TECHNICAL ARTICLE

AS PUBLISHED IN The Journal October 2019 Volume 137 Part 4

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Kerrie Illsley JOURNAL PRODUCTION EDITOR Permanent Way Institution [email protected]

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Targeted asset AUTHORS:

Ian Payne management Freddie Savill Network Rail, Stratford, London Simon Holt approach to Isaac Griffiths Stuart Fielder mitigating railway Atkins Limited earthwork instability

Chitts Hill Embankment, Colchester, Essex

ABSTRACT

Like many other ageing Victorian earthworks, the railway embankment at Chitts Hill, west of Colchester in Essex, has instability problems that are affecting the track and ultimately pose a risk to the public. The site is approximately 520m long between two overbridges on the 100mph busy London to Norwich railway line. The embankment is located within the River Colne valley and underlying deposits include local soft alluvium overlying the London Clay formation. Detailed interrogation of the site has taken place as part of the Control Period (CP) 5 and early CP6 asset management. The designer has undertaken a desk study, geomorphological mapping and a risk management and optioneering exercise to determine the mitigation extents and solutions.

The site has a history of repairs including toe weighting, cess retention and a sheet piled wall but problems persist. In addition to repeated twist faults, alignment issues and frequent maintenance of the track and trackbed, slope movement indicators are evident including a displaced cess walkway, toe bulges, poor drainage, vermin burrowing, tree related desiccation and run-on/run-off transition issues with structures. Understanding the geological complexities, slope movements through inclinometer data, establishing a sound ground model and understanding the robustness of existing interventions is key to selecting the appropriate mitigation. The oversteep clay slopes with ash crests typically show movements of 20-30 mm per year at 2-3 m depth with occasional more sudden movements triggering Temporary Speed Restrictions (TSRs).

Figure 1: Site location plan

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The Anglia Targeted Asset Management (TAM) Category & Definition Risk Rating Risk Definition process includes value engineering principles H HIGH TSR/track closure/delay minutes recorded within past 12 months. to fix the areas posing the highest risk while Service Impacts (TSRs, Track M MEDIUM TSR/track closure/delay minutes recorded within past 5 years continuing to monitor adjacent areas. The A Closures, delay minutes, service approach allows funding to stretch to other disruptions) L LOW No historic TSR/track closure/delay minutes recorded in section. high priority sites across the region meeting ? NO DATA No data available for section. the Network Rail earthworks asset policy Bespoke track maintenance implemented within past 12 months objectives. Optioneering of the proposed (hand jack & pack, temporary cess support etc.); more than 3 engineering solutions, including cess retention H HIGH standard maintenance interventions (e.g. tamping/stoneblowing). and soil nails will be reviewed together with Track dip / twist recorded within past 12 months. overcoming the constraints including access Increased level of track maintenance in section within past 5 years. Bespoke track maintenance implemented within past 5 years; and managing ecological and environmental Track Maintenance and Defects (NR MEDIUM Maintenance intervention run off from adjacent section. Track dip / B track maintenance records and NR M legislation. twist recorded within past 5 years; track dip/twist run off from track geometry records/LADS data) adjacent section. 1 (Track and maintenance data) No historic record of bespoke track maintenance in section. INTRODUCTION L LOW Standard maintenance works only. No historic track dip / twist recorded in section. Past remediation works have often focussed ? NO DATA No data available for section. on a particular section of earthwork exhibiting immediate concerns, rather than assessing Significant rate and/or total movement; deep seated movement in HIGH H inclinometer; shallow movement in upper slope. failure in terms of the overall condition of the Minor rate and/or total movement; shallow movement in midslope asset. However, by not assessing the entire In place inclinometers / MEDIUM A M or toe. instrumentation observations asset, including both sides of the track and L LOW No/negligible movement. the surrounding natural setting, key failure indicators and an appreciation of scale may ? NO DATA No instrumentation available for section. be missed. Furthermore, the presence of Ground conditions deemed indicative of unstable earthwork HIGH H conditions. other failures and / or previously implemented Ground conditions deemed suggestive of possible unstable remediation measures along an asset suggest 2 (existing GI data) MEDIUM M earthwork conditions. B Ground conditions that stability issues may not be limited to the Ground conditions do not indicate any signs of earthwork LOW L instability. area of current failure and that an ongoing ? NO DATA No existing GI data available. or more widely spread problems could be Visual evidence of significant movement to track, ballast shoulder, present. Alternatively, the assessment of HIGH H cess and / or lineside infrastructure. the entire asset can indicate that an issue Visual evidence of minor movement to track, ballast shoulder, cess MEDIUM is localised only, giving confidence in the M and / or lineside infrastructure. A Track level & cess chosen remediation measure. It is therefore L LOW No visual evidence of movement at track level. essential that the asset as a whole is assessed ? NO DATA Not examined (provide reason). to determine the extent of issues at a site and Visual evidence / indicator of significant slope movement on lower, the nature of instability to ensure the correct H HIGH mid or upper slope of embankment / cutting. mitigation type and extent is implemented Visual evidence / indicator of minor slope movement on lower, mid MEDIUM M or upper slope of embankment / cutting. to allow the continued serviceability of the B Slope face earthwork and railway. L LOW No visual evidence of movement on slope face.

? NO DATA Not examined (provide reason). It is recognised however, that due to the large scale of many of the earthworks renewal H HIGH Visual evidence of major drainage issues. projects and the constrained budgets available M MEDIUM Visual evidence of minor drainage issues. for remedial works, large-scale heavy C Drainage L LOW No visual evidence of drainage issues. engineering solutions across entire sites

3 (visual site observations) are not feasible. Therefore, a risk managed ? NO DATA Not examined (provide reason). and targeted approach to renewal works is 3rd Party H HIGH Visual evidence of major issues outside of railway boundary fence. needed to ensure the correct remediation is mass movement feature; ponding; M MEDIUM Visual evidence of minor issues outside of railway boundary fence. D undercutting of embankment toe implemented at the worst affected sections, LOW No visual evidence of issues outside of railway boundary fence. by 3rd party; loading of slope crest L whilst ensuring that a suitable monitoring NO DATA Not examined (provide reason). by 3rd part etc ? system is in place to ensure other areas of H HIGH Visual evidence of major vermin issues. concern do not deteriorate. This allows for M MEDIUM Visual evidence of minor vermin issues. E Vermin L LOW No visual evidence of vermin issues. efficiencies in remediation designs and helps ? NO DATA Not examined (provide reason). to eliminate multiple period spot fixing that will cost proportionately more in the long term (see reference 9 for another case history). Table 1 Definition of TAM failure criteria TARGETED ASSET Definition of TAM Risk Categories Risk Category Definition MANAGEMENT (TAM)

Known evidence of regular high priority track issues and or above average slope High (H) movements with visual signs of instability with high potential to cause public Targeted Asset Management (TAM) has disruption and a threat to the safe operation of the railway. been developed as a proactive way to meet the Earthworks Asset Policy objectives and Potential to cause continued track issues & affect maintenance regime. Slope movements are considered at or below average and steady. No imminent signs of failure optimise budgets. By implementing a risk Medium (M) from visual observations. Continue to monitor and have a future works strategy plan in management approach to undertaking works place should it enter HIGH risk. at the site, the approach moves away from implementing a full fix across an entire site Minor track and maintenance issues with no cause for concern, below average slope Low (L) movements (where data) and no visual indicators of failure. to more sustainable solutions, allowing a larger area of earthwork to be remediated for the available budget by targeting works at Table 2: Definition of TAM risk category

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Figure 2: Detailed geomorphological mapping undertaken on 4th December 2018 and 22nd January 2019.

The detailed map was hand drawn on site based on field observations.

The red annotations indicate track issues, blue indicate drainage issues and green indicate slope issues.

Each of the numbered items records a detailed observation at the site which are not reproduced here.

the worst affected areas. The TAM risk management approach is being utilised to identify the scope of remedial works at key sites within the Anglia Earthworks CP6 workbank. The TAM process targets stabilisation at the highest risk sections of the earthworks to minimise operational disruption and is undertaken in four stages as outlined below.

TAM Stage 1 (GRIP 3-4) – Site mapping, data review, risk zonation, preliminary geotechnical analysis, optioneering and the Approval in Principle AIP (F001) output.

TAM Stage 2 – Cost estimation, scope ratification and confirmation of work scope.

TAM Stage 3 (GRIP 5-6) – Detailed design of agreed remedial works, construction support, further targeted GI and a future works strategy plan to ensure ongoing asset stewardship.

TAM Stage 4 – Continued monitoring and collation of associated data (Network Rail).

Constant dialog between the Client, Contractor and Designer is key to the TAM process and ensures that the final remediation option selected tackles the instability issues at a site, whilst remaining both affordable and constructible. TAM Stage 1 focusses solely on data collection and considers the asset as a whole, enabling the site to be divided into zones based on current condition. Outline analysis can then be completed for these sections allowing an optioneering process to be undertaken, both in terms of the type of remedial measures and extent of remedial works. The options determined during TAM Stage 1 are then scrutinised in terms of cost and constructability during TAM Stage 2 resulting in a finalised agreed scope of works. Detailed design and construction are undertaken during TAM Stage 3 as well as design installation of future monitoring and emergency works designs, enabling an immediate response in the event of an earthworks failure in one of the site zones not remediated as part of the main works.

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Figure 3: LiDAR imagery of the site

TAM STAGE 1 TAM SITE CASE STUDY – CHITTS PUBLISHED GEOLOGY AND HILL EMBANKMENT HYDROGEOLOGY TAM Stage 1 aims to identify and prioritise the key geotechnical risks of the earthwork INTRODUCTION AND SITE LOCATION The British Geological Survey, (BGS), Map with potential to cause problems to the safe Sheet 223 of the 1:50,000 series, (British operation of the railway. A technical risk The Chitts Hill railway embankment is located Geological Survey, 1982), indicates the register is used to record the process and the approximately 2km west of Colchester, Essex geological succession at the site as superficial output enables optimised renewal works at the as shown in figure .1 The site runs southwest River Terrace Deposits overlying the bedrock higher risk areas. Measures such as continued to northeast on the Liverpool Street to Trowse geology of the London Clay Formation. slope movement monitoring are implemented Lower Junction, (Engineers Line Reference Alluvium is mapped at the south western edge at unremediated areas to control residual risks. LTN1), between 49 miles 69.5 chains (49m of the site associated with the River Colne 69.5ch) and 50m 16.5ch and is bounded to the and may extend beneath the London end of The TAM technical risk register has been London end by a seven arch masonry viaduct the embankment. Head deposits and glacial agreed by the Anglia Route Collaboration over the River Colne and at the country end by sand and gravel deposits are also mapped on (ARC) that uses 3 main failure categories: an underbridge. The embankment is 10 m to the higher ground to the north of the site and 15 m high and lies on the edge of a flat flood may extend beneath the Country end of the 1. Track and maintenance issues plain at approximately 10 m AOD. The site is embankment. 2. Ground conditions and slope movement surrounded by agricultural fields to the north monitoring west and a golf course to the south east. HISTORICAL DEVELOPMENT 3. Visual movement indicators The Up side of the embankment has a history The site is located on the Great Eastern Each of the failure criteria is further subdivided of instability with defects recorded on the mainline between London and Norwich which as defined in Table 1 and a risk of low, medium earliest available modern inspections in 2005 was opened in stages between 1839 and 1851, or high defined based on the available data for and historical map evidence indicates past with the section through the site completed a particular zone of the site. Where no data interventions and ongoing instability issues. by 1844. The earliest available historic map is available this is entered into the register, More recently a section of the embankment is from 1877 on which the railway is already allowing areas of concern where limited data between 49m 73ch and 49m 75ch was present. The embankment is shown to have an currently exists also to be identified. The zone repaired during 2011. irregular profile toe from the end of the viaduct is then given an overall TAM Risk Category at 49m 69.5ch through to approximately 50m based on the output of the initial failure criteria This initially comprised a 4 m deep king post 00ch from which point the slopes and toe assessment. and Armco retaining wall at the crest of the become more regular through to the end of the embankment to retain the cess. Following site at 50m 16.5ch, (see figure 2 for reference These categories are used to determine the continued deeper seated movement this was to site mileages). The neighbouring land is extent of remedial works recommended at supplemented by a more substantial sheet pile shown as fields on both sides of the railway the site and are largely based on engineering retaining wall near the toe of the embankment and along the Down side the embankment judgement of the initial failure criteria with the regrading of the slope above the toe toe is indicated to be marshy or wet by the assessment output. The overall risk categories wall. presence of hydrophyllic vegetation symbology, are defined in table 2. suggesting that drainage issues were already prevalent at this early stage.

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A) 49m 73 to 75ch Up. Good condition slope above B) 49m 74ch Up. Hydrophilic vegetation and ponding existing sheet pile wall earthwork renewal at toe beneath sheet pile wall earthwork renewal

C) 50m 01 to 05ch Up. Toe berm – ash and clinker D) 49m 77 to 78ch Down. Ponding at slope toe – construction – possible historic stabilisation opposite worst defect on Up Side

E) 49m 76ch Down. Historic toe bulge blocking toe F) 50m 05 to 12ch Down. Toe berm on slope – possible ditch causing ponding at slope toe historic stabilisation?

Figure 4: Site walkover observations of the earthwork slope

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A) 49m 71ch Up. Exposed sleeper ends, ballast spalling B) 49m 78ch Up. Large dip in cess walkway and down slope and failing cess associated track dip

C) 50m 05ch Up. Cess area in better condition D) 50m 15ch Up. Sleeper ends exposed, poor cess compared to the rest of the site condition. A small king post retaining wall has been constructed to retain ballast at this location. Figure 5: Site walkover observations at track level

A tree filled hollow at the toe of the Down side GEOMORPHOLOGICAL SURVEY Available LiDAR data was used both before slope is also shown at approximately 49m 78ch and during the detailed geomorphological between two toe bulges, which may represent A detailed geomorphological survey of both survey to help identify the location and scale the water concentration feature identified on the Up and Down side of the embankment of features at the site. The toe bulging and site. was undertaken during winter 2018/19, with lateral spreading indicated by the historic map key slope features and defects recorded on data at the London end of the site is visible The 1897 and 1923 maps show no significant a geomorphological map of the site, shown on the LiDAR imagery as shown in figure .3 changes; however, the 1953-1964 map in figure 2. Numerous indicators of stability Historic and recent remediation measures are sheet shows considerable changes to the issues were noted along the length of the also discernible and aided the identification embankment slopes. Two linear benches are embankment on both the Up and Down of these features on site during the detailed now shown on the Up and Down embankment side including toe bulging and failure lobe geomorphological surveys. slopes between 50m01ch to 07ch and features indicating deep seated failure of the 50m10ch to 15.5ch respectively, indicating embankment slope, (figure 4E). Indicators of The embankment slopes at the site are heavily implemented remedial works due to slope groundwater issues were identified in the form vegetated, with large mature deciduous trees instability issues along the site. The toe of the of hydrophilic vegetation and ponding observed present on both the Up and Down sides. These embankment is also shown to extend to the in areas where drainage pathways have large trees are most densely concentrated at boundary fence at the London end of the site been blocked by the presence of toe bulges, the London end of the site where the historic on both the Up and Down sides, which may (figures 4B, D & ).E Several historic repairs failures have led to less steeply graded indicate further failure and lateral spreading of were recorded in the form of toe berms/weights midslopes. the slopes. The most recent detailed historic on both the Up and Down side, (figures 4C & map of the site is from the 1960-1978 period F), and a more recent sheet pile wall and slope Drainage issues are also key to embankment and shows no considerable changes to the regrade, (figure 4A), indicating an ongoing stability and often are the driving factor behind site. history of slope instability issues at the site. failures. Historic slipping on both the Up

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A) 50m 73 to 75ch Up. Distorted cess walkway and B) 50m 73 to 75ch Up. Two rows of cess retention temporary speed restriction (TSR) speed board, in early constructed during May 2011, the slope continued 2011 prior to remediation to deteriorate shortly after construction due to deep seated failure (Ref 11)

C) 50m 73 to 75ch Up. Sheet pile wall constructed in summer 2011 to replace the failing cess retention and address deep seated failure mechanism (Ref 12)

D) 50m 73 to 75ch Up. Recent photo along sheet pile Figure 6: Recent earthwork remediations wall showing good horrizontal allignment.

and Down sides of the embankment, visible country end of the site, meaning that all At both ends of the site, where the on the LiDAR, aerial imagery and historic water draining from the higher ground to the embankment interfaces with adjacent maps, was confirmed during the detailed north of the site must either drain along the structures, the track and cess conditions were geomorphological mapping of the site. This length of the embankment to the London end, observed to be in generally poor condition has led to a number of large toe bulges which where drainage pathways are blocked by with ballast spalling down the embankment have cut off drainage pathways and on the historic failures, or make its way beneath the slopes leading to a low shoulder and exposed Down side has resulted in the formation of a embankment as groundwater flow. sleeper ends (figure 5A & ).D A small king significant pond approximately 10 m wide and post wall has been constructed at the country 2-3 m deep when full. This pond aligns directly TRACKSIDE ISSUES end of the site to restrain the ballast shoulder, with the current worst failure location on the but this was observed to be in poor condition Up side. During the walkover survey the track and cess and out of alignment (figure 5D). Track issues area was observed to be in very poor condition such as these are commonly observed across The Country end of the site is founded on at multiple locations along the embankment. the network at the embankment/structure sidelong ground where the embankment The worst observed area was at 49m 78ch on interfaces. runs parallel with the River Colne and at this the Up side where a large dip in the track and point natural drainage from the high ground cess walkway was recorded along with ballast Access to the full length of the Down side to the north into the valley is blocked by the shoulder thickening up to 1.5m and a lack of embankment at track level was not possible railway embankment. There are no culverts ballast shoulder above sleeper level (figure due to no walking route being present; recorded between the viaduct at the London 5B). nevertheless, it was possible to view the track end of the site and the underbridge at the area periodically along the length of the asset.

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A) LLAMAR Rig set up at embankment crest at Country B) Rotary rig set up at embankment toe at London end end of site for window sampling of site for continuous sampling

Figure 7: Ground investigation utilising a LLAMAR drilling rig and a rotary drilling rig

Some issues were observed on the Down side the large track and cess dip observed during led to the implementation of a temporary such as small track dips, loss of the ballast the walkover survey, (see figure 5B). The poor speed restriction, (TSR), and was causing shoulder and exposed sleeper ends, however, earthwork condition at this location is causing maintenance issues at the site. The two king the condition of the Down side appeared the twist faults observed at track level. Further post walls were constructed with 4 m long significantly better than the Up side. twist faults are also recorded approximately king piles and with two separate rows of piles 50m 16ch adjacent to the underbridge at the to allow the wall to be constructed around an TRACK MONITORING country end of the site, which correlate to the OLE stanchion on site. During construction it defects observed during the walkover survey, was identified that the slope failure was also NR periodically monitor track condition through (see figure 5D), indicative of the embankment/ deeper seated than originally thought and train mounted equipment which records track structure interface issues previously therefore the small cess retaining wall was not geometry and associated ground penetrating discussed. sufficient to arrest the deeper slope movement. radar (GPR) from which the condition of the Subsequently a larger lower slope sheet pile trackbed can be inferred. The frequency of RECENT EARTHWORK REMEDIATION retaining wall was designed by URS and monitoring across the network is dependent on installed by CML in summer 201111, (see figure the frequency of use of the line, with the results An Armco type king post retaining wall, 6C). The wall comprised a 10 m long L605 of the monitoring presented as Linear Asset designed by URS, was constructed by CML sheet pile with a 2 m retained height. The slope Decision Support (LADS) data2. LADS data at 49m 73ch to 75ch on the Up side of the behind the retaining wall was regraded with has been consulted for the site and indicates a embankment in May 201111, (see figure 6B). general granular fill to form a uniform slope. history of twist faults around 50m 00ch on the This structure was designed to remediate a Following installation of the sheet pile retaining Up side between 2016 and 2018, correlating to large dip in the track, (figure 6A), which had wall maintenance issues have abated at this

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Figure 8: Ground models at 49m 70ch and 49m 77.5ch

Figure 9: Embankment fill material encountered during 2015 GI works

section of the site. The sheet pile retaining wall A robust approach to ecology is required to holes were undertaken using Topdrill Limited, was also observed during a walkover survey in ensure that unexpected issues do not arise (Topdrill), Lightweight Limited Access Modular December 2018 and appeared to be effective, that delay a construction project. The majority Rig, (LLAMR), (see figure 7A). This rig (figure 6D), however a small amount of vermin of potential ecological issues have been enabled good samples of the embankment burrowing was observed in the granular fill managed through detailed surveys, with results fill to be obtained; however, samples of the above the sheet pile wall. for dormice and bats the only two outstanding underlying natural ground were limited to 1-3 surveys. m due to the presence of sand and gravel ECOLOGY SURVEYS deposits which caused early refusal in some of GROUND MODEL the positions. Upon completion inclinometers Ecology constraints are ever present at all were installed in all 8 exploratory holes. sites across the network. Chitts Hill was GROUND INVESTIGATION (GI) initially subject to a Phase 1 habitat survey by Further supplementary GI works were specified Ecus in September 2018. This identified the Several phases of recent GI have been by Atkins and undertaken at the site by Topdrill potential for several protected species and undertaken at the site; an initial phase of GI during 2019 comprising 9 windowless sample recommended further surveys for great crested was specified by Atkins1 and undertaken on exploratory holes and 3 dynamic/rotary newts (GCN), bats, dormice and badgers, as the Up side embankment slopes in 2015 by core boreholes. The exploratory holes were well as a precautionary method of working in Topdrill10 to inform design work undertaken in undertaken to a maximum depth of 12mbgl and respect to common reptile species and nesting CP5. The GI work comprised of 8 windowless inclinometers were installed in all exploratory birds. sample exploratory holes to a maximum depth holes upon completion. The supplementary GI of 12mbgl undertaken at varying heights allowed the underlying natural ground profile up the embankment slope. The exploratory to be confirmed with the use of a rotary drilling

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rig, (see figure 7B), at the toe to penetrate the deep-seated movement was also observed in the option to consider smaller localised fixes underlying sand and gravel layers. a number of the other inclinometer installations in some areas, (such as run on / run off of at the site. structures), to maintain the serviceability of GROUND CONDITIONS the railway without implementing a full scale A different failure mechanism was observed embankment remediation throughout. The The GI works undertaken to date have enabled adjacent to the viaduct at 49m 70ch. At this zoning and remedial works options for the site the development of a detailed ground model location a high magnitude of movement was will be further refined during the final phases for the site, which will be subject to further observed, (57 mm max), but this progressive of TAM Stage 1 once the ongoing GI works are refinement during the detailed design process movement was observed at shallow depth at completed and during the TAM Stage 2 scope following the most recently installed slope up to 2mbgl at the crest of the embankment, ratification process, ensuring that the chosen monitoring data becoming available. Two (figure 11A). It is thought that this shallow remedial works are robust and fulfil Network sections were produced to allow stability failure is caused by a loose ash layer which Rail’s requirements. analysis to be undertaken at 49m 70ch and is confined to the upper shoulder of the 49m 77.5ch are shown in figure .8 embankment slope with some desiccation DESIGN OPTIONEERING related movement evident in figure 11B. Typically, the embankment slopes at the Following completion of the TAM Stage 1 site feature an over steepened upper slope Twelve additional inclinometers were installed Risk zonation, initial analysis and design comprised of loose ash and clinker deposits, at the site at the start of 2019 to allow more optioneering was undertaken for the overlying the embankment core, (see figure detailed examination of the slope movement earthworks renewal at the site. Initial design 9). Due to the nature of construction used on at the site. The monitoring results from these options were considered for all high (red) and the early railway construction across the UK, installations are not yet available. medium (orange) areas of the site, (figure and subsequent historic stability problems 12). Three options were considered in the at the site, the composition of embankment CHITTS HILL TAM STAGE 1 optioneering exercise; toe berm with slope fill does show variations across the site, but OUTPUT regrade, soil nails with cess retention and a typically was found to comprise slightly gravelly sheet pile retaining wall with slope regrade. sandy clays. At some sections along the site, The Chitts Hill site has been subjected to The design solutions need to manage both the horizons of softer clay were logged within the the TAM Stage 1 process and following the deeper seated failure in the embankment and embankment fill which from the initial analysis detailed site mapping the site was subdivided the near surface failure in the upper layer of appear to align with areas of movement into zones. The zones were then assessed in ash fill. identified in the inclinometers installed upon line with the TAM failure criteria and assigned completion. an overall TAM Risk Category as can be seen Outline geotechnical analysis for each option in figure 12. was undertaken in accordance with BS EN The underlying natural ground conditions, as 1997-1:2004: Geotechnical design (including anticipated based on interpretation of slope The Up side has been subdivided into eight National Annex)7,4, NR’s design guidance failure mechanisms and slope morphology, zones, the division of these zones being document NR/L3/CIV/0718 and BS 8006- were found to vary along the site. At the determined both by physical changes in the 1:2010 Code of practice for strengthened/ London end of the site up to 3 m of soft shape of the earthwork, (indicative of different reinforced soils and other fills5. The design life alluvial soils associated with the River Colne failure mechanisms), and the condition at track of the stabilisation measures was 60 years were encountered, whereas the middle and level, which has the most immediate impact on for the soil nail option and 120 years for the country end of the site was underlain by the operational safety of the railway. Each of slope regrade and sheet pile wall option. Rail more competent sand gravel and stiff sandy these zones has been scrutinised in detail and traffic loading was taken from BS EN 1991- clay representative of river terrace and head all available data analysed to determine the 26 and maintenance loading was considered deposits. These changes are directly linked resultant TAM Risk Category. The quantity of over the cess area. Geotechnical design to the embankment traversing a number of available GI and slope monitoring data on the parameters were derived from the GI works different geological settings, from the base of Up side has enabled the zoning to be targeted, based on in-situ and laboratory test results the river valley at the London end of the site enabling resultant remediation works to target and were checked against back analyses of onto the lower flanks of the valley side slopes the worst affected areas. the embankment slopes. Groundwater level at the country end. was taken at the toe of the embankment due In contrast, the Down side has less individual to ponding observed at various locations along The top of the London Clay formation was also zones as the information available for this the asset and a pore pressure coefficient

confirmed along the length of the site, ranging side of the earthwork is limited, as this did ru=0.2 was considered in the embankment fill from approximately 3 m OD at the London not form part of the original site scope. material. end of the site to approximately 8 m OD at the Therefore, the subdivision is based entirely country end of the site, tying in with the nearby on site observations and available track The outline design output indicated that all historic borehole data along the A12 corridor. data. However, as a result of undertaking three options were feasible for renewal of this assessment, various issues have been the earthwork. The three options were taken SLOPE MONITORING identified on the Down slopes which were not forward to TAM Stage 2 in which construction previously known, including various historic pricing was estimated, the soil nail option was Eight inclinometers were installed at the site failures, historic intervention works and selected as the preferred option following this during the 2015 GI and these installations drainage issues that may be directly affecting assessment. have been monitored since installation. The the Up side condition at the site. Due to the installations are targeted at the worst affected investigation undertaken there are now plans ASSET MANAGERS PERSPECTIVE areas of the site, (see figure ),2 and movement to undertake drainage improvement works on trends observed in the inclinometers varied the Down side to address the identified issues. During Victorian railway construction most across the site. of the fill was end tipped using horse-drawn The TAM zoning process has allowed the earth wagons with little or no compaction. At the worst affected area of the site at 49m proposed remedial works to be tailored to the Settlements were large but accepted as an 77.5ch, total movement of up to 91 mm was specific requirements of each zone, which inevitable consequence of the construction observed over the 4-year monitoring period, ultimately will allow a greater percentage of method, initially mitigated by topping up with (figure 10). This movement was observed to the site to be fixed within the available budget. readily available materials such as locomotive be deep seated with a failure plane identified The data also allows a detailed analysis of ash and more recently mitigated by speed at approximately 7mbgl at the slope crest. This the mode of failure in these areas, giving NR restrictions and ballast packing. Monitoring

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A: (left) 49m 77.5ch WS03 (slope crest) 99 mm max movement between 2014 and 2019. Two potential zones of movement are visible – one at 2.5mbgl and one at 7mbgl.

B: (right) 49m 77.5ch WS04 (mid-slope) 8 mm max movement between 2014 and 2019. Inclinometer is founded at shallow depth (5.5mbgl) therefore deeper-seated movement could be occurring below the base of the inclinometer.

Figure 10: Inclinometer monitoring results indicating deep seated movements at 49m 77.5ch

A: (left) 49m 70ch WS01 (slope crest) 60mm max movement between 2014 and 2019.

B: (right) B) 49m 70ch WS02 (mid-slope) 6mm max movement between 2014 and 2019.

Figure 11: Inclinometer monitoring results indicating shallow depth failure adjacent to viaduct

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Figure 12: Chitts Hill site TAM risk zonation

slope movements at various depths allows the to avoid needing shorter term Reactive or 2. Bentley, 2019. LADS Data to Analyze Rail identification of clear failure modes with deep Emergency management of slope instability and Track Condition. Available from: https:// seated failures in the over-steep unengineered which would result in much less cost-effective www.bentley.com/en/capabilities/apm/analyze- London Clay embankment and shallower asset management. rail-track near surface movements associated with 3. British Geological Survey, 1982. 1:50,000 degradation of the upper embankment layer CONCLUSION Series England and Wales Sheet 223 Braintree of ash fill evident at Chitts Hill in common with Solid and Drift Geology. many other embankments in the Anglia region. The Chitts Hill embankment site demonstrates 4. BSI, 2007. NA to BS EN 1997-1:2004: the challenges of working on large linear National Annex to Eurocode 7: Geotechnical The Targeted Asset Management, (TAM), earthworks that have been in constant design - part 1: General rules. BSI, London, process has proved useful in managing the use for over 150 years. Old earthworks UK. challenges faced by infrastructure managers such as this experience a variety of issues 5. BSI, 2010. BS 8006-1:2010: Code of holding limited budgets and high performance with fundamentally different underlying practice for strengthened/reinforced soils and expectations of rail customers. Detailed causes. A holistic approach is required for other fills. BSI. London, UK. interrogation of slope movement trends, track the investigation of the assets in order to 6. BSI, 2010. BS EN 1991-2:2003: Eurocode 1: problems and the development of a ground fully understand the driving causes behind Actions on structures - Part 2: Traffic loads on model enabled optimised mitigation for the issues to ensure that an appropriate bridges. BSI, London, UK. higher risk zones of the embankment slope, remediation strategy is implemented. Access 7. BSI, 2010. BS EN 1997-1:2004: Eurocode demonstrating sound asset management. difficulties and limited availability of historical 7: Geotechnical design - part 1: General rules. records add to the difficulty of investigating the BSI, London, UK. The Chitts Hill site was subdivided into areas assets. A pragmatic approach is required to 8. Network Rail, 2011. NR/L3/CIV/074: of low, medium and high risk against a set ensure that necessary works are undertaken Geotechnical Design Issue 4. NR, London, UK. of failure criteria with the view that initially to maintain the safe operation of the railway 9. Payne, I. Holt, S. and Griffiths, I., 2018. only the higher risk and some of the medium- within limited available budgets. This has been Railway embankment stabilisation: economical risk areas would be mitigated earlier. The achieved at the Chitts Hill site through the use asset management. Proceedings of the earthworks renewal portfolio is subject to of the Targeted Asset Management, (TAM), Institution of Civil Engineers - Geotechnical budget constraints and the use of a more approach which has allowed renewal works Engineering 171(4): 332-344. innovative risk-based and focused mitigations to be targeted at the worst affected areas of 10. Topdrill, 2015. CP5 Anglia Critical releases funding to maximise the number the asset whilst risk control measures are Earthworks Report for Ground Investigation at: of site that can be treated across the wider implemented at other high priority sections. Site 12 - Chitts Hill. Doc ID: TOP1829 - 137959 regional earthworks portfolio. Chitts Hil FR01. REFERENCES 11. URS, 2011. Chitts Hill Embankment LTN1: Risks associated with other adjacent 49m 1620 yards Form B Cess Retention untreated areas are controlled through 1. Atkins, 2014. CP5 Anglia Critical Earthworks General Arrangement Drg. Doc Ref: 46382007/ ongoing observational and instrumentation Risk Management Package 1 Particular FORM B/001. monitoring. With increased rail traffic and Specification to the Master Specification 12. URS, 2011. Chitts Hill Embankment Repair continued unpredictable weather in the future, for Chitts Hill. Doc Ref: 137959-ATK-SPE- Works - Sheet Pile Wall. Site Layout Plan And it is important for Network Rail to have such EGE-000023 Rev01. General Arrangement. Drg No: 46382007/ a proactive strategy of Planned interventions FORM B/101.

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