High Level Feasibility Study for the Remediation of Cadshaw Embankment and Retaining Wall
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HIGH LEVEL FEASIBILITY STUDY FOR THE REMEDIATION OF CADSHAW EMBANKMENT AND RETAINING WALL PREPARED BY: Wilde Consultants Ltd DATE: October 2019 WILDE REF: 762-014-R01/P2 CADSHAW EMBANKMENT & RETAINING WALL HIGH LEVEL FEASIBILITY STUDY BLACKBURN WITH DARWEN BOROUGH COUNCIL 762-014-R01/P2 OCTOBER 2019 HIGH LEVEL FEASIBILITY STUDY FOR THE REMEDIATION OF CADSHAW EMBANKMENT & RETAINING WALL Wall Name: Cadshaw Road Name: A666 Blackburn Road Location: E 370420, N 418273 (Grid Ref.) (SD704182) BwDBC Ref: 930.01 Structure Type: Embankment & Retaining Wall REPORT CONTROL SHEET Prepared Version Date Status Checked Approved By October D Freeman D Freeman P1 First Issue O Hawkes 2019 CEng MICE CEng MICE October Second D Freeman D Freeman P2 O Hawkes 2019 Issue CEng MICE CEng MICE i CADSHAW EMBANKMENT & RETAINING WALL HIGH LEVEL FEASIBILITY STUDY BLACKBURN WITH DARWEN BOROUGH COUNCIL 762-014-R01/P2 OCTOBER 2019 CONTENTS REPORT CONTROL SHEET i CONTENTS ii LOCATION PLAN iii 1.0 EXECUTIVE SUMMARY 1 2.0 INTRODUCTION 4 3.0 DESCRIPTION OF EXISTING STRUCTURE 5 4.0 PREVIOUS SITE INVESTIGATION 7 5.0 AVAILABLE INFORMATION 9 6.0 EXISTING UNDERGROUND SERVICES 10 7.0 LAND OWNERSHIP 11 8.0 INDICATIVE DESIGN OPTIONS AND COST ESTIMATES 12 9.0 RECOMMENDATIONS 28 10.0 HEALTH AND SAFETY 30 11.0 TRAFFIC MANAGEMENT 31 12.0 FURTHER WORK REQUIRED 32 13.0 ENVIRONMENTAL IMPACT CONTROL MEASURES AND CUSTOMER CARE ISSUES 33 APPENDIX A – EXISTING ARRANGEMENT A APPENDIX B – OPTIONS SKETCHES B APPENDIX C – STATUTORY UNDERTAKERS INFORMATION C APPENDIX D – LAND OWNERSHIP DETAILS D APPENDIX E – SKETCH BY BwDBC INDICATING EXTENT OF PEAT E APPENDIX F – C3 ESTIMATES F APPENDIX G. PHOTOGRAPHS TAKEN ON SITE G ii CADSHAW EMBANKMENT & RETAINING WALL HIGH LEVEL FEASIBILITY STUDY BLACKBURN WITH DARWEN BOROUGH COUNCIL 762-014-R01/P2 OCTOBER 2019 LOCATION PLAN iii CADSHAW EMBANKMENT & RETAINING WALL HIGH LEVEL FEASIBILITY STUDY BLACKBURN WITH DARWEN BOROUGH COUNCIL 762-014-R01/P2 OCTOBER 2019 1.0 EXECUTIVE SUMMARY 1.1 Wilde Consultants Ltd have been engaged by Blackburn with Darwen Borough Council (BwDBC) to prepare a high-level feasibility report to consider options for the replacement or refurbishment of the highway and retaining wall adjacent to Cadshaw Bridge on the A666 south of Darwen. 1.2 The A666 runs approximately South to North and is the primary route connecting the towns of Bolton and Darwen. Cadshaw Bridge, which carries the A666 over Cadshaw Brook is located approximately 2.5km south of Darwen town centre and 10km North of Bolton town centre. 1.3 Directly to the North of the bridge, approach structures support the highway. The comprise a masonry retaining wall to the East side and an embankment to the West side. 1.4 The highway, retaining wall and embankment to the north of Cadshaw bridge have been found to be in POOR condition, with settlement and break-up of the carriageway surfacing alongside bulging and local collapse of the retaining wall. The cause of this degradation is the presence of a thick “localised layer of alluvial peat and soft alluvium with peat inclusions beneath the highway” (Capita, 2019), this is discussed in detail in the Ground Investigation Report produced by Capita, August 2019. Monthly safety inspections undertaken by BwCBC have highlighted the deteriorating condition of the retaining wall over a relatively short period of time. 1.5 A site walkover was conducted by Wilde Consultants on the 3rd October 2019 and found the retaining wall and carriageway surfacing to be in POOR condition with numerous significant defects. 1.6 The aims of this feasibility report are: • to consider high level options for the refurbishment or replacement of the highway and embankment retaining wall • to provide high level cost estimates • to highlight further works required to deliver the preferred option. 1.7 The following options have been considered; Option 1 – Dig out all of the potentially compressible materials including alluvial peat, alluvium and glacial till, and back fill with engineering fill material. Option 2 – Inject a ground stabilising resin to improve the bearing capacity of the embankment and install a sheet pile wall in front of the existing masonry retaining wall. Option 3 – Inject a ground stabilising resin to improve the bearing capacity of the embankment and install ground anchors/soil nails to support the existing wall. Option 4 – Inject a ground stabilising resin to improve the bearing capacity of the embankment and construct a gravity retaining wall in front of the existing masonry wall. 1 CADSHAW EMBANKMENT & RETAINING WALL HIGH LEVEL FEASIBILITY STUDY BLACKBURN WITH DARWEN BOROUGH COUNCIL 762-014-R01/P2 OCTOBER 2019 Option 5 – Inject a ground stabilising resin to improve the bearing capacity of the embankment and regrade the east embankment with imported material to make the existing retaining wall redundant. Option 6 – Install a piled slab to support the carriageway above the weak strata Option 7 – A do minimum approach where no remedial works are specified and the wall/highway are allowed to degrade in time. 1.8 Option 3 – Inject a ground stabilising resin to improve the bearing capacity of the embankment and install a reinforced concrete facing wall soil nailed into the stabilised embankment, to support the existing retaining wall is recommended due to the following; • Can be constructed mostly under single lane closure traffic management. • Minimal land take required when casting an RC facing against the existing wall. • Service diversions not required. • Minimal temporary works or establishment of plant required to facilitate soil nailing. A summary of the various options presented in this report can be found in the following table. Option Brief Description Estimated Service Approximate Cost Diversions Land Take Dig out all unsuitable material Yes 1 £1,300,000.00 None and back fill with (+£600,000.00) imported fill. Stabilise the embankment using 2 resin injection & £815,000.00 None <100m2 install sheet pile wall. Stabilise the embankment using resin injection & 3 install £925,000.00 None Minimal Gabion/RC facing wall soil nailed into embankment. Stabilise the embankment using 4 resin injection & £920,000.00 None ~200m2 install gravity retaining structure. Stabilise the embankment using resin injection and 5 regrade the east £855,000.00 None ~600m2 embankment burying the existing wall. 2 CADSHAW EMBANKMENT & RETAINING WALL HIGH LEVEL FEASIBILITY STUDY BLACKBURN WITH DARWEN BOROUGH COUNCIL 762-014-R01/P2 OCTOBER 2019 Install pile supported slab to Yes 6 £570,000.00 None support the (+£600,000.00) carriageway. 7 Do nothing. NA NA NA 1.9 It is estimated that it will take 36 weeks to complete the proposed works. 1.10 The estimated construction cost of Option 3 is £925,000.00 1.11 A full list of recommended Further Work is included in Section 12 of this report. 3 CADSHAW EMBANKMENT & RETAINING WALL HIGH LEVEL FEASIBILITY STUDY BLACKBURN WITH DARWEN BOROUGH COUNCIL 762-014-R01/P2 OCTOBER 2019 2.0 INTRODUCTION 2.1 Wilde Consultants Ltd have been engaged by Blackburn with Darwen Borough Council to prepare a high-level feasibility report to consider options for the refurbishment of the A666 adjacent to Cadshaw Bridge and the adjacent embankment retaining wall. 2.2 The existing embankment and retaining wall have had no significant remedial works undertaken to date, and is estimated to be ~170 years old. The wall and highway are in a POOR condition, see photos taken on site by Wilde Consultants in appendix G of this report. BwDBC have provided the results of site investigations consisting of a desk top study and SSL exploratory holes were undertaken in 2019. The results of these are discussed in the Ground Investigation Report. These results are discussed and summarised in section 3.10 and 4.0. 2.3 The aims of this feasibility report are: • to consider high level options for the refurbishment of the A666 and embankment retaining wall. • to provide high level cost estimates. • to highlight further works required to deliver the preferred option. 4 CADSHAW EMBANKMENT & RETAINING WALL HIGH LEVEL FEASIBILITY STUDY BLACKBURN WITH DARWEN BOROUGH COUNCIL 762-014-R01/P2 OCTOBER 2019 3.0 DESCRIPTION OF EXISTING STRUCTURE 3.1 Cadshaw Bridge is a single span masonry arch spanning South to North. The bridge carries the A666 Blackburn Road over Cadshaw Brook approximately 2.5km south of Darwen town centre. Immediately north of the bridge the highway is raised above local ground level to the level of the bridge deck by an embankment, the east side of this embankment is supported by a masonry retaining wall. 3.2 The masonry retaining wall to the north east of Cadshaw bridge is approximately 63m long. The retaining wall and carriageway surfacing are in POOR condition, with multiple significant defects observed indicating differential settlement of the embankment. For the purpose of this report, the site refers to a length of highway approximately 63m long to the north of Cadshaw bridge, encompassing the retaining wall the east to and the embankment slope to the west. 3.3 The retained height of the wall varies along its length, from available topographical information of the site, the maximum retaining height is approximately 1700mm. The wall in in a POOR condition with significant bulging along its length and loose masonry. 3.4 There exists a footway on the east side of the carriageway adjacent to the retaining wall, the footway surfacing is in POOR condition and is subsiding in areas. 3.5 The retaining wall is topped with an 800mm high masonry parapet, this parapet is in POOR condition and exhibiting rotation away from the carriageway and significant bulging. 3.6 The length of the retaining wall and embankment considered in this report is approximately 63m.