Ground Overbridge (962)
6.10.5.1 Existing and proposed structures
The existing overbridge is a 4-span structure, square to the M4 Motorway, with a total span of 48.2m. The abutments and piers have spread foundations at 17.4mAOD.
The preferred solution is to demolish the existing structure and to construct a new single-span composite bridge (36.1m clear span) on-line during temporary road closure. The new bridge will have spread foundations.
Alternatively, if full closure, demolition and construction on-line are not practicable due to traffic requirements or the need to carry utilities, the single-span composite bridge could be constructed off-line to the east. This would require retaining walls on the east side to enable embankment widening within existing land boundaries.
A possible variation of the on-line option would apply if the route was permanently closed to vehicular traffic. The existing bridge could then be demolished and replaced with a truss type structure suitable for foot, cycle and equestrian traffic. It might then be possible to retain the existing abutments and centre pier and their foundations.
6.10.5.2 Ground conditions
The geological survey map of area shows superficial sand and gravel deposits of the Shepperton Gravel Member overlying London Clay.
The geological interpretation drawing, marked as Figure 2.6 (ref. RT-DTP0201-206-02) in HA GDMS Report 3268, shows two boreholes close to the bridge abutments, BHG25:015 (north) and BHG26:015 (south), and a further borehole, BHC25:015, about 110m to the north. However, no detailed records are available. The longitudinal section shows the main strata encountered in BHC25:015 and BHG25:015 but only in legend form, and the legends are unclear.
BGS records include a borehole referenced SU97NE25:160, carried out in 1962, which appears to be the same one shown as BHC25:015 in the geological interpretation drawing (H GDMS report 3268). More detailed strata descriptions are provided. Unfortunately, this is the borehole furthest from the bridge; there are no similar detailed records corresponding to BHG25:015 and BHG26:015 near the abutments.
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Below ground level of 19.7mAOD, Borehole BHC25:015 encountered made ground (described as a rubbish tip) to 1.2m depth, overlying 0.9m of firm clayey sandy silt and 2.9m of gravel with sand and occasional silt content. Brown/blue clay (clearly the London Clay formation) was met at 4.0m depth and was proved to a maximum depth of 11.6m (8.1mAOD). The depth of groundwater is shown as 1.5m (18.2mAOD).
Four numbers marked on the log as ‘blows/ft’ (presumed to be SPT test results) gave values between 15 and 61 in the gravel and sand, indicating a variable but generally medium-dense to dense condition. The London Clay was described as firm at the top but soon becoming stiff and then hard near the bottom of the borehole.
The legend record of Borehole BHG25:015 at the north abutment shows about 0.8m of alluvium, overlying 4.4m of Flood Plain Gravel and 4.0m of London Clay, with no further details. Combining these strata levels with the soil descriptions from BHC25:015 is probably the most appropriate way of utilising the very limited data available.
For preliminary purposes, ground conditions may therefore be assumed as follows.
Top level Base level Thickness (m) Soil Strata Formation (mAOD) (mAOD) 19.0 18.0 1.0 Soft clay(?) Alluvium Medium-dense gravel with 18.0 13.5 4.5 Shepperton Gravel1 sand and occasional silt
Stiff to hard clay, 13.5 (8.0) - London Clay firm in upper levels
Notes: 1. Referred to in previous records as Flood Plain Gravel
6.10.5.3 Alternative foundation types
The existing abutments and piers have spread foundations at about 17.4mAOD, near the top of the Shepperton Gravel. Spread foundations at about the same level are proposed for preferred option of a single-span bridge built on-line. They would be equally suitable for the alternative off-line bridge construction to the east
Bored piles founded in the London Clay would also be feasible.
The approach embankments will need to be raised by about 0.7m and retaining walls will be required to keep the new construction within existing land boundaries. For the preferred on-line solution, this can be achieved most simply by constructing L-shaped concrete walls at the top of the embankment slope. (If the existing bridge was replaced with truss type structure for non- vehicular traffic, embankment raising would be reduced.)
For the alternative off-line solution, retaining walls will be required on the east side to keep the new construction within existing land boundaries. Reinforced soil walls founded in the Gravel stratum would be the most suitable solution but foundation conditions would need to be confirmed by further ground investigation.
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6.10.5.4 Bearing pressures and settlements
The bearing pressures and settlements mentioned below are broad estimates based on very limited information. They are not intended as precise, conservative or limiting values and should not be taken out of this context.
A net allowable bearing pressure of at least 250kPa would be appropriate for spread foundations in the Gravel. The results of SPT tests in the borehole 110m north of the bridge suggest a medium- dense to dense state.
For the preferred on-line option, the new spread foundations will be at a similar location to the existing foundations and at about the same level. The underlying strata (about 4.0m of Gravel over London Clay) will thus have been effectively preloaded and this will reduce settlement, probably to less than 15mm.
This does not apply to the alternative off-line location, where settlements of about 25mm would be expected, including a small long-term component due to consolidation of the London Clay. An alternative foundation of 12m long, 900mm nominal diameter piles would provide an allowable working load of about 1,000kN and pile group settlement would be less than 10mm.
6.10.5.5 Potential geotechnical problems and construction difficulties
For the preferred on-line bridge replacement, the proposed level of the new spread foundations will be about 17.4mAOD. There is no information on groundwater level at the bridge site, but based on other sites in the area, it is likely to be close to the top of the Gravel stratum. More reliable information is required from further ground investigation. However, it is possible that groundwater will need to be lowered to allow foundations to be constructed in the dry.
The depth of alluvial deposits at the bridge site is unclear. For the on-line option, any soft alluvium or other unsuitable soils will almost certainly have been excavated and replaced with engineered fill over most of the footprint area of the structure.
However, the original extent of such deposits and their subsequent removal is unknown, and they may still be present immediately east of the existing bridge in the area of the alternative off-line construction. If so, these materials will need to be excavated and replaced with granular fill to provide a suitable foundation for the reinforced soil walls. For deep deposits, ground treatment would be an alternative solution
6.10.6 Datchet Road Overbridge (963)
6.10.6.1 Existing and proposed structures
The existing overbridge is a 4-span structure with a total span of 46.2m (57.1m skew). The abutments and piers are founded on driven precast concrete piles, 406mm and 356mm square, including a number of raking piles at the abutments. It will be necessary to maintain single-way working during construction of the new bridge due to traffic requirements and utilities. The preferred solution is to construct a single-span composite bridge (36.1m clear span) partly off-line to the east, to demolish the existing structure and complete the on-line part. It is proposed that the new bridge will also have pile foundations.
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6.10.6.2 Ground conditions
The geological survey map of area shows superficial river terrace deposits of sand and gravel of the Kempton Park Gravel formation overlying clays, silts and sands of the Lambeth Group (formerly known as Reading Beds in this area).
The geological interpretation drawing, marked as Figure 2.6 (ref. RT-DTP0201-206-02) in HA GDMS Report 3269, shows ten boreholes at, or in the immediate vicinity of, the existing bridge. The most relevant are Boreholes BHGE23:020, BHGE24:020 and BHGE34:020, for which summary strata descriptions are given in pre-construction drawing A4/298/1 for the Slough- Maidenhead By-pass. Borehole BHGE24:020 was at the south abutment, BHGE34:020 at the centre pier and BHGE23:020 at the toe of embankment slope on the north-west side.
BGS records of other boreholes in the area of the bridge include one borehole, SU97NE131:160, within the footprint of the structure, near the verge pier on the south side. However, information from this is very limited.
Original ground level was about 19.7mAOD, roughly the level of the M4 carriageways. In the ‘GE’ boreholes, made ground was revealed to a maximum depth of about 1.4m, underlain by soft organic alluvium, described as ‘soft silt, peat or sandy clay, often black’ having a thickness of 0.5m to 1.5m. These soft upper soils were underlain by 3m to 4m of gravel and sand, and then clay strata, described as firm brown/grey mottled clay at the top but soon becoming stiff red/grey mottled clay, followed by hard brown/blue mottled clay. The deepest borehole, BHGE23:020, encountered dense silty sand with thin layers of brown clay near its base.
Geological classification of the strata below the gravel and sand is unclear. The longitudinal section (HA GDMS 3269) shows London Clay, but the variability and colouration of the strata suggests that they may be part of the Lambeth Group formation. However, this is unlikely to have a significant effect on design and construction of the new bridge.
For preliminary purposes, ground conditions at the bridge abutments may be assumed as follows. Above 18.5mAOD, original construction materials, including engineered fill, will have replaced the upper strata in most areas. The inferred replacement of alluvium to 17.0mAOD is based on the maximum depth found in the boreholes.
The highest recorded groundwater level was 18.5mAOD.
No test data are available at the bridge site, but SPT results in boreholes over 100m away suggest an average undrained shear strength of about 150kPa for the cohesive strata below the gravel and sand.
Top level Base level Thickness Soil Strata Formation (mAOD) (mAOD) (m) 18.5 17.0 1.5 Replacement granular fill (Alluvium) 17.0 14.0 3.0 Gravel and sand Kempton Park Gravel1
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London Clay/ Lambeth 14.0 8.0 6.0 Stiff/hard mottled clay Group2(?) 8.0 (7.5) - Silty sand with clay layers Lambeth Group2(?)
Notes: 1. Referred to in previous records as Flood Plain Gravel 2. Formerly known as the Reading Beds formation in this area
6.10.6.3 Alternative foundation types
Bored piles founded in the stiff to hard clays present below the Kempton Park Gravel are proposed for the new bridge abutments.
Spread foundations in the Gravel would also be feasible, although this might require additional excavation below groundwater level. Alternatively, they could be founded in the granular fill that is assumed to have been placed after excavation of the soft alluvial soils. However, the quality and state of compaction of this material between existing pile caps would need to be investigated.
Deck level of the new bridge will be 0.9m higher than that of the existing bridge. The approach embankments will therefore need to be raised and retaining walls will be required to keep the new construction within existing land boundaries. For the preferred part off-line solution, reinforced soil walls at the toe of the existing slope to the east are proposed as the most suitable solution. Foundation conditions will need to be confirmed by further ground investigation.
6.10.6.4 Bearing pressures and settlements
The bearing pressures and settlements mentioned below are broad estimates based on very limited information. They are not intended as precise, conservative or limiting values and should not be taken out of this context.
Nominal 900mm diameter piles of 10m length would provide an allowable working load of about 1,000kN and pile group settlement would be expected to be less than 10mm.
A net allowable bearing pressure of at least 250kPa would be appropriate for alternative spread foundations in the Gravel. Settlements would be of the order of 25mm and would take place largely during construction, although there might be a small long-term component due to consolidation of the underlying cohesive soils.
6.10.6.5 Potential geotechnical problems and construction difficulties
Existing pile cap soffit levels are about 18.7mAOD. The existing piles were fabricated 12.2m long. If fully driven, their installed length would have been about 11.2m (after allowance for building into the pile cap) and pile toe levels would be about 7.5mAOD, which is approximately the level of the dense silty sand in Borehole BHGE23:020. It is also possible that the piles would have achieved an acceptable set at a higher level.
The new pile layout will need to be designed to avoid a clash with the existing piles, particularly the 6 no. raking piles (raked at 1:4) shown in the pre-construction drawings at each abutment. The positions of the existing vertical piles can be confirmed when the pile caps are removed. Alignment of the raking piles will be more difficult to determine as they are prone to deviation
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The new pile cap soffit levels will be similar to those of the existing bridge and close to the highest groundwater level shown in the boreholes. Groundwater levels will be subject to seasonal variation but are not expected to be a problem.
The boreholes indicate a variable depth of soft organic alluvial soils, including peat, extending down to 17.0mAOD, almost 3m below original ground level. Greater thicknesses may be present between borehole positions. It seems almost certain that these unsuitable soils were removed during construction of the existing bridge.
Section A-A of pre-construction drawing no. A4/298/2 shows a dashed line marked ‘level to which mass excavation is to be carried out’, indicating a clear intention to remove and replace the soft alluvium. A further note states ‘granular fill to be placed and compacted before piling commences’. The proposed excavation line slopes down from about 18.5mAOD at the south abutment to 17.0mAOD at the north abutment, which is corresponds with the findings of the ‘GE’ boreholes.
It is almost certain that the soft alluvium was removed and replaced with granular fill at least over the footprint area of the structure. However, the full areal extent of removal is unknown.
It is possible that soft ground still exists in the area of the part off-line construction east of the existing bridge, particularly near the toe of the embankment side slope where a reinforced soil retaining wall is proposed. If so, these soils will need to be excavated and replaced with granular fill to provide a suitable foundation for the wall. Excavation below groundwater level alongside the M4 carriageways should be avoided, however. Should deep soft deposits be found in this area, ground treatment or a piled slab might be required, although this is not considered likely.
6.10.7 Windsor Railway Underbridge (966)
6.10.7.1 Existing and proposed structures
The existing underbridge is a five-span railway bridge with an overall span of 71.4m (skew) and a width of approximately 36m, including edge beams. The existing piers and abutments have spread foundations.
It is proposed to widen the existing bridge by 16.5m on the south side only and to support the extended deck on pile foundations.
The existing approach embankments, which have a maximum height of about 8.0m at the abutments and side slopes of about 1(v):2(h), will need to be widened accordingly. To keep the southern limit of the works within existing boundaries, it is proposed to construct full height retaining walls, approximately along the line of the existing embankment toes.
6.10.7.2 Ground conditions
The geological survey map shows river terrace deposits of the Shepperton Gravel Member overlying the Lambeth Group formation (formerly known as the Reading Beds in this area).
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The geological interpretation drawing in HA GDMS report 3269, marked as Figure 2.5 (ref. RT- DTP0201-206-02), shows ten boreholes located within about 100m of the bridge. Of those for which records are available, the most relevant appear to be BHGE15:020 and BHGE16:020 Their positions are ambiguous in the Acer Consultants drawing (HA GDMS 3269) but are shown more clearly in pre-construction drawing A4/287/4W for the Slough-Maidenhead By-pass: BHGE15:020 was about 5m east of the outer pier (B) on the west side of the underbridge, and BHGE16:020 was about 25m east of the eastern abutment (F). Both were a few metres north of the M4 Motorway centre-line.
Drawing A4/287/4W also shows summary strata logs. From a ground level of 20.4mAOD, both boreholes revealed 1.9m of topsoil and firm sandy clay (probably alluvium) overlying 5.8m to 6.1m of mainly gravel and sand (Shepperton Gravel). Hard red/grey mottled clay (Lambeth Beds) was met at 7.7m to 8.0m depth (about 12.5mAOD).
At roughly the mid-depth of Gravel, both borehole records show a thin band (0.1m to 0.2m) of firm silty clay. In BHGE15:020, this was underlain by about 1m of medium sand, before gravel and sand was again encountered. In BHGE16:020, the underlying Lambeth Beds were noted to have silt and sand laminations.
The BGS records include two boreholes, SU97NE116:160 and SU97NE117:160, said to have been carried out in 1960, in positions close to those described above. The basic strata summaries are sufficiently similar to suggest that the two sets of records are of the same ‘GE’ boreholes, which were done in May 1958 during the early stages of work on the M4. The BGS date is almost certainly incorrect. Unlike the ‘GE’ borehole records, those for SU97NE116:160 and SU97NE117:160 show the depth of penetration into the Lambeth Beds, 2.3m to 2.9m, and the final borehole depths.
BGS records of other investigations in the area include five boreholes put down in the commercial premises to the north, between the M4 and White Hart Road. A similar thickness of upper alluvium and/or made ground was encountered but the thickness of Gravel was more variable (3.1m to 5.0m) and less than at the bridge site, and the Lambeth Beds occurred at a higher level (13.8mAOD to 15.5mAOD). Whilst the locations of BHGE15:020 and BHGE16:020 make them more directly relevant for preliminary design, these other boreholes may be indicative of the possible variation in level and thickness of the strata in this area, including the southern side of the bridge where the widening works will be undertaken.
Acer Consultants longitudinal section (HA GDMS 3269) also shows two deeper boreholes, marked BH31:053 and BH32:053, carried out south of the bridge (for MEWFAS) at distances of about 80m and 120m. Strata summaries are shown in legend form only. The levels of the Gravel and the Lambeth Beds are in the range found in the boreholes described above. However, these two holes were taken to greater depth and the Chalk horizon was revealed at about -0.5mOD. Water strikes occurred in most of the boreholes at between about 1.5m and 2.5 depth, usually near the top of the Gravel. ‘Standing water levels’ are indicated in the logs of BHGE15:020 and BHGE16:020 at 17.7mAOD and 18.7mAOD, respectively.
For preliminary purposes, ground conditions may be assumed as follows. (In the longitudinal section and in some of the records in the BGS archive, the bedrock geology is described as London Clay, whereas the geological survey map indicates the Lambeth Group formation. Whilst there are no test data, the strata descriptions suggest that the latter is correct.)
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Top level Base level Thickness Soil Strata Formation (mAOD) (mAOD) (m) 20.5 18.5 2.0 Soft/firm(?) cohesive soils Alluvium
Gravel and sand, 18.5 12.5 6.0 with sand layers Shepperton Gravel1 and thin bands of clay
Very stiff/hard clay, 12.5 -0.5 13.0 Lambeth Group2 with silt/sand laminations
Seaford and Newhaven -0.5 (-5.5) - Chalk Chalk Formations3
Notes: 1. Referred to in previous records as Flood Plain Gravel 2. Formerly known as the Reading Beds formation in this area 3. Formerly known as Upper Chalk in this area
No soil test data are available for this bridge site. Based on limited information from other sites along this stretch of the M4, it is assumed that the Gravel is in a generally medium-dense to dense state and that the Lambeth Group clays are very stiff to hard (as indicated in the logs of Boreholes BHGE15:020 and BHGE16:020).
6.10.7.3 Alternative foundation types
To limit differential settlement between the new and existing foundations, bored piles will be used to support the extended deck. It is proposed to set the pile cap at a minimum depth in order to limit excavation, possibly at a soffit level of about 20.0mAOD.
The pre-construction drawings show that the existing piers and abutments were intended to have at founding levels ranging between 18.7mAOD at the west abutment (A) and 18.4mAOD at the east abutment (F). This corresponds with the level of the top of the Gravel stratum (18.5mAOD) revealed in the two most relevant boreholes and also the likely level of groundwater.
Piles with a head level of 20.0mAOD, might therefore have the top 1.5m of their shaft length in alluvium, followed by 6.0m in mainly gravel and sand and their base in very stiff to hard clays of the Lambeth Beds. Piles longer than about 20m would probably penetrate into the underlying Chalk.
Spread foundations in the Gravel would be a feasible alternative for the extended deck, solely from the viewpoint of bearing capacity. However, excavation would be required down to the level of the existing foundations, or perhaps below, in order to achieve an adequate founding level and/or to remove and replace the soft alluvium with compacted granular fill to form a suitable bearing layer. Total and differential settlements also would be significantly greater than for piles. Spread foundations are not therefore recommended.
It is proposed to use reinforced soil retaining walls to support the new works along their southern edge in the region of the existing embankment toes. It seems very probable that soft alluvial deposits were present in this area prior to construction of the embankments, but it is not certain that they were fully removed and replaced beneath the embankment side slopes.
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The required retaining walls will have a maximum height of about 8.0m and although reinforced soil is inherently flexible, an adequate foundation is essential for stability. Further ground investigation will be necessary in these areas to establish suitable founding levels.
6.10.7.4 Bearing pressures and settlements
The bearing pressures and settlements mentioned below are broad estimates based on very limited information. They are not intended as precise, conservative or limiting values and should not be taken out of this context.
Spread foundations are not recommended for the new works.
A pile of nominal diameter 750mm, a length of 14m should provide a net allowable working load of about 1,000kN in the Lambeth Beds. This would increase to about 1,250kN for a pile length of 17.5m and 1,500kN for a length of about 20.0m. However, these estimates are based on the assumption that the properties of the upper levels of the Chalk would no adverse effect on pile capacity. Further information from ground investigation will be essential for detailed pile design.
Pile group settlement should not exceed about 10mm, including post-construction settlement resulting from consolidation of the predominantly cohesive Lambeth Group soils.
6.10.7.5 Potential geotechnical problems and construction difficulties
No as-built drawings are available to confirm that the existing piers and abutments were founded at their intended levels of between 18.7mAOD and 18.4mAOD. In view of the presence of soft alluvium in boreholes at around this level, it is possible that the intended levels might have had to be lowered or replacement compacted granular fill introduced.
Setting the pile cap of the proposed foundations at a much higher level (possibly 20.0mAOD) may not reduce the volume of excavation if the upper alluvial soils are unsuitable as a subgrade for the pile caps, since excavation and replacement with granular fill will still be necessary. However, a high pile cap level would at least avoid the need to lower groundwater levels to enable construction in the dry.
It is also possible that the alluvial soils in the proposed deck extension area were removed and replaced during the original bridge construction. Pre-construction drawing A4/287/3W for the Slough-Maidenhead By-pass shows the southern elevation of the bridge, with lines drawn at about 20.4mAOD and 18.3mAOD to show ‘existing ground level’ and ‘mass excavation level’. It is unclear, however, whether mass excavation would have extended laterally to the full width (16.5m) of the proposed deck extension works on the south side.
Similarly, it is unclear whether soft ground is present in the areas of the proposed reinforced soil walls. Pre-construction drawing A4/287/3W shows the mass excavation line extending (as a dashed line) behind the abutments into the areas of ‘approach filling’, where the removal of such deposits would have been essential to reduce differential settlement. It is not certain, however, that mass excavation (to 2m or more) would have been carried out over the full area of the embankment side slopes, including the area of the proposed reinforced soil walls near the existing toes.
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In view of the height of the walls (and hence their required base width), the existing embankments will need to be cut back at least 5m from the toe to provide a foundation platform from which the walls will be raised. It is unlikely that this could be achieved simply by steepening the existing 1:2 side slopes, even temporarily. A steepened slope angle of at least 35° would be required and this may jeopardise stability.
Temporary support will therefore be needed, probably in the form of steel sheet piling. Should soft ground be found still to exist in these areas, deeper excavation would be required to provide a suitable wall foundation and this would make temporary support more onerous. Ground treatment, possibly using stone columns, might then be a more suitable options. Alternatively, an embedded pile wall, designed to take account of the presence of soft ground, could be used.
Further detailed investigation in these areas will be essential to establish the nature and properties of the near-surface deposits.
6.10.8 Wood Lane Overbridge (971)
6.10.8.1 Existing and proposed structures
The existing overbridge is a 4-span structure with a total span of 46.9m (48.8m skew). The abutments and piers are founded on 356mm (14 inch) square driven precast concrete piles, most of those at the abutments being raked at 1:4.
The preferred solution is construct a new single-span bridge of 36.1m clear span (37.6m skew) fully off-line to the east. This will allow traffic to be maintained on the existing bridge while the new bridge is constructed in a single stage within the land boundaries. This will, however, require full- height retaining walls on the east side of the approach embankments.
It is proposed that the new bridge will have pile foundations.
6.10.8.2 Ground conditions
The geological survey map of area shows superficial sand and gravel deposits of the Shepperton Gravel Member overlying the London Clay formation.
The geological interpretation drawing, marked as Figure 2.4 (ref. RT-DTP0201-206-02) in HA GDMS Report 3269, shows several boreholes close to the existing bridge, including Borehole BHGE10:020, at the south abutment, for which summary strata descriptions are given in pre- construction drawing A4/284/2 for the Slough-Maidenhead By-pass.
BGS records of other boreholes in the area include two on the north side of the structure at distances of about 30m and 60m from the north abutment, but these records are basic and the maximum hole depth was only 2.1m. Similarly, the geological interpretation drawing (HA GDMS REPORT 3269) shows the strata (in legend form only) found in two further boreholes close to the bridge site but these were less than 2m deep.
Borehole BHGE10:020 is therefore the most useful source of information on ground conditions at this structure. From a ground level of 20.8mAOD, firm sandy clay (of uncertain origin) was revealed below topsoil to a depth of 1.7m. This was underlain by 2.7m of gravel and sand, 1.8m of firm white and grey laminated silt and a further 1.7m of gravel and sand. At 7.9m depth
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(12.9mAOD) hard red/grey mottled clay was encountered and was proved for a depth of 1.2m before the borehole was terminated.
In the absence of detailed strata descriptions and soil test data, geological classification of these strata is unclear. The gravel and sand layers and the intervening layer of firm silt are probably all part of the Shepperton Gravel Member. The underlying hard red/clay mottled clay is more typical of the Lambeth Group formation (formerly known as Readings Beds in this area) than the London Clay indicated on the geological survey map. Given the wide variability in the Lambeth Beds, it is also possible that the firm silt and lower sand and gravel layers are part of this formation.
Groundwater is marked on the log at 0.5m depth (20.3mAOD) in the upper firm sandy clay. It is not known whether this was the equilibrium level at the time of boring.
The other four boreholes mentioned above are useful only in providing a general indication of the top level of the Shepperton Gravel (19.5mAOD to 21.2mAOD) and the level at which groundwater was encountered (about 20.6mAOD). The depth of the upper soils overlying the Gravel varied from 0.8m to 1.5m and the BGS records show this to be mainly cohesive soil. However, there is no indication of its origin or its stiffness.
None of the boreholes at Wood Lane bridge site were deep enough to determine the level of the Chalk formation that underlies the Lambeth Beds. Deep boreholes near the Oldway Lane overbridge site, about 1.1km to the west, and near the Windsor Branch railway underbridge, some 1.7km to the east, indicate average Chalk levels of about -8mAOD and 0mAOD, respectively. Whilst linear interpolation of geological formations is unreliable, in the absence of further information it is tentatively assumed that the Chalk horizon might be at a level of about -5mAOD at Wood Lane bridge.
For preliminary purposes, ground conditions may therefore be assumed as follows. (The firm laminated silt has been included as part of the Shepperton Gravel.) No test data are available for this site.
Top level Base level Thickness (m) Soil Strata Formation (mAOD) (mAOD)
21.0 19.0 2.0 Firm clays and silts Alluvium/ Made Ground(?)
Gravel and sand, 19.0 13.0 6.0 Shepperton Gravel1 with firm silt layers
Stiff/hard mottled clay with silt 13.0 (-5.0) (18.0) Lambeth Group2 and sand layers
Seaford and Newhaven (-5.0) - - Chalk Chalk Formations3
Notes: 1. Referred to in previous records as Flood Plain Gravel 2. Formerly known as the Reading Beds formation in this area 3. Formerly known as Upper Chalk in this area 6.10.8.3 Alternative foundation types
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For the preferred option of a new single-span bridge constructed off-line to the east, bored piles founded in the predominantly cohesive strata of the Lambeth Group formation are proposed for the new bridge abutments.
Spread foundations in the overlying Gravel might be feasible, although the presence of a 1.8m thick layer of firm laminated silt within the Gravel, as indicated in Borehole BHGE10:020, would both decrease the allowable bearing pressure and increase foundation settlement.
Deck level of the new bridge will be about 1.1m higher than that of the existing bridge. The re- aligned approach embankments will therefore need to be raised accordingly. It is expected that the retaining walls required on the east side of the embankments will be founded in the Gravel layer.
6.10.8.4 Bearing pressures and settlements
The bearing pressures and settlements mentioned below are broad estimates based on very limited information. They are not intended as precise, conservative or limiting values and should not be taken out of this context.
Proposed pile cap soffit level for the new bridge is about 20.5mAOD, as for the existing bridge. Bored piles of 900mm nominal diameter and 12m length would provide an allowable working load of about 1,000kN and pile group settlement would be less than 10mm.
Nearly 2m of firm laminated silt was shown to exist at the south abutment of the existing bridge within the depth of influence of spread foundations. Similar soils may well be present, perhaps in greater thickness, at the site of the new bridge. Should the use of spread foundations be considered as an alternative to piles, detailed ground investigation and testing would be needed to determine the allowable bearing pressure and to estimate the likely settlement.
6.10.8.5 Potential geotechnical problems and construction difficulties
The existing bridge abutments are founded on 356mm square precast concrete piles: 4 no. vertical piles and 16 no. piles raked at 1:4 in the direction of the bridge axis. As there are no transverse raker piles, there should be no possibility of clashes between old and new piles.
Firm sandy clay was found to a depth of 1.7m at the south abutment of the existing bridge, at a level of about 19.0mAOD. It is unclear whether it was considered necessary to excavate and replace this material during construction of the piles and pile caps or elsewhere within the footprint area of the existing bridge and/or the approach embankments.
Further ground investigation will be necessary to determine whether similar material or possibly softer alluvial deposits are present in the areas of proposed reinforced soil retaining walls and embankment raising on the east side of the existing bridge. If so, they may need to be excavated and replaced with granular fill to provide a suitable foundation for the walls. Should deep soft deposits be found in these areas, ground treatment or a piled slab might be required.
Owing to the limited distance to the land boundary on the east side, the capping beam of the reinforced soil walls will align with the outer edge of the new footway and the wall height will be the full embankment height, a maximum of about 6.6m at the abutments. To allow excavation to the required base width of the walls, the existing embankments will require temporary support,
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There is no reliable information on groundwater levels at this bridge, although the various borehole records suggest a level similar to the proposed pile cap soffit level of 20.5mAOD. Groundwater will be subject seasonal variations but is not expected to pose any significant problems for bridge construction.
6.10.9 Oldway Lane Overbridge (972)
6.10.9.1 Existing and proposed structures
The existing overbridge is a 4-span structure with a total span of 47.5m. The centre and outer piers are each founded on 8 no. driven precast concrete piles, 356mm square. Each abutment is founded on four pairs of 356mm square raking piles (marked as 1:3 rake) and two pairs of 303mm square transverse raking piles (marked as 1:8 rake).
The preferred solution is to demolish the existing structure and to construct a new single-span composite bridge (36.1m clear span) on-line during temporary road closure and traffic diversion. It is proposed that the new bridge will have pile foundations.
Alternatively, it may be possible to replace the existing bridge, during road closure, with a lightweight bridleway truss bridge supported on the existing abutments.
If it is necessary to maintain traffic flow, the new bridge could instead be constructed off-line to the west.
6.10.9.2 Ground conditions
The geological survey map of area shows superficial alluvial deposits of clay, silt, sand and gravel overlying London Clay.
The geological interpretation drawing, marked as Figure 2.4 (ref. RT-DTP0201-206-02) in HA GDMS Report 3269, shows several boreholes in the immediate vicinity of the existing bridge. The most useful is Borehole BHGE9:020, at the north abutment, for which summary strata descriptions are given in pre-construction drawing A4/283/2 for the Slough-Maidenhead By-pass.
BGS records of other boreholes in the area include a number at Slough Sewage Treatment Works drilled in 1986. The most relevant being Boreholes B/19 (SU97NW117:174) and B/20 (SU97NW118:174), that are located approximately 70m south-east of the existing bridge.
Original ground level varied between about 22.2 and 23.0mAOD. In Borehole BHGE9:020, 4.1m of river terrace gravel and sand was encountered beneath topsoil, underlain by 0.3m of firm silt, followed by stiff red/grey mottled clay of the Reading Beds (now considered part of the Lambeth Group formation), which was proved for 6.7m without penetration. The intervening thin layer of ‘firm silt’ could conceivably belong to either formation.
A similar thickness of gravel and sand was revealed in Boreholes B/19 (SU97NW117:174) and B/20 (SU97NW118:174) beneath thin layers of topsoil and alluvium. Again, this was underlain by
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Highways Agency — M4 J3-12 MM-ALR predominantly cohesive strata, the upper 1.5m to 2.5m of which was considered possibly to belong to the London Clay formation. The underlying Reading Beds were proved to a maximum depth of 18.6m. The strata were more variable than in Borehole BHGE9:020 and contained layers of clayey silt and clayey sand. Both boreholes were terminated in silty fine sand.
The other boreholes marked on the longitudinal section (HA GDMS 3269) (for which no details are available) show at least 5m of gravel and sand above the Readings Beds. The two deepest boreholes encountered the underlying Chalk at -6.0m and -9.5m.
Groundwater level in Borehole BHGE9:020 is shown at 20.7mAOD in the gravel and sand, about 2m below original ground level. Piezometers were installed at greater depth in Boreholes B/19 B/19 (SU97NW117:174) and B/20 (SU97NW118:174) but no monitoring records are available.
For preliminary purposes, ground conditions may be assumed as follows. Above about 22.0mAOD, original construction materials, including engineered fill, will have replaced the upper strata over most of the area.
No test data are available at the bridge site.
Top level Base level Thickness (m) Soil Strata Formation (mAOD) (mAOD) 22.0 17.0 5.0 Gravel and sand River Terrace Gravel1 London Clay/ 17.0 15.0 2.0 Stiff sandy clay Lambeth Group2(?)
Stiff mottled clay with silt and 15.0 -8.0 23.0 Lambeth Group2 sand layers
Seaford and Newhaven -8.0 (-10.0) - Chalk Chalk Formations3
Notes: 1. Referred to in previous records as Flood Plain Gravel 2. Formerly known as the Reading Beds formation in this area 3. Formerly known as Upper Chalk in this area
6.10.9.3 Alternative foundation types
For the preferred option of a new single-span bridge, bored piles founded in the predominantly stiff cohesive strata of the Lambeth Group formation are proposed for the new bridge abutments. Spread foundations in the overlying Gravel would also be feasible. An alternative lightweight bridleway truss bridge would be supported on the existing piled abutments.
Deck level of the new bridge will be about 0.6m higher than that of the existing bridge. The approach embankments will therefore need to be raised and small concrete retaining walls will be required at the top of the embankment slope to keep the new construction within existing land boundaries. Embankment stability is not expected to be a problem.
6.10.9.4 Bearing pressures and settlements
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The bearing pressures and settlements mentioned below are broad estimates based on very limited information. They are not intended as precise, conservative or limiting values and should not be taken out of this context.
Nominal 900mm diameter piles of 10m length would provide an allowable working load of about 1,000kN. This would increase to about 1,400kN for piles of 15m length. Pile group settlement would be less than 10mm.
A net allowable bearing pressure of at least 250kPa would be appropriate for alternative spread foundations in the Gravel. Settlements would be of the order of 25mm and would take place largely during construction, although there might be a small long-term component due to consolidation of the underlying cohesive soils.
The existing 356mm square abutment piles that would be used to support the alternative lightweight truss bridge are estimated to have a net allowable axial load capacity of about 350kN, which would be adequate. The applied loads from the new bridge will, in any case, be less than the loads on the existing abutments.
6.10.9.5 Potential geotechnical problems and construction difficulties
Existing pile cap soffit levels at the abutments are 21.9mAOD and the 356mm square piles were fabricated 12.2m long. If fully driven, their installed length would have been about 11.2m (after allowance for building into the pile cap) and pile toe levels would be in the region of 11mAOD. (The piles are marked on the pre-construction drawings as having a 1:3 rake but are drawn at about 1:4.5.)
The new pile layout of the preferred scheme will need to be designed to avoid a clash with the existing piles, which are arranged in four balanced pairs with nominal clear gaps between each pair of 1.70m, 0.76m and 1.70m. However, the gaps between piles may well be smaller than these nominal figures due to drift of the piles during driving and this will need to be taken into account.
The new pile cap soffit levels at the abutments will be about 21.2mAOD, about 0.7m deeper than those of the existing bridge and about 0.5m above the only groundwater level shown in the boreholes. Groundwater levels will be subject to seasonal variation but are not expected to be a problem.
6.10.10 Huntercombe Spur Overbridge (973)
6.10.10.1 Existing and proposed structures
The existing overbridge is an asymmetric 4-span structure with a total span of 49.7m. The abutments and piers have spread foundations at levels of between 20.9mAOD and 21.2mAOD.
The preferred option is to construct a new two-span asymmetric bridge on-line, while maintaining one lane of traffic in each direction. The existing abutments and centre pier will be modified to support the new deck, which will have the same overall span as the existing bridge. The verge piers will be removed and new retaining walls will be constructed in front of the abutments on both sides to enable the carriageways to be widened.
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Demolition and construction will be carried out in two halves while maintaining traffic and utilities on the remaining deck at successive stages. It is expected that the existing spread foundations will be retained, although the centre pier foundation could, if necessary, be replaced with a single row of bored piles
To simplify construction, an alternative solution would be to construct the new bridge partly off-line to the east (offset by approximately 10m) during temporary road closure and to realign the highway accordingly. The bridge would have a single clear span of 41.7m. The new abutments would have spread foundations at a level similar to the existing foundations.
6.10.10.2 Ground conditions
The geological survey map of area shows superficial alluvial deposits of clay, silt, sand and gravel overlying clays, silts and sands of the Lambeth Group (formerly known as Reading Beds in this area).
The geological interpretation drawing, marked as Figure 2.3 (ref. RT-DTP0201-206-02) in HA GDMS report 3269, shows two boreholes, BHGE5:020 and BHGE6:020, of unknown source and uncertain location, close to the existing bridge. However, further details are given in pre- construction drawing A4/271/2 for the Slough-Maidenhead By-pass, which shows BHGE5:020 at the west end of the south abutment and BHGE6:020 at the east end of the north abutment.
From a ground level of 22.8mAOD, Borehole BHGE5:020 encountered 1.1m of topsoil and soft sandy clay underlain by 4.8m of gravel and sand, clayey in upper levels. These superficial deposits were underlain by Lambeth Beds consisting of 4.2m of stiff becoming hard red-grey sandy clay, followed by 2.1m of grey sand in which the borehole was terminated.
Borehole BHGE6:020 revealed similar strata at the north abutment, 0.8m of soft alluvium overlying 4.8m of gravel and sand, sometimes slightly clayey. The Lambeth Beds consisted of 6.3m of clays, firm in upper levels but soon becoming hard.
Groundwater was encountered at 1.8m depth in both boreholes, about 21.0mAOD, within 1m of the top of the gravel and sand.
These two holes were bored to depths of only about 12m and were terminated in the Lambeth Group strata at 10.5mOD. A deeper borehole (BH123:020) in the geological interpretation drawing (HA GDMS report 3269), about 320m to the south-east, shows this formation to extend to at least 5mOD. The level of the underlying Chalk formation at the bridge site is unknown. In boreholes near Oldway Lane overbridge, some 600m to the east, it was met at between 6.0mOD and 9.5mOD. The Chalk horizon rises to the west but is considered unlikely to be above 0.0mOD at Huntercombe Spur.
For preliminary purposes, ground conditions may be assumed as follows. The levels of the spread foundations of the existing bridge are at about 21.0mAOD, near the top of the gravel and sand. The overlying soft alluvium, and any other unsuitable soils, will have been replaced with engineered fill above this level during the original construction.
Top level Base level Thickness Soil Strata Formation (mAOD) (mAOD) (m)
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22.5 21.5 1.0 Replacement granular fill (Alluvium) Gravel and sand 21.5 17.0 4.5 River Terrace Gravel1 with clayey zones
Stiff/hard clays (firm in upper 17.0 (0.0)* >12.0 Lambeth Group Formation2 levels) with sand layers
Seaford and Newhaven (0.0)* - - Chalk Chalk Formations3
(*estimated level)
Notes: 1. Referred to in previous records as Flood Plain Gravel 2. Formerly known as the Reading Beds formation in this area 3. Formerly known as Upper Chalk in this area
No information is available on the state of compaction of the granular layers. A medium-dense state would be a reasonable assumption at this stage.
6.10.10.3 Alternative foundation types
The new bridge will have spread foundations at about 21.0mAOD in the upper levels of the Gravel layer found in both boreholes at the existing abutments. The foundations will be underlain by about 4m of gravel and sand and then the variable Lambeth Group strata.
For the preferred two-span on-line structure, bored piles founded in the Lambeth Beds could be used as an alternative foundation for the centre pier.
The approach embankments will need to be raised by about 0.8m and retaining walls will be required to keep the new construction within existing land boundaries. For the preferred on-line solution, this can be achieved most simply by constructing L-shaped concrete walls at the top of the embankment slope.
For the alternative single-span partly off-line solution, retaining walls of up to about 6.6m height will be needed at the toe of the existing embankment slopes on the east side. Reinforced soil or concrete gravity retaining walls would be founded in the Gravel stratum.
6.10.10.4 Bearing pressures and settlements
The bearing pressures and settlements mentioned below are broad estimates based on very limited information. They are not intended as precise, conservative or limiting values and should not be taken out of this context.
The gravel and sand was noted to be slightly clayey at some levels but this is unlikely to affect bearing capacity. Assuming a medium-dense condition, a net allowable bearing pressure of at least 250kPa would be appropriate for spread foundations for the abutments and centre pier. Settlements would be of the order of 25mm and would take place largely during construction, although there might be a small long-term component due to consolidation of the Lambeth Group strata. Bored piles would be a suitable alternative foundation for the centre pier. Piles of nominal diameter 900mm and a length of 10m would provide an allowable working load of about 1,000kN. Smaller diameter piles might be more suitable for the restricted working area around the centre
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Highways Agency — M4 J3-12 MM-ALR pier. For 750mm nominal diameter piles, a length of 12m would provide about the same allowable load. This would increase to 1,800kN for a pile length of about 22m. Pile group settlement would be expected to be less than 10mm.
These estimates are based on the assumption that piles will founded in the Lambeth Group strata. Boreholes BHGE5:020 and BHGE6:020 had a maximum depth of about 12m. Whilst it seems likely that the hard clays and sand layers of the Lambeth Beds are present to at least twice this depth, this will need to be confirmed by further ground investigation. It is possible that the underlying Chalk formation would influence the bearing capacity of piles greater than 20m in length, though not necessarily adversely. Further information would be essential for detailed pile design.
6.10.10.5 Potential geotechnical problems and construction difficulties
Boreholes BHGE5:020 and BHGE6:020 at the bridge abutments indicate a limited depth of soft alluvium, about 1m or less. This will have no consequence for the preferred on-line option, as it will almost certainly have been removed from the footprint area of the existing bridge. For the alternative part off-line option, it is possible that soft ground might still exist in the area of the proposed retaining walls on the east side. If so, it will need to be removed and replaced with suitable fill.
The highest groundwater level in the boreholes was at 21.0mAOD, close to the top of the Gravel stratum, which is also the level of the spread foundations. It is not known whether this was an equilibrium water level at the time of boring but groundwater levels will, in any case, vary seasonally. To construct new abutment spread foundations at this level in the off-line area may therefore require excavation below groundwater alongside the existing foundations and the M4 carriageways. This will need to be done with care to avoid disturbance due to water flow.
The preferred on-line two-span option requires new retaining walls in front of the north and south abutments at distances of approximately 4.0m and 2.0m, respectively. The corresponding retained heights will be about 2.3m (north) and 4.0m (south), and there will be 1(v):2(h) soil slopes on top. Steel sheet piling (or another form of embedded wall) is proposed.
To avoid unacceptable lateral displacement and/or excessive embedment of heavy section sheet piles, it is anticipated that the walls may need to be restrained by permanent ground anchors at capping beam level. (The need is clearly likely to be greater on the south side.) The ground anchors would be drilled at a suitable angle through the gaps in the abutment substructure to miss the abutment foundations. Resistance would be derived from fixed anchor lengths grouted into the Gravel layer beneath the approach embankments.
6.10.11 Huntercombe Lane Overbridge (974)
6.10.11.1 Existing and proposed structures
The existing overbridge is a 4-span structure with a total span of 50.7m (56.9m skew). The abutments and piers have spread foundations, founded at between 21.9mAOD and 21.0mAOD.
The preferred solution is to construct a new single-span composite bridge (39.8m clear span) on line during temporary road closure and to demolish the existing structure. It is proposed that the new bridge will also have spread foundations.
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If it is necessary to maintain one lane of traffic flow, the new bridge could be constructed alternatively part off-line to the east.
6.10.11.2 Ground conditions
The geological survey map of area shows superficial alluvial deposits of clay, silt, sand and gravel overlying clays, silts and sands of the Lambeth Group (formerly known as Reading Beds in this area).
The geological interpretation drawing, marked as Figure 2.3 (ref. RT-DTP0201-206-02) in HA GDMS Report 3269, shows Borehole BHGE4:020 at the bridge site. This is noted to be of unknown source and uncertain location. However, further details are given in pre-construction drawing A4/270/2 for the Slough-Maidenhead By-pass, which shows its location at the east corner of the north abutment and provides a summary of the strata.
About 5m of river terrace deposits of gravel and sand was encountered at 23.3mAOD, immediately beneath topsoil. The underlying Lambeth Beds consisted of 2m of stiff red-grey sandy clay underlain by a further 6m of sand containing thin layers of stiff clay. Hard clay and claystone was met near the bottom of the borehole.
A groundwater level of 21.0mAOD, about 2.5m below original ground level, is indicated in the gravel and sand.
For preliminary purposes, ground conditions at the bridge abutments may be assumed as follows. Above 21.0mAOD, original construction materials, including engineered fill, will have replaced the upper strata in most areas.
Top level Base level Thickness (m) Soil Strata Formation (mAOD) (mAOD) 21.0 18.0 3.0 Gravel and sand River Terrace Gravel1 18.0 16.0 2.0 Stiff sandy clay 16.0 10.5 5.5 Sand Lambeth Group Formation2 10.5 (10.0) - Hard clay/claystone
Notes: 1. Referred to in previous records as Flood Plain Gravel 2. Formerly known as the Reading Beds formation in this area
No information is available on the state of compaction of the granular layers. A medium-dense state would be a reasonable assumption at this stage
6.10.11.3 Alternative foundation types
Spread foundations for the new bridge are proposed at a similar level to those of the existing structure, 1m to 2m below the top of the Gravel stratum found in Borehole BHGE4:020. The new foundations will therefore be underlain by about 3m of gravel and sand and then the variable strata of the Lambeth Group formation.
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Deck level of the new bridge will be 1.4m higher than that of the existing bridge. The approach embankments will therefore need to be raised and retaining walls will be required to keep the new construction within existing land boundaries.
For the preferred on-line solution, this can be achieved most simply by constructing L-shaped concrete walls at the top of the embankment slope. Stability will need to checked but is not expected to be problem for the mature embankments. Cantilever sheet piling would be an alternative solution but potential long-term lateral displacement would need to be assessed.
For the alternative part off-line solution, reinforced soil walls at the toe of the existing slope to the east are proposed as the most suitable solution.
6.10.11.4 Bearing pressures and settlements
The bearing pressures and settlements mentioned below are broad estimates based on very limited information. They are not intended as precise, conservative or limiting values and should not be taken out of this context.
A net allowable bearing pressure of at least 250kPa would be appropriate for spread foundations for the bridge abutments. Settlements would be of the order of 25mm and would take place largely during construction, although there might be a small long-term component due to consolidation of the Lambeth Beds.
Bored piles would be a suitable alternative foundation. Nominal 900mm diameter piles of 12m length would provide an allowable working load of about 1,000kN and pile group settlement would be expected to be less than 10mm.
6.10.11.5 Potential geotechnical problems and construction difficulties
Borehole BHGE4:020 suggests a depth of alluvium of less than 1m. However, this is likely to vary. The difference of about 1m in foundation levels of the existing bridge might be attributable to such variation. It is presumed that soft, or otherwise unsuitable, soils would have been removed and replaced with suitable fill, at least over the footprint area of the structure. However, the full areal extent of removal is unclear.
If the part off-line solution were adopted, soft ground might be present in the area of the proposed reinforced soil retaining wall to the east and would need to be removed and replaced. Should any such deposits extend to much greater depth adjacent to the M4 carriageways, ground treatment or a piled slab might be required as an alternative to deep excavation and replacement below groundwater level, but this is not considered likely.
Groundwater level in Borehole BHGE4:020 at the time of boring was at 21mAOD. This is also the level of the lowest spread foundation of the existing bridge, at the north abutment. Whilst levels will vary seasonally in the Gravel stratum, groundwater is not expected to be a problem for foundation construction
6.10.12 Marsh Lane Overbridge (977)
Existing and proposed structures
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The existing overbridge is a 4-span structure with a total span of 48.0m (51.2m skew). The abutments and piers are on spread foundations founded at 21.1mAOD, about 3.5m below the M4 carriageway levels.
The preferred solution is to demolish the existing structure and to construct a new single-span composite bridge (36.1m clear span) on-line during temporary road closure and traffic diversion. It is proposed that the new bridge will have spread foundations.
Alternatively, if it is necessary to maintain single-way working due to traffic requirements and utilities, the new bridge could instead be constructed part off-line to the west.
6.10.12.1 Ground conditions
The geological survey map of area shows superficial sand and gravel deposits of the Shepperton Gravel Member overlying clays, silts and sands of the Lambeth Group (formerly known as Reading Beds in this area).
The geological interpretation drawing, marked as Figure 2.3 (ref. RT-DTP0201-206-02) in HA GDMS Report 3269, shows Borehole BHGE1:020 (of unknown source) at the bridge site. Summary strata descriptions are given in pre-construction drawing A4/269/2 for the Slough- Maidenhead By-pass, which also shows its location at the eastern end of the south abutment.
Below a ground level of 22.3mAOD, the borehole revealed a thin layer of topsoil underlain by 6.9m of river terrace gravel and sand, followed by 1.2m of hard fissured clay and 3.4m of hard mottled sandy clay, which was proved to a depth of 12.2m (10.1mAOD) without penetration. Groundwater level was recorded as 20.0mAOD, 1.5m below the top of the gravel and sand.
A group of about ten boreholes were put down some 400m to the east of Marsh Lane bridge for MEWFAS, in 1990 and 1991, and are also recorded in HA GDMS Report 3269. No detailed records are available but the longitudinal section shows a similar succession to Borehole BHGE1:020, with gravel and sand underlying a thin layer of alluvium or made ground. The gravel and sand appears consistently thicker (9m to 10m) in this area and extends to about 13mAOD. The Lambeth Beds were shown to be about 8m thick and to be underlain by Chalk at 5.5mAOD, which was proved to about -2.5mAOD in the deepest holes.
Whilst extrapolation of geological horizons is unreliable, the MEWFAS boreholes provide some guide as to the possible thickness of the Lambeth Group strata at Marsh Lane bridge. The following strata levels are considered reasonable for preliminary design. Above the level of the existing spread foundations, 21.1mAOD, original construction materials, including engineered fill, will have replaced the upper strata in most areas.
The only recorded groundwater level was at 20.0mAOD. Groundwater can be expected to be close to the top of the gravel and sand.
In the absence of test data, it would be reasonable to assume a medium-dense state for the gravel and sand. Whilst the Lambeth Group clays are described as ‘hard’ in the summary log of borehole BHGE1:020, this may not be accurate. Until further investigation is carried out, it would be prudent to assume a ‘stiff becoming hard’ condition for preliminary design purposes.
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Top level Base level Thickness (m) Soil Strata Formation (mAOD) (mAOD) 22.5 21.5 1.0 Replacement granular fill (Alluvium) 21.5 14.5 7.0 Gravel and sand Shepperton Gravel1 Stiff/hard clays and 14.5 5.5 9.0 Lambeth Group2 sandy clays
Seaford and Newhaven 5.5 (-2.5) - Chalk Chalk Formations3
Notes: 1. Referred to in previous records as Flood Plain Gravel 2. Formerly known as the Reading Beds formation in this area 3. Formerly known as Upper Chalk in this area
6.10.12.2 Alternative foundation types
Spread foundations for the new bridge are proposed at a similar level to those of the existing structure, in the upper levels of the Shepperton Gravel. Based on the findings of borehole BHGE1:020, it is expected that the abutments will be underlain by at least 5m or 6m of medium dense gravel and sand, which will provide a suitable foundation. Bored piles founded in the underlying Lambeth Beds would be a suitable alternative foundation for the abutments.
Deck level of the new bridge will be 1.3m higher than that of the existing bridge. The approach embankments will therefore need to be raised and retaining walls will be required to keep the new construction within existing land boundaries.
For the preferred on-line solution, this can be achieved most simply by constructing L-shaped concrete walls at the top of the embankment slope. Stability will need to checked but is not expected to be problematic for the mature embankments. Cantilever sheet piling would be an alternative solution but potential long-term lateral displacement would need to be assessed.
For the alternative part off-line solution, reinforced soil walls at the toe of the existing slope to the west would be the most suitable solution.
6.10.12.3 Bearing pressures and settlements
The bearing pressures and settlements mentioned below are broad estimates based on very limited information. They are not intended as precise, conservative or limiting values and should not be taken out of this context.
A net allowable bearing pressure of at least 250kPa would be appropriate for spread foundations for the bridge abutments. Settlements would be of the order of 25mm and would take place largely during construction. In view of the thickness of the Gravel layer, long-term settlement due to consolidation of the underlying Lambeth Group strata is unlikely to be significant.
Alternatively, nominal 900mm diameter bored piles could be used, and a pile length of 10m would provide an allowable working load of about 1,000kN. Pile group settlement would be expected to be less than 10mm.
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6.10.12.4 Potential geotechnical problems and construction difficulties
It is presumed that any soft, or otherwise unsuitable, soils would have been removed and replaced with suitable fill, at least over the footprint area of the structure during the construction of the existing bridge. However, the full areal extent of removal is unclear.
If the part off-line solution were adopted, it is possible that soft ground might be present in the area of the proposed reinforced soil retaining wall on the west side and would need to be removed and replaced. It is unlikely that soft soils extend to such depth that ground treatment or a piled slab would be required for the retaining wall foundations.
Groundwater level in Borehole BHGE1:020 at the time of boring was at 20.0mAOD, which is below the level of the proposed spread foundations. Whilst levels will vary seasonally in the Gravel layer, groundwater is not expected to be a problem for foundation construction
6.10.13 Thames Bray Underbridge (978)
6.10.13.1 Existing and proposed structures
The existing underbridge is a three-span river crossing with an overall span of 105.9m and a span of 82.3m between intermediate piers. Deck width is 31.0m, including edge beams. The bridge carries three running lanes in each direction with no hard shoulders.
The existing piers and abutments are supported on driven piles of 457mm (18 inch) nominal diameter, many of which are raked at 1:8. The pile caps are about 2.4m thick and have soffit levels of approximately 18.6mAOD.
It is proposed to extend the existing bridge deck on the north side to provide a total width of 39.5m, including edge beams. The existing abutment walls, intermediate piers and pile foundations will be extended to support the new girders.
The existing approach embankments, which have a height of about 8.7m at the abutments, will similarly be widened on the north side. To keep the new works within existing land boundaries and to provide working space for construction, it is proposed to construct reinforced soil retaining walls to a maximum height of about 5.4m, above which a new upper slope will be formed at 1(v):2(h) to embankment crest level.
6.10.13.2 Ground conditions
The geological survey map shows alluvial and river terrace deposits of clay, silt, sand and gravel overlying the Lambeth Group formation (formerly known as the Reading Beds formation in this area).
There is no information available from exploratory holes within the footprint area of the bridge. However, BGS records include three water well boreholes put down in 1988 for Mid-Southern Water Company at distances ranging from 35m to 65m from the abutments. Ground level at all three boreholes was close to 22.0mAOD. Borehole 105G (SU97NW146:173), about 35m from the southern end of the east abutment, encountered 6.0m of sand and gravel
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(probably the Shepperton Gravel Member) overlying 1.0m of uniform coarse sand, at which point clay was reportedly ‘touched’ and the hole terminated.
A layer of brown clay (probably alluvium) was present in both boreholes on the west side, to a maximum depth of 1.5m. Borehole 105F (SU97NW145:173), about 50m from the southern end of the abutment, encountered 6.5m of sand and gravel with an intervening 0.4m thick layer of silt. These superficial deposits were underlain by 2.5m of clayey silty sand and 0.6m of hard mottled clay, thought to be part of the Lambeth Group formation.
Only 4.7m of sand and gravel was found beneath the alluvial clays and clayey sands in Borehole 105C (SU97NW142:173), about 65m from the northern end of the abutment, and this was directly underlain by hard mottled clay of the Lambeth Beds.
914mm (36 inch) diameter water wells were installed with slotted sections within the sand and gravel, and groundwater ‘rest’ levels were recorded at between 18.5mAOD and 18.8mAOD.
The maximum depth of the Lambeth Group strata was not determined in these boreholes. Data from ground investigations about 900m to the east and 800m to the west of the site show the top of the underlying Chalk formation at levels of +5.5mAOD and -3.5mAOD, respectively, which suggests a Chalk level in the region of +1.0mAOD at this site. Whilst linear interpolation of geological horizons is unlikely to be reliable, this is considered to be a reasonable assumption at this stage in the absence of other data. On this basis, the thickness of the Lambeth Beds is therefore assumed to be about 14m.
For preliminary purposes, ground conditions at the bridge abutments may be assumed as follows. (All of the alluvium and about 2m or the sand and gravel deposits will have been excavated for construction of the pile caps.)
Top level Base level Thickness Soil Strata Formation (mAOD) (mAOD) (m) 22.0 20.5 1.5 Clay, silt and sand Alluvium Sand and gravel with 20.5 15.0 5.5 Shepperton Gravel1 some silt layers
Mainly stiff to hard clays with 15.0 (1.0) (14.0) Lambeth Group2 clayey sand layers
Seaford and Newhaven (1.0) - - Chalk (assumed) Chalk Formations3
Notes: 1. Referred to in previous records as Flood Plain Gravel 2. Formerly known as the Reading Beds formation in this area 3. Formerly known as Upper Chalk in this area
The highest recorded groundwater level in the water well boreholes was 18.8mAOD, about 2m to 3m below ground level at the boreholes locations. However, groundwater levels adjacent to the existing piers and abutments will be influenced by river water levels and are therefore likely to be higher.
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No information is available on the state of compaction of the sand and gravel at the bridge site. Based on data from boreholes carried out near Bray Lake, about 800m to the west, a medium- dense state would be a reasonable assumption at this stage.
6.10.13.3 Alternative foundation types
To limit differential settlement between the new and existing foundations, it is proposed to use bored piles to support the extended deck. The new piles will have head levels of about 20.5mAOD. Approximately 5m of their upper shaft will be in sand and gravel but the piles will derive resistance mainly from the variable stiff to hard clays and clayey sand layers of the Lambeth Group.
Spread foundations would be a feasible alternative for the extended deck, solely from the viewpoint of bearing capacity. However, total and differential settlement would be significantly greater than for piles and they are not therefore recommended.
In the areas of the proposed widened approach embankments and reinforced soil walls on the north side, it is possible that soft cohesive alluvium may be present above the sands and gravels. Further ground investigation will be necessary in these areas to establish suitable founding levels.
6.10.13.4 Bearing pressures and settlements
The bearing pressures and settlements mentioned below are broad estimates based on very limited information. They are not intended as precise, conservative or limiting values and should not be taken out of this context.
For a pile length of 15m, nominal diameter 600mm and 750mm piles would be expected to provide net allowable working loads of about 900kN and 1,200kN, respectively, in the Lambeth Beds. For pile lengths greater than 15m, the underlying Chalk formation could influence the bearing capacity of individual piles, though not necessarily adversely. Further information is required from ground investigation for detailed pile design.
In view of the area of the extended foundations, pile group settlement will depend upon the properties of strata well below pile base level, including the Chalk. A reasonable expectation for preliminary purposes is that maximum pile group settlement will not exceed 20mm, of which up to 10mm may occur as post-construction settlement resulting from consolidation of the predominantly cohesive Lambeth Group soils.
6.10.13.5 Potential geotechnical problems and construction difficulties
No information is available on the level or properties of the Chalk formation. Based on boreholes 900m and 800m to the east and west, it is estimated that the top of the Chalk may be at about +1mAOD, but it could be several metres higher or lower. This is unlikely to cause any significant problems with design or construction of the proposed pile foundations but will need to be taken into account in detailed design.
Whilst the upper levels of the Chalk could have lower strength and stiffness than the hard clays at the base of the Lambeth Beds, consolidation will take place more rapidly and should not therefore increase long-term settlement or differential settlement.
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The increased loads from the bridge extension, transferred to the ground through the new piles, will cause stress increases in the strata from which the original piles derive their resistance, and this will induce a small settlement of the existing foundations. The effect will clearly be greatest on the north side, immediately adjacent to the new loading, and will moderate differential settlement between new and existing parts of the structure. Potential ground movements will need to be examined at the detailed design stage.
The reinforced soil retaining walls proposed in the areas of embankment widening on the north side are inherently flexible structures. Even so, any soft alluvium or other unsuitable soils present in these areas will need to be removed and replaced with compacted granular fill, both to ensure stability and to prevent undue displacement of the retaining walls and embankment fill. In the unlikely event of deep soft deposits being encountered in these areas, ground treatment or a piled slab could be used to avoid the need for deep excavation
6.10.14 Monkey Island Overbridge (979)
6.10.14.1 Existing and proposed structures
The existing overbridge is a 4-span structure with a total span of 47.4m (48.0m skew). The piers and abutments have spread foundations. Founding levels of the centre and outer piers are 21.0mAOD and those of the abutments are 19.4mAOD (south) and 19.1mAOD (north). Existing box culverts, 4.3m by 3.2m (external dimensions), are located immediately behind the abutment walls on both sides, just above the foundation pads.
It will be necessary to maintain one lane of traffic. The preferred solution is therefore to construct a single-span composite bridge of 36.1m clear span (36.5m skew) partly off-line to the west, to demolish the existing structure and complete the on-line part. It is proposed that the new bridge will also have spread foundations.
Alternatively, it may be possible to realign the bridge to the east and, in view of its relatively narrow width, to construct full-width.
6.10.14.2 Ground conditions
No information is available from previous ground investigations in the immediate vicinity of this structure.
The geological survey map of area shows superficial deposits of alluvium and sand and gravel of the Shepperton Gravel Member overlying clay, silt and sand of the Lambeth Group (formerly known as Reading Beds in this area). This is underlain by undifferentiated Seaford Chalk and Newhaven Chalk Formations (formerly known as Upper Chalk).
There are records from two boreholes, BH01:035 and BH02;035, in HA GDMS Report 13468 at a site about 480m to the west of Monkey Island Lane Bridge, where the M4 motorway skirts the northern side of Bray Lake. There are also records of three boreholes (SU97NW146:173, SU97NW145:173, SU97NW142:173) put down in 1988 for Mid-Southern Water Company in the area of Thames Bray underbridge, between 300m and 420m to the east.
From ground levels of about 21.5mAOD, the 1995 boreholes encountered 1.1m to 2.2m of firm cohesive soil, considered to be fill material, overlying 0.6m to 0.9m of alluvium, which included
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SPTs in the sandy gravel gave N-values of between 9 and 26, with an average of 19, indicating a generally medium-dense condition. The Lambeth Group strata consisted mainly of stiff to very stiff clays but contained distinct sandy horizons, including a 1.9m layer of dense silty fine sand at 7.5m depth in Borehole 1. The Chalk was described as ‘medium soft to hard fractured chalk with occasional flints’ with SPT N-values ranging from 38 to 78.
Groundwater strikes occurred at 1.3m below the top of the sand and gravel in both boreholes and the water rose by about 0.5m in 20 minutes.
From ground levels of about 22.0mAOD, the 1988 boreholes at Thames Bray encountered up to 1.5m of mixed cohesive strata (probably alluvium) overlying 4.7m to 6.5m of granular soils, consisting mainly of sand and gravel but with 0.4m of silt in one borehole. These deposits were underlain by variable hard clays and clayey sands of the Lambeth Beds, which were proved to a maximum depth of 11.0m. The underlying Chalk was not encountered.
914mm (36 inch) diameter water wells were installed with slotted sections within the sandy gravel. Groundwater ‘rest’ levels were recorded at between 18.5mAOD and 18.8mAOD.
Extrapolation of the findings of these two investigations to the Monkey Island Lane bridge site must be tentative. The strata sequence is expected to be similar but the levels of the main horizons are unknown. Until reliable information is obtained from ground investigation at this structure, the following tentative assumptions are considered reasonable for preliminary purposes.
Above the level of the existing spread foundations, it is likely that original construction materials, including engineered fill, will have replaced the upper strata in most areas.
Top level Base level Thickness Soil Strata Formation (mAOD) (mAOD) (m) 21.5 20.0 1.5 Replacement Fill (Alluvium) 20.0 15.0 5.0 Sandy gravel Shepperton Gravel1 Mainly stiff to hard clays with 15.0 7.0 8.0 Lambeth Group2 clayey sand layers
Seaford and Newhaven 7.0 (-3.0) - Chalk Chalk Formations3
Notes: 1. Referred to in previous records as Flood Plain Gravel 2. Formerly known as the Reading Beds formation in this area 3. Formerly known as Upper Chalk in this area
The highest recorded groundwater level in the five boreholes described above was 18.8mAOD, about 1m below the top of the Shepperton Gravel.
No information is available on the state of compaction of the granular layers at the bridge site. Based on the 1995 boreholes near Bray Lake, a medium-dense state would be a reasonable assumption at this stage.
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6.10.14.3 Alternative foundation types
It is proposed to construct spread foundations for the new abutments at about 21.6mAOD, which is 2.0m below finished road level. Foundation level will thus be 2.2m higher than that of the existing south abutment and 2.5m higher than that of the existing north abutment.
In the on-line area, the soils are likely to consist of granular fill placed during the original construction. In the off-line area to the west, it is possible that soft alluvium may be present overlying the natural sands and gravels. Further ground investigation is essential to confirm suitable founding levels and determine bearing capacity.
Bored pile foundations would be a feasible alternative. Deck level of the new bridge will be 1.1m higher than that of the existing bridge. The approach embankments will therefore need to be raised and retaining walls will be required to keep the new construction within existing land boundaries. For the preferred part off-line solution, reinforced soil walls at the toe of the existing slope on the west side are proposed as the most suitable solution.
6.10.14.4 Bearing pressures and settlements
The bearing pressures and settlements mentioned below are broad estimates based on very limited information. They are not intended as precise, conservative or limiting values and should not be taken out of this context.
In view of the lack of information of ground conditions in the neighbourhood of this structure, the following preliminary estimates should be regarded as tentative. Foundation conditions will need to be established by detailed ground investigation.
A net allowable bearing pressure of at least 250kPa is likely to be appropriate for spread foundations in the Gravel or in compacted granular fill placed above this layer after excavation of soft alluvium or other unsuitable soils. Settlements would be of the order of 25mm and are likely to take place largely during construction, although there might be a small long-term component due to consolidation of the underlying cohesive soils.
Nominal 900mm diameter piles might provide an allowable working load of about 1,000kN in the Lambeth Group strata for a pile length of about 12m. Pile group settlement would be expected to be less than 10mm.
6.10.14.5 Potential geotechnical problems and construction difficulties
It is not known whether soft alluvium or other unsuitable soils were present in this area prior to construction of the existing bridge and, if so, their likely depth. At the sites of other bridges along this stretch of the M4 motorway there is evidence that soft ground beneath the footprint of the structure was excavated and replaced with compacted granular fill, although the full areal extent of such replacement is unknown. It is probable that a similar policy was adopted for this bridge.
The existing abutment foundations are much deeper (by 1.6m to 1.9m) than those of the existing piers. This seems almost certainly due to the need to accommodate box culverts above the foundation pads rather than to any factors related to ground conditions.
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Proposed foundation level of the new abutments is 21.6mAOD, about 2.0m below carriageway level. The required width of the spread foundations will be dependent on the strata levels and bearing capacity to be determined from ground investigation. However, the new foundations are likely to overlap the existing foundations in the on-line area, possibly by about 1.0m, and their base levels might be between 0.9m and 1.3m above the top of the existing pads.
The nature and state of compaction of the existing fill material in these areas will need to be investigated and replacement carried out as necessary. However, it is not expected that overlapping of new and old foundations will cause any further problems.
It is possible that soft ground may be present in the area of the part off-line construction to the west of the existing bridge, particularly near the toe of the embankment side slope where a reinforced soil retaining wall is proposed. If so, these soils will need to be excavated and replaced with granular fill to provide a suitable foundation for the wall. Excavation below groundwater level alongside the M4 carriageways should be avoided.
No information is available on groundwater levels in the immediate area of this bridge and it is essential that this is obtained during the required ground investigation. Should deep soft deposits be found in the area of the reinforced soil wall, ground treatment or a piled slab might be required to provide a suitable foundation, but this is considered unlikely
6.10.15 Ascot Road Overbridge (986)
6.10.15.1 Existing and proposed structures
The existing overbridge is a 4-span structure with a total span of 47.4m (48.0m skew). The piers and abutments have spread foundations. Founding levels of the centre and outer piers are about 24.9mAOD and the abutments are founded at 25.6mAOD.
It will be necessary to maintain two lanes of traffic with one lane in each direction. The preferred solution is therefore to construct a single-span composite bridge of 45.4m clear span (46.1m skew) partly off-line to the east, to demolish the existing structure and complete the on-line part. This span is necessary to accommodate slip roads on both carriageways. It is proposed that the new bridge will have spread foundations.
6.10.15.2 Ground conditions
There is no information available from previous ground investigations in the neighbourhood of this bridge.
The geological survey map of area shows superficial deposits of sand and gravel of the Kempton Park Gravel formation overlying clay, silt and sand of the Lambeth Group (formerly known as Reading Beds in this area). This is underlain by undifferentiated Seaford Chalk and Newhaven Chalk Formations (formerly known as Upper Chalk).
Detailed records are available of two boreholes, BH01:035 and BH02;035, in HA GDMS report 13468. These were put down by Contest Melbourne Weeks Ltd in 1995 at a site approximately 1.25km to the east where the M4 motorway skirts the northern side of Bray Lake. The exact locations of the boreholes are unknown.
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From a ground level of 21.5mAOD, 2.0m to 2.6m of made ground and/or alluvium was encountered above 3.8m to 4.8m of sandy gravel followed by 6.8m to 8.2m of variable strata of the Lambeth Group formation. Chalk was revealed at about 14.7m depth and was proved to the maximum borehole depth of 25.0m.
The soft alluvium in one of the boreholes was noted to contain peat. SPT’s in the sandy gravel gave an average N-value of 19, indicating a generally medium-dense condition. The Lambeth Beds were found to consist predominantly of stiff to very stiff silty clays but with distinct sandy horizons, including a 1.9m layer of dense silty fine sand. The Chalk was described as ‘medium soft to hard fractured chalk with occasional flints’ and had SPT N-values ranging from 38 to 78.
Groundwater strikes occurred at 1.3m below the top of the sandy gravel in both boreholes and rose by about 0.5m in 20 minutes.
Extrapolation of this data to the Ascot Road bridge, over 1km to the west, must clearly be extremely tentative, particularly as there is a difference of nearly 4m in elevation of original ground level in the two areas. The geological survey map suggests that the sequence of strata might be similar, but their levels are unknown.
In the absence of further information and until the necessary ground investigation is carried out at this structure, the following tentative assumptions are considered reasonable for preliminary purposes. Above the level of the existing spread foundations (25.6mAOD to 24.9mAOD), the upper strata will have been replaced by construction materials, including engineered fill, in most areas.
Based on observations elsewhere, it is expected that groundwater will be present within the upper levels of the sandy gravel.
Top level Base level Thickness Soil Strata Formation (mAOD) (mAOD) (m) 25.5 21.0 4.5 Sand and gravel Kempton Park Gravel1 Mainly stiff/very stiff clays with 21.0 7.0 14.0 Lambeth Group2 clayey sand layers
Seaford and Newhaven 7.0 - - Chalk Chalk Formations3
Notes: 1. Referred to in previous records as Flood Plain Gravel 2. Formerly known as the Reading Beds formation in this area 3. Formerly known as Upper Chalk in this area
A medium-dense state would be a reasonable assumption for the sand and gravel at this stage.
6.10.15.3 Alternative foundation types
It is proposed to construct the new abutment foundations about 2m below carriageway level at levels similar to those of the pier foundations of the existing bridge (between 25.6mAOD and 24.9mAOD). The fact that the existing foundations were constructed at these levels suggests that sands and gravels were known to be present, although it is also possible that the existing foundations were constructed on compacted granular fill following excavation of soft or otherwise
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Bored pile foundations would be a feasible alternative.
Deck level of the new bridge will be 1.5m higher than that of the existing bridge. The approach embankments will therefore need to be raised and retaining walls will be required to keep the new construction within existing land boundaries. For the preferred part off-line solution, reinforced soil walls at the toe of the existing slope on the east side are proposed as the most suitable solution.
6.10.15.4 Bearing pressures and settlements
The bearing pressures and settlements mentioned below are broad estimates based on very limited information. They are not intended as precise, conservative or limiting values and should not be taken out of this context.
In view of the lack of information of ground conditions in the neighbourhood of this structure, the following preliminary estimates should be regarded as extremely tentative. Foundation conditions will need to be established by detailed ground investigation.
A net allowable bearing pressure of at least 250kPa is likely to be appropriate for spread foundations in the Gravel. Settlements would be of the order of 25mm and are likely to take place largely during construction, although there might be a small long-term component due to consolidation of the underlying cohesive soils.
Nominal 900mm diameter piles might provide an allowable working load of about 1,000kN in the Lambeth Group strata for a pile length of between 10m and 15m. Pile group settlement would be expected to be less than 10mm.
6.10.15.5 Potential geotechnical problems and construction difficulties
It is not known whether soft alluvium or other unsuitable soils were present in this area prior to construction of the existing bridge and, if so, their likely depth. At the sites of other bridges along this stretch of the M4 motorway there is evidence that soft ground beneath the footprint of the structure was excavated and replaced with compacted granular fill, although the full areal extent of such replacement is unknown. It is probable that a similar policy was adopted for the Ascot Road Bridge.
Nevertheless, it is possible that soft ground may still exist in the area of the part off-line construction to the east of the existing bridge, particularly near the toe of the embankment side slope where a reinforced soil retaining wall is proposed. If so, these soils will need to be excavated and replaced with granular fill to provide a suitable foundation for the wall. However, excavation below groundwater level alongside the M4 carriageways should be avoided.
No information is available on groundwater levels in this area and it is essential that this is obtained during the necessary ground investigation. Should deep soft deposits be found in the area of the reinforced soil wall, it is possible that ground treatment or a piled slab would be required to provide a suitable foundation. Alternatively, a concrete or steel sheet pile wall could be employed
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6.11 Structure Foundations
6.11.1 Introduction
The scheme as envisaged will contain approximately 28 super cantilever sign/signal gantries, 28 cantilever sign gantries, 58 “hockey stick” MS4 cantilever gantries, 13 portal frame gantries (including 8 with superspan dimensions, >20m?) and 4 MS3 cantilever signal gantries. For costing purposes and in advance of any surveys it is also anticipated that discrete 12-14m long retaining walls (varying between 1-3m in height) will be required at each proposed gantry location. The scheme works are also anticipated to require the construction of 34 Emergency Refuge Areas, however, 22 of these are deemed not to require retaining walls at this stage. Further to this there are potentially 49 retaining walls required with individual lengths varying between 15 and 515m and retained heights varying between 1.0 and 3.5m. These retaining walls are provisionally required to accommodate proposed widening at sections of existing reduced hard shoulder widths. This equates to approximately 6417m on the westbound carriageway and 5741m on the eastbound carriageway. The proposed geotechnical constraints affecting the structures are summarised in Table 6.1 in Appendix J
6.11.2 Super cantilever sign and signal gantries, cantilever sign gantries and cantilever (MS3, MS4 “hockey stick”) signal gantries
The type of foundation at each location will be determined based on an assessment of the ground and topographical conditions at each site, including review of historical information as appropriate. For preliminary design purposes it can be assumed that the majority of super cantilever and cantilever sign / signal masts will be sited on piled foundations. Screw piles or micropiles may be considered particularly for the smaller MS4 signal gantries. These may be able to minimise the timescales on site due to their relatively rapid installation, the potential for significantly reduced temporary works and the prefabrication of several components. It is unlikely that any benefit may arise from their use on the larger super cantilever structures, particularly where these are sited adjacent to the edge of the running lanes with no significant setback, due to the size of the loads imparted by the structure on the foundations. The potential exists for spread foundations, particularly where the existing motorway is at-grade. However, based on the relatively light gantry structures and large cantilevers, there are likely to be large eccentric loads generated. These may require large shallow foundations with significant temporary works excavations. The depth and type of foundation should be determined. This may require a site specific ground investigation at the proposed gantry location, depending on available historical information.
6.11.3 Portal Frame Gantries (incl. Superspan)
The type of foundation at each location will be determined based on an assessment of the ground and topographical conditions at each site, including review of historical information as appropriate. For preliminary design purposes it can be assumed that the majority of the Superspan Portal Frame Gantries will be sited on piled foundations.
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Screw piles or micropiles may be considered as these may be able to minimise the timescales on site due to their relatively rapid installation, the potential for significantly reduced temporary works and the prefabrication of several components. The potential exists for spread foundations, particularly where the existing motorway is at-grade. However, based on the relatively light gantry structures and large eccentric loads (from wind loading) these may require large shallow foundations with significant temporary works excavations. In areas of existing embankment slope instability, foundations should be designed to minimise detrimental loading, so far as is reasonably practicable. The depth and type of foundation should be determined. This may require a site specific ground investigation at the proposed gantry location, depending on available historical information.
6.11.4 Emergency Refuge Areas – Retaining Walls
Following geometry requirements within IAN161/12 the ERA’s will be level areas approximately 100m long and 4.6m wide. It is likely that a retaining structure will be required at the majority of the sites. Even sections of earthwork recorded as “at-grade” could be up to 2.5m high/deep, with “structures” falling under the remit of BD2/12 when they reach a retained height of 1.5m. The retaining walls will be required to retain the widened cutting slope or embankments and limit the proposed works to within the required land areas (assumed to be within the existing motorway boundaries). Prior to any detailed design, the site specific ground and groundwater conditions along with the site geometry and actions affecting the proposed ERA’s will need to be confirmed.
6.11.5 Widening of reduced width hardshouder / hard strip – Retaining Walls
To comply with geometry requirements within IAN161/12 there are several sections of the scheme where localised widening of the pavement will be required due to existing reduced width hard shoulders (less than 3.3m) and hard strips (approximately 1m wide). These are generally located on the merges and diverges at junctions where the historical addition of extra lanes has reduced the hard shoulder width. It is likely that a retaining structure will be required at the majority of the sites. The retaining walls will be required to retain the widened cutting slope or embankments and limit the proposed works to within the required land areas (assumed to be within the existing motorway boundaries). Prior to any detailed design, the site specific ground and groundwater conditions along with the site geometry and actions affecting the proposed areas of widening will need to be confirmed.
6.11.6 Central Reserve Vertical Concrete Barrier (VCB)
Ground investigations comprising confirmatory Dynamic Cone Penetrometer (DCP) tests to verify design assumptions are to be undertaken following possession of the site for construction.
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6.11.7 Culverts and subways
Where culverts and subways are to be widened consideration and assessment of the potential for excessive differential settlement between the old and new sections will be required with mitigation by design.
6.11.8 Environmental barrier
The TAR (Mouchel, 2011b) notes “13km of environmental barrier and 3km of anti dazzle fence is present in the scheme area. The level of replacement will be dependent on the new infrastructure locations and drainage provisions.” Depending on final scheme proposals some areas of environmental barrier replacement may not be coincident with other structural works (e.g. gantries, widening etc.). In these areas, depending on the proposed environmental barrier location and height, the site specific ground and groundwater conditions along with the site geometry, may need to be confirmed.
6.12 Cross Carriageway Ducts (CCD’s)
Any cross carriageway ducts (CCD’s) are likely be installed by directional and/or auger drilling through either natural ground or engineered fill underlying the motorway. The drilling and installations should be undertaken in accordance with the relevant HA guidance. These include: Manual of Contract Documents For Highway Works, Volume 3, Section 3a, Drawing MCX 0814 ‘Installation Drawing NMCS (Ducted Cable), Duct Installation, Transverse Ducts’ Design Manual for Roads & Bridges, Volume 4 – Geotechnics and Drainage, Section 1 – Earthworks, Part 8, HA120/08, ‘Guidance on the trenchless installation of services beneath motorways & trunk roads’. A method statement for Directional Drilling outlining proposed methods to prevent heave in the road surface should also be prepared by the Contractor and submitted to the Geotechnical Advisor of the Overseeing Organisation for approval prior to the start of the works.
6.13 Contaminated Land/Soil Chemistry
As noted in Section 4.9 there are several areas bounding the existing motorway with historical evidence of contaminative land use, particularly the abundance of landfill sites (see also drawings M4MM-MUH-ML-ZZ-DR-GE-200001 to 200030. Whilst there is no clear evidence of landfill material being present beneath the M4 motorway corridor the potential for localised occurrences being encountered is relatively high, particularly on associated link/slip roads. There is also the potential of the migration of ground gas and soil/groundwater contamination from these areas towards and under the motorway footprint. Targeted chemical testing is likely to be required on recovered samples (soil and groundwater) to determine the potential for contamination along the scheme route (at proposed construction locations). This should be considered both in the context of any potential affect of the contamination on the proposed works but also on the exposure of any site workers to contamination.
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As noted in Section 4.7 the scheme route runs through several source protection zones related to groundwater abstraction and overlies principal aquifers. The implications of certain construction methodologies and their potential affect on the migration of contaminants should be considered during the design stage and appropriate measures proposed to reduce/mitigate the impact where these methodologies cannot be eliminated. Aside from the potential for migration of external contaminants into the site area, discussed above, contaminants such petroleum hydrocarbons might be anticipated. Should such materials be encountered during the works, suitable chemical screening and potentially WAC (Waste Acceptance Criteria) testing should be undertaken to classify the material and determine the appropriate disposal route.
6.14 Existing Geotechnical Problems
As outlined in Section 4.12 and presented in Tables H.1 to H.4 in Appendix H there are areas of significant earthworks defects (and repairs) in the scheme route. Drawings M4MM-MUH-ML-ZZ-DR-GE-200001 to 200030 record the location of the earthworks defects extracted from HA GDMS, in relation to the scheme chainage. Table 6.1 in Appendix J presents a summary of the geotechnical constraints relating to the scheme chainage of the proposed structures planned structures (Gantries, ERA’s, and retaining walls for widening). Of the 227 discrete sites listed in Table D.5, nearly 40% (86 sites) have a recorded earthwork defect within their scheme chainage limits. Only six sites however are associated with major defects (Class 1A, 1B & 1C as defined in HD41/03). Thirty sites are associated with minor defects (1D). Forty two sites are associated with Class 2B defects. These are areas at risk for a reason not related to soil slope instability. The majority of these will relate to the presence of a landfill site in the vicinity of an earthwork. Thirteen sites are associated with Class 3A defects. These will generally relate to previous repairs of Class 1A defects (e.g. granular slope replacement and areas of remedial slope drains). Thirteen sites are noted to contain Class 3C defects. This could include strengthened earthworks and previous 1C defects that have been repaired. The impact of the proposed scheme works on existing earthworks defects should be considered at each discrete location. It may be prudent to consider integrating any planned earthworks renewals (through the Managing Agent Contractor (MAC) or subsequent to Nov 2013 the new Asset Support Contract (ASC)), in areas affected by the proposed scheme, within the M4MM scheme programme. Efficiencies may be gained particularly in the use of combined traffic management, integration of temporary works and in the economies of scale. The presence of several flooded areas of former clay and/or gravel workings along the scheme route, sometimes directly adjacent to the motorway boundary, presents a risk of instability in the submerged slopes of the old workings affecting the existing motorway infrastructure. Similarly, the impact of the construction of any new sign/signal gantries and their foundations on the slopes of the old workings should be considered, where appropriate. Section 4.3 of this report considers hydrology and flood risk issues and highlights the risks of flooding up to and sometimes within the M4 corridor from either the 1:100 year or 1:1000 year flood event. Mitigation of the risk of earthworks erosion from flood waters should be considered as part of design works.
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6.15 Effects of Man-made Obstacles/Site History
As described in Section 2, there are several underground and overhead services present in close proximity to the proposed works. These services may require relocation/removal to facilitate the works. Any relocation/ removal will require agreement of the service providers and any costs and programme implications should be taken into account in the overall timing of the scheme. Overhead electricity transmission and distribution lines should be considered when locating gantries and ERA’s as plant required for construction and maintenance may have access issues beneath the lines depending on their height above the carriageway. The overhead electricity lines are included with other services details on the utility drawings in Appendix C. The TAR (Mouchel, 2011b) notes that, “there are 103 significant structures comprising overbridges, underbridges, major drainage structures and subways between J3 and J12”. The location and position of these structures, especially underground structures, will need to be considered during the planned works (including any ground investigations). Table 4.10 (Appendix F) contains details of all under and over structures as well as retaining walls where a structure number has been assigned by the Highways Agency. This includes culverts but may not be comprehensive particularly in respect of pipes with a diameter of less that 600mm. Site inspections should be undertaken prior to finalisation of all Managed Motorway ITS infrastructure and completion of associated ground investigations and potential piling operations to prevent damage to any recorded or unrecorded understructures.
Between approximate chainage 13740 to 13780 the twin tunnels of the Heathrow Express Rail Link run north to south beneath the M4 motorway. Any planned excavation works within the easement of the tunnels will need to comply with the owners requirements.
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7 COMPARISON OF PROJECT OPTIONS AND RISKS
7.1 Introduction This section presents the Geotechnical Risk Register for the Scheme (as developed from the initial register included in the Statement of Intent), together with details of other construction related risks known at this time based on information contained in the Preliminary Sources Study Report.
The Statement of Intent produced by Mouchel in 2011 did not make an initial assessment of which Geotechnical Category the Scheme falls under. Based on the information included within this report, the Halcrow-Hyder Joint Venture has assessed the proposed Scheme to fall into Geotechnical Category 2 as it includes “conventional types of geotechnical structures, earthworks and activities with no exceptional geotechnical risks, unusual or difficult ground conditions or loading conditions”.
A qualitative approach has been used based on the procedures set out in Managing Geotechnical Risk (2001). Under a qualitative risk assessment, the degree of risk is the expected impact of damage, loss or harm for a given hazard, under particular circumstances which is expressed as:
Degree of Risk = Probability (P) x Impact (I)
The probability and the impact (ie consequence to the scheme) has determined using Table 7.1 and 7.2 respectively, below, which together then provide the degree of risk based in Table 7.3
Table 7.1: Scale of Probability Table 7.2: Scale of Impact
Probability (P) Scale Impact (I) Scale
Negligible 1 Very low 1 Unlikely 2 Low 2 Likely 3 Medium 3 Probable 4 High 4 Very likely 5 Very high 5
Table 7.3: Degrees of risk
Degree of Risk Risk Level Recommended Response 1 to 4 Trivial Monitor. 5 to 8 Tolerable Regular attention. 9 to 12 Substantial Early attention. 13 to 16 Intolerable Unacceptable. 17 - 25 Intolerable Unacceptable.
The risk register is presented in Table 7.4.
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Table 7.4: Geotechnical Risk Register
SECTION RISK LEVEL BEFORE MITIGATION RISK LEVEL AFTER MITIGATION Next Formal OF CONSEQUENCE OF HAZARD Risk MITIGATION MEASURE TO BE TAKEN BY No. HAZARD Risk Review SCHEME (RISK) Owner DESIGNER Probability Impact Degree of Risk Date Probability Impact Degree of Risk IMPACTED Geotechnical Risks
1 All Sections Unknown/variable Founding stratum failure or Establish ground and groundwater conditions from ground and deformation in excess of initial desk study review supplemented by appropriate groundwater serviceability limits resulting in ground investigation as required. During conditions structure/ infrastructure damage. 4 4 16 Designer detailed 2 4 8 Perched water tables leading to design slope instability and failure. Disruption to construction owing to unforeseen ground conditions. 2 All Sections Competent rock Mechanical excavation difficult. Implement site specific ground investigation. shallower than Program delays and additional Determine weathering profile of rock and identify During estimated costs due to pre-treatment areas of competent rock at shallow depth early to 4 3 12 Designer detailed 2 3 6 incorporate in foundation options and design design
3 All Sections Weak and/or soft Founding stratum failure or Establish ground conditions from initial desk study compressible deformation in excess of review supplemented by appropriate ground strata inc poorly serviceability limits resulting in During investigation as required. Design suitable foundation compacted or structure/ infrastructure damage. 4 4 16 Designer detailed to mitigate identified risks to acceptable levels. 2 4 8 softened Disruption to construction. design previously engineered fill. 4 Sections 1,2 Contaminated Risk to human health, controlled Assess potential contamination risk from past & 4 ground water receptors, buried service and historical land use from desk study review structures. During supplemented by ground investigation as needed to 4 4 16 Designer detailed enable appropriate treatment measures, if any, to be 2 4 8 design designed and implemented.
5 Sections 1,2 Generation of Excessive amount of material for Establish amount of unacceptable material from initial & 4 excessive disposal off site and associated desk study review supplemented by appropriate quantities of disposal costs ground investigation as required. Amend scheme During unacceptable designs if practicable to minimise arisings and 4 3 12 Designer detailed 2 3 6 material, including requirement for off-site disposal. design material impacted with contamination 6 Sections 1,2 Presence of Founding stratum failure, Establish presence of landfills from initial desk study & 4 landfills at deformation or long term review supplemented with appropriate ground numerous degradation of concrete in investigation as required. If practicable, adjust During locations along foundations and substructure in locations of structures to avoid areas of landfill. 4 4 16 Designer detailed 2 4 8 the Scheme excess of serviceability limits design resulting in structure/ infrastructure damage. Issues of leachate or methane production.
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SECTION RISK LEVEL BEFORE MITIGATION RISK LEVEL AFTER MITIGATION Next Formal OF CONSEQUENCE OF HAZARD Risk MITIGATION MEASURE TO BE TAKEN BY No. HAZARD Risk Review SCHEME (RISK) Owner DESIGNER Probability Impact Degree of Risk Date Probability Impact Degree of Risk IMPACTED 7 All Sections Presence of Risk to human health of site Establish presence of peat/alluvium deposits and hazardous soil workers. Ignition of methane gas landfills from initial desk study review supplemented borne gas arising leading to injury/death of site with appropriate ground investigation as required. Use from alluvium/peat workers. During gas monitor during ground investigation and deposits or 4 5 20 Designer detailed construction to monitor presence of gases. 1 5 5 landfills design Emergency procedures for evacuation if gases breach trigger levels given in appropriate published literature.
8 All Sections Aggressivity of the Long term degradation of concrete Establish aggressivity characteristics during ground ground, chemical in foundations and substructure investigation. Design and specify concrete and screw During attack on buried and screwpiles or similar as pile design in accordance with industry standards. 4 3 12 Designer detailed 1 3 3 structural proposed, in excess of structure design elements. serviceability limits resulting in structure/ infrastructure damage. 9 All Sections Groundwater Contamination of aquifers and Design appropriate drainage containment systems to Protection Zones water supplies during construction During collect/treat/redirect contaminated surface water away and during operational life of the 4 4 16 Designer detailed from protected zone. 1 4 4 road. design
10 All Sections Permeability of Water entering excavations leading Undertake location-specific assessment to identify ground higher to possible erosion of fines and likelihood of water ingress. Design earthworks than anticipated ultimately collapse. Potential During drainage as appropriate. disruption of program due to 3 4 12 Designer detailed 2 4 8 delays. Requirement to obtain design necessary consents to discharge pumped water. 11 All Sections Planned future Extraction may give rise to stability Consider any areas of planned future extraction and extraction of local and groundwater issues on highlight within scheme documentation. mineral resources adjacent sections of the earthworks During (gravel) infrastructure. Temporary 2 4 8 Designer detailed 1 4 4 groundwater lowering for extraction design may lead to localised settlement issues. 12 All Sections Locations within Flooding of sites affecting All parties involved to be aware of weather conditions areas of high temporary works and long term and Environment Agency adverse weather warnings. flooding risk performance of the geotechnical Plan temporary works to avoid working in adverse Designer During structures. Disruption to weather conditions. Plan emergency procedures for 4 3 12 and detailed 2 3 6 construction owing to flood events. flooding events. Site staff to be notified via Designer's Contractor design Delays to program and increased Risk Register. costs. Erosion and subsequent instability. 13 All Sections Proximity to Falls into the water, risk of Where practicable locate works away from surface water drowning/injury/death to site watercourses and water pools. Otherwise Contractor features workers to design work area to restrict access of site staff with Designer During (watercourses and watercourses. Emergency procedures in place for 3 5 15 and detailed 2 4 8 water pools) falls into water. Site staff to be notified via Designer's Contractor design Risk Assessment. Long term risk of earthworks erosion to be assessed and development of designs to mitigate risk to acceptable level.
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SECTION RISK LEVEL BEFORE MITIGATION RISK LEVEL AFTER MITIGATION Next Formal OF CONSEQUENCE OF HAZARD Risk MITIGATION MEASURE TO BE TAKEN BY No. HAZARD Risk Review SCHEME (RISK) Owner DESIGNER Probability Impact Degree of Risk Date Probability Impact Degree of Risk IMPACTED 14 All Sections High groundwater Contamination of aquifer and water Design appropriate drainage containment systems to vulnerability supply during construction and During collect/treat/redirect contaminated surface water away during operational life of the road 4 4 16 Designer detailed from protected zone 1 4 4 design
15 All Sections Areas of previous Re-activation of previous instability Review existing available reports/ HA GDMS records earthwork due to disturbance of adjacent to identify areas of previous instability. Understand instability ground during construction or previous instability issues and mechanisms. loading crest of embankments. Inspect/monitor condition of embankments during During Damage to structures, highway construction, take remedial action as needed. 3 4 12 Designer detailed 2 4 8 infrastructure and adjoining design properties. Acceleration of existing earthworks instability through adjacent disturbance, change in profile or loading conditions. 16 All Sections Areas of previous Possible risk of further instability as Review existing available reports/ HA GDMS records earthwork repairs remedial works become life expired to identify areas of previous instability repairs. Review or if previous remedial works were method of remedial works undertaken and assess if insufficient compared to current During they are suitable. Understand previous instability standards 3 4 12 Designer detailed issues and mechanisms. Inspect/monitor condition of 2 4 8 design embankments during construction, take remedial action as needed.
17 All Sections Slope instability Damage to structures, highway, Assess stability of all slopes likely to be affected by during and/or infrastructure and adjoining the works and design structures accordingly. immediately after properties. construction Designed During instigated by 3 4 12 and detailed 1 4 4 temporary works Contractor design and loading the crests of existing highway embankments 18 All Sections Damage to Additional costs to repair damage Establish ground and groundwater conditions from existing and delays to program. initial desk study review supplemented by appropriate carriageway and ground investigation as required. Assess stability of During neighbouring Designed all earthworks likely to be affected by the works. detailed structures due to 3 4 12 and Ensure care is taken during construction to avoid 2 4 8 design and foundation works Contractor damage to the carriageway. Make sure works close to construction and/or ground existing structures are undertaken with due care. movement during construction 19 All Sections Running on hard Failure of earthwork leading to Establish ground and groundwater conditions from shoulder affecting closure of carriageway. Additional initial desk study review supplemented by appropriate loading of existing costs to repair damage and During ground investigation as required. Assess stability of structures. strengthen earthwork. 3 4 12 Designer detailed all earthworks likely to be affected by the works and 2 4 8 design design mitigation measures, if any, accordingly.
20 All Sections Obstructions to Foundation cannot be installed. Review existing as-built plans to establish extent of construction due Additional costs and delays to During existing foundations. Undertake ground investigation. to existing program. 3 4 12 Designer detailed 2 4 8 foundations design
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SECTION RISK LEVEL BEFORE MITIGATION RISK LEVEL AFTER MITIGATION Next Formal OF CONSEQUENCE OF HAZARD Risk MITIGATION MEASURE TO BE TAKEN BY No. HAZARD Risk Review SCHEME (RISK) Owner DESIGNER Probability Impact Degree of Risk Date Probability Impact Degree of Risk IMPACTED 21 All Sections Constrained Requirement for earth retaining or Identify areas where the highway boundary is highway boundary reinforced earth solutions During constrained and design solutions accordingly. 4 4 16 Designer detailed 1 4 4 design
22 All Sections Ground Possessions required. Undertake desk study and identify areas where investigation and During ground investigation and construction works will construction 4 1 4 Designer detailed interact with railways. Liaise with network rail. 1 4 4 interacting with design railways 23 All sections Recorded and un- Damage and consequential cost Consider substructure locations during optioneering. recorded and delay due to damaged caused During Site inspections immediately prior to ground Designed understructures by ground investigation or detailed investigation and foundation construction especially 4 3 12 and 2 3 6 including culverts foundation (esp piling) works hitting design and piling. Contractor and pipes. substructures construction
24 All sections Natural ground Delays to ground investigations Complete necessary ground investigations and obstructions eg and construction assess the potential for natural ground obstructions to During boulders be intersected during construction and highlight to 4 3 12 Designer detailed 2 3 6 Contractor to include construction responses in design method statements.
25 Section 4 Natural cavities Delays to construction and PSSR has identified a low risk of natural cavities additional cost affecting the scheme and hence the residual risk has During will be addressed in detailed design. Contractor to 3 3 9 Contractor 1 3 3 construction include risk in construction risk register and identify risk mitigation procedures in method statements.
26 Section 3 Potential for Head Risk of instability Ground investigation to confirm presence of sheared containing shear head deposits and subsequent appropriate design to During surfaces mantling meet design criteria. 3 3 9 Designer detailed 1 3 3 Lambeth Group design
27 Section 4 Brickearth Under pressures in excess of Ground investigation to confirm the presence of 200kPa collapse of the soil may Brickearth and appropriate design to mitigate occur. Brickearth deposits are also identified risks associated with its presence. During liable to collapse of their structure Contractor to be aware of wetting risk during 3 3 9 Designer detailed 1 3 3 on wetting which may occur as a construction. design result of flooding or broken water mains. Potential additional cost and delay 28 Section 4 Underground Extensive history of chalk mining Targeted ground investigation if considered mining identified in the Reading area appropriate based on final structures layout and During (between chainages 34100 and appropriate subsequent design. 2 5 10 Designer detailed 1 5 5 62900). design
29 All sections Regional Increases in pore water pressures Regional groundwater rise to be reviewed and risk to groundwater rise triggering instability the scheme to be assessed. Finalise designs to During due to reduction in mitigate residual risks as appropriate. 2 3 6 Designer detailed 1 3 3 groundwater design abstraction.
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SECTION RISK LEVEL BEFORE MITIGATION RISK LEVEL AFTER MITIGATION Next Formal OF CONSEQUENCE OF HAZARD Risk MITIGATION MEASURE TO BE TAKEN BY No. HAZARD Risk Review SCHEME (RISK) Owner DESIGNER Probability Impact Degree of Risk Date Probability Impact Degree of Risk IMPACTED 30 Section 4 Junction 11 Design based on incorrect ground Designer to ensure that the ground model is up to mapping out of model date. During date 2 3 6 Designer detailed 1 3 3 design
31 All sections Recorded Instability and associated delay Designer to undertake review of BGS National landslides and increased cost Landslide Database and identify any landslides that During may impact on the works and investigate/design so as 2 5 10 Designer detailed 1 5 5 to mitigate risks of instability to acceptable level. design
32 All sections Excessive loading Potential bearing failure and slope Designer to ensure that earthwork strength is of engineered instability triggered by use of heavy During sufficient for envisaged construction methodology. Designer earthworks. plant (eg 200 tonne cranes) for detailed Contractor to revise construction methodology should 3 5 15 and 1 5 5 structure construction design and required strengths not be achievable. Contractor construction
33 All sections Archaeology Delay and cost Consultation with an archaeological consultant during During the ground investigation and detailed design stages of Designer detailed the project 2 2 4 and 1 2 2 design and Contractor construction
34 All sections Unexploded Explosions, risk of injury/death to It is a reasonable assumption that UXO risks within ordnance (UXO) site workers. the current M4 footprint were mitigate when earlier During GI scheme phases were completed including original Designer planning, construction. Contractor to ensure all site operatives 2 5 10 and before any are aware of any residual UXO risks. If the proposed 1 5 5 Contractor site work is works are located outside the footprint of the current commenced M4, ordnance reports should be obtained and geophysical survey should be undertaken to identify possible areas of risk. 35 All sections River Terrace High water table, possible Designer to take account of the potential Deposits undocumented and filled former consequences of founding in or immediately above During quarries, variable ground river terrace deposits and design to mitigate identified 3 4 12 Designer detailed 1 4 4 conditions. risks to acceptable levels. design
36 All sections London Clay Potential for ground heave, high Designer to take account of the potential sulphate, perched water table, consequences of founding in or immediately above During selenite crystal formation resulting London Clay and design to mitigate identified risks to 3 4 12 Designer detailed 1 4 4 in volume increase and strata acceptable levels. design weakening
37 Sections 2, 3 Lambeth Group Potential for ground heave, Designer to take account of the potential and 4 perched water tables, variable consequences of founding in or immediately above During foundation conditions and sink the Lambeth Group and design to mitigate identified 3 4 12 Designer detailed 1 4 4 holes close to the contact with the risks to acceptable levels. design Upper Chalk
Other Key Related Construction Risks
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SECTION RISK LEVEL BEFORE MITIGATION RISK LEVEL AFTER MITIGATION Next Formal OF CONSEQUENCE OF HAZARD Risk MITIGATION MEASURE TO BE TAKEN BY No. HAZARD Risk Review SCHEME (RISK) Owner DESIGNER Probability Impact Degree of Risk Date Probability Impact Degree of Risk IMPACTED 38 All Sections Services Conflict with proposed new Identify existing services from desk study review construction, damage during During supplemented by on site investigation as needed to Designer construction works, delays to detailed enable appropriate protection or diversion of services 5 5 25 and 1 5 5 construction and additional costs, design and to be designed and implemented. Use CAT scanning Contractor risk of injury/death to construction construction and hand dug inspection pits for all excavations. personnel 39 All Sections Road traffic Injury/ death to site workers/ Where practicable locate works away from live traffic. members of the public Otherwise traffic management in accordance with During Designer Chapter 8. Residual risks for management by the detailed 5 5 25 and contractor and site staff to be notified via Designer’s 2 5 10 design and Contractor Risk Assessment. construction
40 All Sections Narrow verge Difficult/restricted access for Assess potential for minor variations in locations of width at plant/machinery. Increased risk of structures to avoid areas of restricted work area. During embankment slips/falls. Increased risk of Adjust foundation solution accordingly to 5 3 15 Designer Detailed 2 3 6 crests damage to equipment and risk to accommodate restricted areas of working. Design human health
Opportunities
41 All sections Cost of retaining Excessive construction cost due to N/A N/A N/A Designer and contractor to be watchful of the potential N/A N/A N/A structures over reliance on retaining During to reduce cost and construction programme if Designer structures for design to minimise detailed additional land purchase can be facilitated to minimise and land take design and use of structural solutions. Contractor construction
42 All sections Cost of foundation Excessive construction cost from N/A N/A N/A Designer to maximise use where feasible of N/A N/A N/A construction long construction possessions and During construction methodologies that minimise possession Designer associate traffic management costs detailed times and hence traffic management costs. and design and Contractor construction
43 All sections Whole life cost Excessive cost by dealing with N/A N/A N/A Designer to liaise with the Managing Agent Contractor N/A N/A N/A earthworks defects within the and (after Nov 2013) the Asset Support Contractor as During scheme separately to the main well as the M25 DBFO Contractor to identify Designer Detailed works earthworks defects that could be costs effectively Design rectified alongside the main works.
Probability: Impact/Consequence: Degree of Probability x Impact/Consequence Risk 1 - Negligible 5 - Very high 2 - Unlikely 4 - High 3 - Likely 3 - Medium 12-25 Intolerable 4 - Probable 2 - Low 5 - Very likely 1 - Very low
9-12 Substantial
5-8 Tolerable
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SECTION RISK LEVEL BEFORE MITIGATION RISK LEVEL AFTER MITIGATION Next Formal OF CONSEQUENCE OF HAZARD Risk MITIGATION MEASURE TO BE TAKEN BY No. HAZARD Risk Review SCHEME (RISK) Owner DESIGNER Probability Impact Degree of Risk Date Probability Impact Degree of Risk IMPACTED
1-4 Trivial
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8 FIGURES, DRAWINGS AND PHOTOGRAPHS
Figures and drawings can be found in Appendix K
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9 REFERENCES Previous Geotechnical Studies
HA GDMS reports A full list of HA GDMS reports reviewed during the study is presented as Table 9.1(sorted by HA GDMS report number) and as Table 9.2 (sorted by HHJV unique report reference number) in Appendix L.
Additional HA GDMS reports (not in Table 9.1 & 9.2): LG Mouchel & Partners, 1991. M25 Provision of Motorway Signals Mark 2 and Enhanced message Signs, Geotechnical Desk Study, January 1991. HA GDMS Report Reference: 12677
BGS borehole records The following are references for the schemes / ground investigations used for BGS borehole records. A list of the relevant BGS borehole records corresponding to each report is presented in Table 1.3 in Appendix A.
Unknown Author, 1943, Slough Southern Intercepting Sewer, HHJV Report Reference: 172 Unknown Author, 1959, 1959 – MOT Chiswick – Langley Road Scheme, HHJV Report Reference: 159 Unknown Author, 1960, 1960 - Slough-Maidenhead By-Pass Contract No.2 (M4), HHJV Report Reference: 160 Ground Explorations, 1967, Construction of Pulverising Plant - Slough, HHJV Report Reference: 171 Le Grand Adsco, 1967a, London to South Wales Motorway - Theale to Winnersh Section, HHJV Report Reference: 161 Le Grand Adsco, 1967b, London to South Wales Motorway Holyport Section, HHJV Report Reference: 162 Unknown Author, 1983, M4 Motorway Heathrow Spur to M25 Interchange, HHJV Report Reference: 164 Unknown Author, 1986, Slough Sewage Treatment Works - Manor Farm, HHJV Report Reference: 174 Unknown Author, 1987, Maidenhead Flood Study, HHJV Report Reference: 165 Soil Mechanics, 1989, Heathrow Access Road, HHJV Report Reference: 166 Norwest Holst Soil Engineering, 1991, Heathrow Surface Access Studies, HHJV Report Reference: 167 Fugro-McCelland, 1991, M25 VMS Gantries Phase 2 Ground Investigation, HHJV Report Reference: 168 Norwest Holst Soil Engineering, 2001a, M4 Junction 11 to 12 , HHJV Report Reference: 169 Unknown Author, 2005, Area 3 Class 1 Earthworks Defects 2005, HHJV Report Reference: 170 Various Authors, Various, BGS Records of Water wells (various), HHJV Report Reference: 173
Scheme Specific Studies - M4 Junctions 3-12 Managed Motorways SGAR-1 Mouchel, 2011a. Geotechnical Statement of Intent M4 Junctions 3 – 12 Managed Motorways. November 2011. Mouchel, 2011b. Technical Appraisal Report (TAR) for M4 J3 to 12 Managed Motorways. December 2011. SGAR-2 URS, 2013. M4 Junction 3 to 12 Managed Motorway All Lane running, Extended Options Phase Report, June 2013 (Draft - in preparation 12 June 2013)
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Highways Agency Highways Agency, 2003. HD 41/03 Maintenance of Highway Geotechnical Assets, Design Manual for Roads and Bridges, Volume 4: Geotechnics and Drainage, Section 1: Earthworks, Part 3; HMSO Highways Agency, 2008. HD 22/08 Managing Geotechnical Risk, Design Manual for Roads and Bridges, Volume 4: Geotechnics and Drainage, Section 1: Earthworks Part 2; HMSO Highways Agency, 2012. BD 2/12 Technical Approval of Highway Structures, Design Manual for Roads and Bridges, Volume 1: Highway Structures Approval Procedures and General Design, Section 1: Approval Procedures; HMSO Highways Agency. 2013. Highways Agency Geotechnical Data Management System (HA GDMS) http://www.hagdms.com/ [Accessed between 12 March 2013 and 3 April 2013].
British Geological Survey British Geological Survey. 1981. 1:50,000 Map Sheet 270 South London (Solid and Drift Edition) British Geological Survey. 1999. 1:50,000 Map Sheet 269 Windsor (Solid and Drift Edition) British Geological Survey. 2000. 1:50,000 Map Sheet 268 Reading (Solid and Drift Edition) British Geological Survey. 2013. GeoIndex http://www.bgs.ac.uk/geoindex/ [Accessed between 12 March 2013 and 3 April 2013]. British Geological Survey. 2013. Borehole Scans http://www.bgs.ac.uk/data/boreholescans/. [Accessed between 12 March 2013 and 3 April 2013]. British Geological Survey. 2013. Mining Plans Portal http://www.bgs.ac.uk/nocomico/. [Accessed 4 April 2013]. Ellison, R A & Williamson, I T. 1999. Geology of the Windsor and Bracknell district – a brief explanation of the geological map. Sheet Explanation of the British Geological Survey. 1:50,000 Sheet 269 Windsor (England and Wales). Mathers, S J & Smith, N J P. 2000. Geology of the Reading district – a brief explanation of the geological map. Sheet Explanation of the British Geological Survey. 1:50,000 Sheet 268 Reading (England and Wales). McEvoy, F M, and 7 others. 2003. Mineral Resource Information in Support of National, Regional and Local Planning: Berkshire (comprising West Berkshire, Reading, Wokingham, Windsor & Maidenhead, Bracknell Forest and Slough). British Geological Survey Commissioned Report CR/03/074N. Benham, A.J., and 6 others. 2003a. Mineral Resource Information in Support of National, Regional and Local Planning: Hertforshire and NW London boroughs (comprising Hertfordshire, London Boroughs of Barnet, Enfield, Harrow and Hillingdon). British Geological Survey Commissioned Report CR/03/075N. Benham, A.J., and 8 others. 2003b. Mineral Resource Information in Support of National, Regional and Local Planning: Buckinghamshire (comprising Buckinghamshire and Milton Keynes). British Geological Survey Commissioned Report CR/03/077N.
Landmark Information Group Landmark Information Group. 2006. Envirocheck report ref. 20689417-1-1 dated 07/12/2006. Landmark Information Group. 2006. Envirocheck report ref. 20689418-1-1 dated 07/12/2006. Landmark Information Group. 2008. Envirocheck report ref. 26013375-1-1 dated 05/08/2008. Landmark Information Group. 2008. Envirocheck report ref. 26013377-1-1 dated 05/08/2008.
Environment Agency Environment Agency. 2013. What is in your backyard? http://www.environment- agency.gov.uk/wiyby [Accessed between 12 March 2013 and 3 April 2013].
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Environmental Sources Multi-Agency Geographic Information for the Countryside (MAGIC). 2013. http://magic.defra.gov.uk/. [Accessed between 12 March 2013 and 3 April 2013]. Nature on the Map. 2013. http://www.natureonthemap.naturalengland.org.uk/. [Accessed between 12 March 2013 and 3 April 2013].
Web based Resources Bing Maps. 2013. www.bing.com/maps [Accessed between 12 March 2013 and 3 April 2013]. Google Earth. 2013. http://earth.google.co.uk [Accessed between 12 March 2013 and 3 April 2013].
Technical References Hight, D W; Ellison, R A & Page, D P. 2004. Engineering in the Lambeth Group. CIRIA C583. British Drilling Association. 1992. Guidance Notes for the Safe Drilling of Landfills and Contaminated Land Northmore, K J; Bell, F G & Culshaw, M G, May 1996. The engineering properties and behaviour of the brickearth of south Essex, Quarterly Journal of Engineering Geology and Hydrogeology, 29, 147-161. Tomlinson, M J, 2001. Foundation Design and Construction (7th Edition), Pearson
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