Croxley Rail Link Environmental Statement. Appendix 11D. Geology and Soils - Geotechnical Interpretative Memo. Appendix 11D

Transcription

1 Croxley Rail Link Environmental Statement Appendix 11D Geology and Soils - Geotechnical Interpretative Memo Appendix 11D Mouchel 2011

2 Croxley Rail Link Geotechnical Interpretative Memo November 2011 * Severn House Lime Kiln Close Stoke Gifford Bristol BS34 8SQ T F For Hertfordshire County Council

3 Croxley Rail Link Geotechnical Interpretative Memo Job_Name4 Document Control Sheet Project Title Report Title Report Reference Version Croxley Rail Link Geotechnical Interpretative Memo GEO-EWK-RPT-AJL-117 B Issue Date November 2011 Record of Issue Version Status Author & Date Checked & Date Authorised & Date A For Information K Hartles A Lamb A Paraskeva B For Information K Hartles A Lamb A Paraskeva Distribution Organisation Contact Format Copies London Underground Limited D Povey Electronic PDF 1 Hertfordshire County Council T Duckmanton Electronic PDF 1 Sinclair Knight Merz S Parkinson Electronic PDF 1 Mouchel Ground Engineering A Paraskeva Electronic PDF 1 Mouchel GEO-EWK-RPT-AJL November 2011 ii

4 Croxley Rail Link Geotechnical Interpretative Memo Job_Name4 LIMITATIONS This report is presented to London Underground Limited in respect of Croxley Rail Link and may not be used or relied on by any other person or by the client in relation to any other matters not covered specifically by the scope of this Report. Notwithstanding anything to the contrary contained in the report, Mouchel Limited is obliged to exercise reasonable skill, care and diligence in the performance of the services required by London Underground Limited and Mouchel Limited shall not be liable except to the extent that it has failed to exercise reasonable skill, care and diligence, and this report shall be read and construed accordingly. This report has been prepared by Mouchel Limited. No individual is personally liable in connection with the preparation of this report. By receiving this report and acting on it, the client or any other person accepts that no individual is personally liable whether in contract, tort, for breach of statutory duty or otherwise. Mouchel has used reasonable skill, care and diligence in the design and interpretation of the ground investigation, however, the inherent variability of ground conditions allows only definition of the actual conditions at the location and depths of exploratory holes and samples/tests therefrom, while at intermediate locations conditions can only be inferred. New information, changed practices or new legislation may necessitate revised interpretation of the report after the date of its submission. Mouchel GEO-EWK-RPT-AJL November 2011 iii

5 Croxley Rail Link Geotechnical Interpretative Memo Job_Name4 Contents 1 Introduction Overview - Scheme Ground Investigations Ground Summary Overview of Route Geology Made Ground Embankment Fill Alluvium Glacial Sands and Gravels Chalk Ground Conditions and Material Properties Topsoil Made Ground Embankment Fill Alluvium Glacial Sands and Gravels Chalk Groundwater Levels Land Contamination Risks Conceptual Model Waste and Sustainability Considerations Summary of Chemical Laboratory Testing Potential Reuse of Soils Waste Assessment Recommendations Tables Table 1: Summary of Geotechnical Parameters for Embankment Fill...13 Table 2: Summary of Geotechnical Parameters for Alluvium associated with the River Gade...18 Table 3: Summary of Geotechnical Parameters for Glacial Sands and Gravels...19 Table 4: Summary of Geotechnical Parameters for Upper Chalk...21 Mouchel GEO-EWK-RPT-AJL November 2011 iv

6 Croxley Rail Link Geotechnical Interpretative Memo Job_Name4 Figures Figure 1 A-Line Plot for Embankment Fill...10 Figure 2 Particle Size Distribution for Embankment Fill...11 Figure 3 Moisture content, Liquid Limit and Plastic Limit against Elevation for Embankment Fill...11 Figure 4 - A Line Plot for Alluvium...15 Figure 5 Moisture content, Liquid Limit and Plastic Limit against Elevation for Alluvium...16 Figure 6 Particle Size Distribution for Alluvium...17 Figure 7 - Particle Size Distribution for Glacial Sands and Gravels...19 Appendices Appendix A Appendix B Appendix C Extracts from Soil Mechanics Ground Investigation Factual Report CLEA Soil Screening for a Commercial End Use Waste Assessment for Contaminated Soils Mouchel GEO-EWK-RPT-AJL November 2011 v

7 Croxley Rail Link Geotechnical Interpretative Memo Job_Name4 1 Introduction Croxley Rail Link is a major project within Hertfordshire County Council s (HCC) Local Transport Plan and has been outlined in the Watford Borough Council s District Plan for a number of years. The scheme is also supported by the Mayor of London and Transport for London (TfL) as it improves the regional connectivity of northwest London. The project seeks to extend the Metropolitan line via the now closed Croxley Green branch line to provide an east-west public transport link for Watford, connecting the communities around Croxley Green with Watford town centre. It also connects Watford Junction with the Underground network providing passengers wishing to travel on the West Coast Main Line with an alternative route. For details of the route and structures required please refer to the Geotechnical Desk Study 1 produced by Mouchel in May Mouchel were commissioned by HCC to carry out initial feasibility design work to assist in the Transport and Works Act (TWA) process associated with the Croxley Rail Link Scheme. As part of this commission Mouchel were required to produce a geotechnical interpretative memo to: 1. Report on the TWA phase site investigation designed and specified by Mouchel and undertaken by Soil Mechanics in April This GI was specifically targeted to allow the required geotechnical design development of the proposed viaduct foundations. 2. Review all other available geotechnical and geo-environmental information within the scheme extents. 3. Present an overall ground model for the area and provide characteristic parameters for feasibility design. 1 Mouchel Limited, /GEO/NNN/RPT/AP/001 rev B, Croxley Rail Link Geotechnical Desk Study, May Mouchel GEO-EWK-RPT-AJL November

8 Croxley Rail Link Geotechnical Interpretative Memo Job_Name4 1.1 Overview - Scheme Ground Investigations As alluded to above a ground investigation was specified as part of the TWA process and to aid in the feasibility design process. The ground investigation comprised the drilling of two boreholes (BHTWA02 and BHTWA03) to a maximum depth of 41m bgl. The field work was undertaken between the 7 and 16 March The two boreholes were located along the route of the proposed viaduct spanning the A412 (Watford Road) and immediately east of the Grand Union Canal / River Gade. The location of the two boreholes are shown on Drawing /GEO/GEN/GEN/FOD/00/20/006. The boreholes were progressed using cable percussion to 20.0 m bgl and then to the termination depths quoted above using rotary core drilling (120 mm diameter core). On completion of the drilling works standpipe piezometers were installed two to 20.0 m bgl (within the Chalk) and one to 6.0 m bgl (within the Superficial Deposits). Geotechnical and chemical testing was carried out on representative samples taken during the drilling works. The Ground Investigation Factual Report (by Soil Mechanics) was issued under separate cover on 13 th September However, the laboratory testing and borehole logs are presented in Appendix A of this report, along with the location drawing /GEO/GEN/GEN/FOD/00/20/006. In addition to the April 2011 TWA specific ground investigation, the site has been subject to previous Ground Investigations. These investigations were carried out during previous scheme designs for the proposed rail link and on London Underground Limited (LUL) earthwork assets. Numerous historical borehole logs are also available from the British Geological Survey (BGS). The previous Ground Investigations are summarised below and detailed in the Geotechnical Desk Study 1. Mouchel GEO-EWK-RPT-AJL November

9 Croxley Rail Link Geotechnical Interpretative Memo Job_Name4 Costain Ground Investigation Report, Phase 1 site specific investigation. STATS Supplementary Contaminated Land Investigation, 2005 and At adjacent waste transfer site. Trans4m Earth Structures Assessment Report Slope Stability Assessment of London Underground Limited (LUL) assets. White Young Green Environment Ground Investigation, investigation for Slope Stability Assessment of LUL assets. It should be noted that the Trans4m document is an interpretative report, and the original borehole logs and testing data were not available. 2 Costain Geotechnical Services Ltd, 936/3256, The Croxley Rail Link Stage One Factual Report, July STATS Ltd, , Supplementary Contaminated Land Investigation Ascot Road, Croxley, Watford, September Trans4m Limited, RPT-EST-M , Watford to Croxley (Baldwins Lane) M042 EM3 and M042 EM6 Earth Structures Assessment Report, January WYG Environment, A053160, M042 EM5 (Croxley to Watford) Ground Investigation Report, February Mouchel GEO-EWK-RPT-AJL November

10 Croxley Rail Link Geotechnical Interpretative Memo Job_Name4 2 Ground Summary 2.1 Overview of Route Geology The published BGS Geological mapping (Sheets 255 and 256) indicate that the bedrock comprises the White Chalk Subgroup (hereafter referred to as Chalk ). Superficial deposits locally overlie the Chalk and comprise cohesive and granular Alluvium and Glacial Sands and Gravels. Drawing No /GEO/GEN/GEN/FOD/00/20003 provides an indicative geological plan for the Scheme. The summary of ground conditions that follows is based on the historic geotechnical data and the results of the April 2011 TWA Phase GI. Made Ground was encountered along the route, associated with the routes former use as an operating railway and its location within a built up urban environment. In the areas of existing embankments and redundant stations engineered fill is encountered. At the western end of the route the existing LUL embankment (comparing ash, clay and chalk Made Ground) is underlain with the Chalk. To the south east (beneath the proposed viaduct) the proposed alignment crosses the flood plain of the River Gade. Here the Chalk is overlain with alluvial deposits, comprising clay and gravels in varying proportions, with occasional areas of organic material and peat. As the alignment joins the existing (disused) Croxley Branch Line the existing embankments (composition unknown, but assumed to comprise ballast and ash with locally sourced glacial sands and gravels from the adjacent cuttings) are underlain directly by the Chalk. As the existing alignment continues to the east, it enters a cutting. No borehole logs are available for this section, but from the BGS mapping it is believed that cuttings are constructed in Glacial Sands and Gravels, underlain at depth by the Chalk. Following the cutting, the alignment returns to embankment (composition unknown, but assumed to comprise ballast and ash with locally sourced glacial sands and gravels from the cuttings). The embankment is underlain with Alluvium associated with the River Colne, overlying the Chalk. Mouchel GEO-EWK-RPT-AJL November

11 Croxley Rail Link Geotechnical Interpretative Memo Job_Name4 The final section of the alignment runs At-Grade underlain by Upper Chalk, with an outcrop of Glacial Sand and Gravel at the northern extent of the route. 2.2 Made Ground Made Ground is found along the length of the route, associated with the railway and general urban development. As a stratum, Made Ground is inherently variable. Over the scheme length it was encountered at depths of between 0.0 m and 3.0 m below ground level (m bgl) with a minimum thickness of 0.25 m. Along the route the level of the base of the Made Ground varied between 67.7 mod and 47.9 mod. Ash, ballast, brick, coal, clinker, concrete, glass and macadam were found in varying proportions within a matrix of clayey silty sandy gravel. There is little spatial pattern to the thickness of the Made Ground encountered along the route. 2.3 Embankment Fill From the investigations carried out to date, Embankment Fill was only encountered in boreholes conducted on the LUL earthworks at the western extent of the route. However, it can be assumed that embankment fill will also be encountered within the disused sections of the route where the alignment is above natural ground level. The data and assessments in respect of Embankment Fill apply only to the LUL Metropolitan line embankments at the western end of the scheme. The Embankment Fill identified at the LUL embankments consisted of interbedded layers of granular, cohesive and chalk material. The granular layers are typically 0.1 m to 2.8 m thick. The chalk and cohesive layers are typically 0.2 m to 5.6 m thick, however chalk embankment fill was found to 9.6 m and cohesive embankment fill was found to 8.8 m in asset EM03. The granular layers generally consist of loose clayey sandy angular to subangular fine to coarse gravel of flint, ballast, coal, chalk and clinker. The cohesive layers generally consist of firm slightly sandy gravelly clay or chalk with angular to subrounded flint chalk and ballast. Mouchel GEO-EWK-RPT-AJL November

12 Croxley Rail Link Geotechnical Interpretative Memo Job_Name4 2.4 Alluvium Alluvium is encountered at two locations along the route; towards the western extent associated with the River Gade and at the centre of the route associated with the River Colne. Alluvium is typically soft or loose and its composition is generally variable both vertically and horizontally due its depositional environment. The Alluvium associated with the River Gade is typically 0.15 m to 3.5 m thick, extending to a maximum depth of 8.4 mbgl. The material is generally described soft to firm slightly sandy slightly gravelly clay, with the gravel consisting of subangular to subrounded fine to coarse flint. The Alluvium associated with the River Colne is typically described as a soft silty clay becoming a clayey gravelly sand with depth. The soft silty clay was encountered between 0.4 m 1.7 m thick, and persisted to depths of 3.7 m bgl. The clayey gravelly sand varied in thickness between 2 m and 6 m. The gravel is of chalk and flint. 2.5 Glacial Sands and Gravels Glacial Sands and Gravels were encountered in boreholes adjacent to much of the route. However, from the BGS mapping of the superficial deposits it is believed that the route is only underlain by the Glacial Sands and Gravels at Watford Junction Station and in the disused cutting section. Similar to the Alluvium, the material is generally variable both vertically and horizontally which may be due its depositional environment. Typically thicknesses along the route vary between 0.45 m and 7.0 m, at depths of up to 8.0 m bgl. This material is generally described as medium to very dense sandy gravel of fine to coarse angular flint with occasional chalk. 2.6 Chalk Chalk underlies the entire route, and was encountered at depths ranging from ground level to m bgl, or between 67.0 m AOD to m AOD. No boreholes carried out to date have proved the thickness of the chalk. Mouchel GEO-EWK-RPT-AJL November

13 Croxley Rail Link Geotechnical Interpretative Memo Job_Name4 Generally the nature of the chalk varies with depth, typically as the degree of weathering lessens. At ground surface, or immediately below the superficial deposits, the chalk is generally described as extremely weak to weak low density cream yellow, brownish cream and white Chalk with frequent to occasional subangular fine to course flint gravel and low to medium cobble content. Where the Chalk has been completely weathered the material is described as a soil i.e. very soft to hard. The density of the chalk ranges from low to medium. Only two boreholes (Soil Mechanics Viaduct specific TWA investigation 2011), used rotary drilling with core recovery techniques. Therefore, at shallow depths where cable percussive drilling techniques were used the Chalk was recovered as a silty sandy sub angular to subrounded gravel, silty gravel and slightly sandy slightly gravelly silt. Below approximately 10.0 m bgl, the Chalk becomes off white to white in colour but with frequent orange brown staining in places. Below 20.0 m bgl where rotary follow on was used, fractures are identified as closely spaced and infilled with comminuted chalk (Grade B3). Fractures range between planar smooth to irregular and undulose. Rare to frequent manganese speckling is noted on fracture surfaces. Medium spaced bands of subangular course flint gravel and cobbles are present. Below approximately 17.0 m OD the chalk changes from weak low density off white chalk to medium strong medium density off white, in places yellow, and occasionally grey stained Chalk. Fractures are very closely to closely spaced with white comminuted chalk infill, but with no obvious infill in other places. Given the different phases of drilling at the site the chalk has been classified using both the original Mundford grading system and the revised approach using the CIRIA chalk grading scheme (CIRIA C574 6 ). Under the Mundford grading system the chalk across the site is classified between Grade V to Grade I, i.e. from Grade I, extremely soft, structureless chalk containing small lumps of intact chalk to Grade V, a blocky hard and brittle chalk with joints more than 200mm apart and closed. Generally the weathered grade decreases with depth. Under the new CIRIA grading system within the boreholes drilled at the location of the proposed viaduct, the chalk is classified as a Grade B3. Within the boreholes drilled to depth there is no obvious evidence of dissolution features. 6 CIRIA C654 Engineering in Chalk Mouchel GEO-EWK-RPT-AJL November

14 Croxley Rail Link Geotechnical Interpretative Memo Job_Name4 3 Ground Conditions and Material Properties Geotechnical characteristic parameters used for the feasibility design works have been derived from: The results of the available geotechnical laboratory testing. Atterberg Limit results and their published correlations of Plasticity Index and effective angle of shearing resistance (BS ) Standard Penetration Test N values and published correlations 8 for undrained shear strength (cu), Co-efficient of volume compressibility (mv) and drained and undrained angle of shearing resistance (for non-cohesive soils) Where no field or laboratory data was available geotechnical parameters were estimated from published reference data Topsoil Topsoil was found in various locations along the route, associated with the cutting and embankment faces. No topsoil was encountered adjacent to Stadium Approach Underbridge or at Watford Junction Stadium. 3.2 Made Ground Minimal test data was available for the Made Ground along the scheme. Two Atterberg Limit tests were carried out; one within material from the LUL Embankment and one near the A411 crossing. The Liquid Limit varied between 25 and 49%, the Plastic Limit varied between 16 and 22%, and the Plasticity Index between 9 and 27%, indicating a clay of intermediate plasticity, while the 7 British Standards Institute, BS8002:1994 Code of Practice for Earth Retaining Structures, CIRIA, R143, SPT Methods and Use, CIRIA R574, Engineering in Chalk, 2002 Mouchel GEO-EWK-RPT-AJL November

15 Croxley Rail Link Geotechnical Interpretative Memo Job_Name4 Made Ground at the A411 crossing is clay of low plasticity. The moisture content of each sample was at or below the Plastic Limit, suggesting that the materials are brittle and non plastic. One Particle Size Distribution (PSD) test was carried out in material found at the base of the LUL embankments, at the same depth as the Atterberg limit test was carried out. This indicates the material to be slightly sandy gravelly clay / silt. Within the Made Ground of the LUL embankment Nine ph and SO 4 tests were carried out in order to provide classifications in accordance with BRE SD1 10. ph varied between 7.47 and 8.43 along the route, and SO 4 varied between 50 and 104 mg/kg. The design sulphate class is therefore DS1 and the aggressive chemical environment for concrete class is AC-1 in these locations. Testing should be carried out along the remainder of the route to determine the concrete class at each structure location. The Made Ground will not provide a suitable bearing stratum for foundations, and as such will act solely as a surcharge. A unit weight of 19 kn/m 3 is proposed. 3.3 Embankment Fill Classification testing was carried out in granular, cohesive and chalk Embankment Fill, as shown in Table 1. No strength or chemical testing is currently available Granular Embankment Fill The A-Line Plot shows that the granular material is consistent with an intermediate plasticity (see Figure 1). A moderately conservative characteristic Plasticity Index was determined. Given that the granular material has a plastic limit it can be assumed that there is a significant clay / silt content, and that the mass may behave as a cohesive soil. 10 Building Research Establishment, Special Digest 1, Concrete in Aggressive Ground, 2005 Mouchel GEO-EWK-RPT-AJL November

16 Croxley Rail Link Geotechnical Interpretative Memo Job_Name4 PSD testing (Figure 2) shows that the materials are generally highly variable. One test was carried out in granular fill, which showed the material to contain 33% cohesive materials and be generally moderately well graded. Published correlations 7 between density, grading, angularity and SPT N value have been used to determine a characteristic internal angle of friction and unit weight. Figure 1 A-Line Plot for Embankment Fill CE CV 40 CH Plasticity Index (%) 30 CI ME 20 MV CL MH Liquid Limit (%) Embankment Fill Granular Embankment Fill Chalk Embankment Fill Cohesive Mouchel GEO-EWK-RPT-AJL November

17 Croxley Rail Link Geotechnical Interpretative Memo Job_Name Cohesive Embankment Fill The A Line Plot (Figure 1) shows that the cohesive material is of intermediate plasticity. Figure 3 shows that the moisture content of the cohesive materials are at or below the plastic limit, suggesting they will exhibit brittle behaviour. Figure 2 Particle Size Distribution for Embankment Fill Particle Size Distribution Curves Embankment Fill Granular (Red), Cohesive (Blue) and Chalk (Green) Percentage Passing (%) Clay Silt Sand Gravel Cobbles Particle Size (mm) M042H05WA 0 M042H07W 0.3 M042H01WA 0.3 M042H02W 0.5 M042H04W 1 M042H01WA 1.0 M042H01WA 2.2 M042H04W 2.5 M042H01WA 4.6 M042H04W 4.7 Figure 3 Moisture content, Liquid Limit and Plastic Limit against Elevation for Embankment Fill 65 Moisture Content, Plastic Limit and Liquid Limit (%) Elevation (m OD) WYG M042H05WA WYG M042H02W WYG M042H04W WYG M042H07W WYG M042H01WA Mouchel GEO-EWK-RPT-AJL November 2011

18 Croxley Rail Link Geotechnical Interpretative Memo Job_Name4 A characteristic value of plasticity index has been determined (see Table 1). Due to the low number of tests statistical methods could not be used so a moderately conservative values have been adopted. The SPT N value of the cohesive embankment fill ranged between 7 and 17, indicating a soft to stiff material. Stroud proposed a relationship between undrained shear strength and SPT N value; Undrained Shear strength (c u ) = SPT N * f 1, where f 1 can be taken as 4.5. This gives an undrained shear strength of the cohesive material of between 32 and 77 kpa. A moderately conservative value of 45 kpa is considered appropriate. A characteristic unit weight has been determined using published values 7 for firm clay. Five consolidated undrained triaxial tests were undertaken in the cohesive embankment fill. The average internal angle of friction of 33.5 is very high for cohesive material, however this is likely to be due to the large amount of granular material present, and the fact that the material is generally derived from chalk. A characteristic cohesion of 1 kpa is considered conservative due to the small amount of data and the non-linear behaviour of low strength materials. Published correlations 7 suggest an internal angle of friction of 21, and this will be taken as a moderately conservative characteristic value Chalk Embankment Fill The A Line Plot (Figure 1) shows that the chalk fill is very variable, with a plasticity ranging from low to high. Figure 3 shows that the water contents of the chalk materials are very close to the plastic limit, suggesting they will exhibit brittle behaviour. A characteristic value of plasticity index has been determined (Table 1). Due to the low number of tests statistical methods could not be used and moderately conservative values have been adopted. For the chalk fill the proportion of cohesive material varies between 11 and 33%, however this may be an overestimate due to the tendency of chalk particles to break down to a smaller size during sampling and testing. This Mouchel GEO-EWK-RPT-AJL November

19 Croxley Rail Link Geotechnical Interpretative Memo Job_Name4 suggests that the material will act as a moderately well graded granular material. The SPT N value of the chalk embankment fill ranged between 21 to 36 above 5m bgl, and between 10 and 15 below 5m bgl. The chalk embankment material is therefore medium dense. One consolidated undrained triaxial test was also undertaken in the chalk embankment fill, resulting in a cohesion of c = 2.6 kpa and an angle of shear resistance of Due to the non-linear behaviour of low strength materials a characteristic cohesion of zero is considered appropriate. This agrees with published recommendations 9. Published correlations 7 between density, grading, angularity and SPT N value have been used to determine a characteristic unit weight and internal angle of friction Geotechnical Parameters Table 1: Summary of Geotechnical Parameters for Embankment Fill Properties Constituents Min Max Mean No. of Tests Characteristic Value / Classification Natural Moisture Content (%) Granular Cohesive Chalk Plastic Limit (%) Granular Cohesive Chalk Liquid Limit (%) Granular Cohesive Chalk Plasticity Index (%) Granular Cohesive Chalk Plasticity Classification Granular CI CI Cohesive CL CI - 10 CI Chalk CL CH CI 5 CI Mouchel GEO-EWK-RPT-AJL November

20 Croxley Rail Link Geotechnical Interpretative Memo Job_Name4 Properties Constituents Min Max Mean No. of Tests Characteristic Value / Classification Liquidity Index Granular Cohesive Chalk Unit Weight, γ (kn/m 3 ) Granular Cohesive Chalk 19* 19* 19* SPT N Values Cohesive Chalk Undrained Shear Strength c u (kpa) Cohesive Internal Angle of Friction, φ ( ) Granular Cohesive * 21 Chalk Cohesion, c (kpa) Granular 0 Cohesive Chalk *No lab results available, derived value only It should be noted that as the various types of embankment fill are interbedded, the cohesive embankment fill will tend to dominate the behaviour of the embankment as a whole. It is therefore recommended that the parameter for the cohesive embankment fill are adopted for the earthwork mass for the purposes of slope stability calculations. 3.4 Alluvium No testing is available for the Alluvium associated with the River Colne. The difference in composition noted in section 2.4 shows that the materials were laid at differing depositional rates, and therefore these materials are likely to have differing properties. As such the properties below should only be applied to Alluvium associated with the River Gade. The A Line Plot (Figure 4) shows that while the Alluvium varies in plasticity, the material is generally high plasticity clay. In the majority of cases the measured Mouchel GEO-EWK-RPT-AJL November

21 Croxley Rail Link Geotechnical Interpretative Memo Job_Name4 moisture content was lower than the plastic limit, suggesting that the material may be brittle (Figure 5). A characteristic value of plasticity index has been determined for the Alluvium (see Table 2). Due to the low number of tests statistical methods could not be used and a moderately conservative value of 39% is considered appropriate. The Particle Size Distribution (Figure 6) shows a large variation in the Alluvium, with percentage cohesive material varying from 56 to 82%, with an average of 71% cohesive material. The individual clay and silt fractions are unknown. The gravel content of the material varies between 7 and 32%, and the sand fraction varies between 9 and 14%, suggesting the material is slightly sandy slightly gravelly silt / clay. Figure 4 - A Line Plot for Alluvium CE 50 CV CH Plasticity Index (%) CI ME MV 20 CL MH Liquid Limit (%) Mouchel GEO-EWK-RPT-AJL November

22 Croxley Rail Link Geotechnical Interpretative Memo Job_Name4 Testing, carried out during the 2008 STATS Ltd ground investigation, for aggressive ground to BRE SD1 10 was carried out on two samples. ph varied between 7.83 and 7.91, while SO 4 was consistent at 50 mg/kg. A design class of DS1 and an aggressive chemical environment for concrete class of AC-1 is therefore appropriate at this location. Testing should be carried out along the remainder of the route to determine the concrete class at each structure location. Figure 5 Moisture content, Liquid Limit and Plastic Limit against Elevation for Alluvium 61 Moisture Content, Plastic Limit and Liquid Limit (%) Elevation (m OD) WYG M042H03W WYG M042H05WA WYG M042H06W WYG M042H07W WYG M042H08W Mouchel GEO-EWK-RPT-AJL November

23 Croxley Rail Link Geotechnical Interpretative Memo Job_Name4 Figure 6 Particle Size Distribution for Alluvium Percentage Passing (%) Clay Silt Sand Gravel Cobbles Particle Size (mm) M042H03W 0.3 M042H06W 0.6 M042H06W 1.0 M042H07W 3.8 M042H04W 7.5 No SPT testing or laboratory strength testing was carried out on the Alluvium. Borehole logs describe the Alluvium as generally soft or soft to firm. It is therefore suggested that the undrained shear strength is likely to vary between 20 kpa and 75 kpa. A conservative characteristic value is therefore 30 kpa. A characteristic unit weight was determined using published recommendations 7 for soft clay. Based on the characteristic PI value a characteristic value of internal friction angle has been derived 7. Due to the non-linear behaviour of low strength materials a characteristic value of 1 kpa was determined for cohesion. Mouchel GEO-EWK-RPT-AJL November 2011

24 Croxley Rail Link Geotechnical Interpretative Memo Job_Name4 Table 2: Summary of Geotechnical Parameters for Alluvium associated with the River Gade Properties Min Max Mean No. of Tests Characteristic Value / Classification Natural Moisture Content (%) Plastic Limit (%) Liquid Limit (%) Plasticity Index (%) Plasticity Classification CI CV CH 9 CH Liquidity Index Unit Weight, γ (kn/m 3 ) 19* Undrained Shear Strength 30* c u (kpa) Internal Angle of Friction, φ ( ) 25* Cohesion, c (kpa) 1* * No lab results available, derived value only 3.5 Glacial Sands and Gravels Particle Size Distribution testing (Figure 7) was carried out in Glacial Sands and Gravels in the location of the proposed viaduct and show the material to contain predominantly gravel. The gravel content varies between 82% and 95%. The remaining material is sand, although two tests contained 1% of cohesive material. The uniformity coefficient was calculated for each test and showed the material to be well graded in all but one test. Borehole log descriptions show the Glacial Sands and Gravels to be generally medium to very dense. SPT N values taken along the route show the material to vary from very loose to very dense, with an average density of medium dense. Spatially, the material varies from an average SPT value of 23 (medium dense) in the area of the viaduct to an average of 44 (Dense) along the remainder of the route. A characteristic N value has therefore been defined for both the area adjacent to the viaduct, and the remainder of the route (Table 3). Mouchel GEO-EWK-RPT-AJL November

25 Croxley Rail Link Geotechnical Interpretative Memo Job_Name4 Figure 7 - Particle Size Distribution for Glacial Sands and Gravels Gravel 80 Percentage Passing (%) Sand 10 Clay Silt Cobbles Particle Size (mm) BHTWA BHTWA BHTWA BHTWA One ph and SO 4 test to BRE SD1 10 was carried out in the Viaduct area. The ph was 9.3 and the Sulphate was 1040mh/kg, resulting in a design class of DS1 and an aggressive chemical environment for concrete class of AC-1. A characteristic unit weight was derived based on the material being dense gravel. A characteristic internal angle of friction was also derived 7, based on a well graded angular material. Table 3: Summary of Geotechnical Parameters for Glacial Sands and Gravels Properties Route Section Min Max Mean No. of Tests Characteristic Value / Classification SPT N Value Viaduct Remainder Uniformity coefficient, C U >6 Unit Weight, γ (kn/m 3 ) 19* Internal Angle of Friction, φ ( ) Viaduct Remainder 35* 35* * No lab results available, derived value only Mouchel GEO-EWK-RPT-AJL November 2011

26 Croxley Rail Link Geotechnical Interpretative Memo Job_Name4 3.6 Chalk A summary of the geotechnical parameters derived from laboratory testing is presented in Table 4. Whilst chalk was recovered as putty/slurry this was in part a function of the method of cable percussive drilling and the addition of water during drilling, this changed rapidly to a very weak to weak low density Chalk recovered as a gravel, before becoming intact Chalk with depth. Therefore it is considered that the Chalk will behave as a weak rock rather than a soil throughout. SPT N values generally increase with depth, supporting the exploratory hole descriptions that dissolution features/infilled dissolution features do not appear to be present at depth. A graph of SPT N value plotted against elevation (in mod) is presented in Figure 8. Figure 8 SPT N Values plotted against depth for Chalk 70 SPT N value (blows/300mm) Elevation (m OD) Soil Mechanics BHTWA02 Soil Mechanics BHTWA03 Costain BH2 Costain BH3 Costain BH4 Costain BH5 Costain BH7 BGS 10 TQ19NW25B BGS 14 TQ09NW180 BGS 15 TQ09NW181 BGS 17 TQ09NW182 BGS 18 TQ09NW257 BGS 19 TQ09NW259 BGS 7 TQ19NW27B BGS 9 TQ19NW26A design minimum SPT design mean SPT Mouchel GEO-EWK-RPT-AJL November

27 Croxley Rail Link Geotechnical Interpretative Memo Job_Name4 Table 4: Summary of Geotechnical Parameters for Upper Chalk Properties Min Max Mean No. of Tests Characteristic Value / Classification Natural Moisture Content (%) Plastic Limit (%) Liquid Limit (%) Plasticity Index (%) Plasticity Classification (CL, CI, CH, ML, MI, MH etc) CL CL Liquidity Index SPT N Value (blows per 300mm) Saturated Moisture Content (%) See Figure Porosity (%) Bulk Density (Mg/m3) Dry Density (Mg/m3) Unconfirmed Compressive Strength (MPa) Internal Angle of Friction, φ ( ) Groundwater Levels General Groundwater Regime The Chalk is a Principal aquifer (formerly classified as a Major Aquifer) which may be highly productive and able to support large abstractions for public water supply and other purposes. The superficial deposits (Glacial Sands and Gravels) are classified as a Secondary A Aquifer, i.e. a permeable layer capable of supporting water supplies at a local rather than a strategic scale. Mouchel GEO-EWK-RPT-AJL November

28 Croxley Rail Link Geotechnical Interpretative Memo Job_Name4 The Grand Union Canal, River Gade and River Colne all pass under the proposed alignment and it is likely the rivers (and potentially the canal if hydraulically connected) are receiving water from the Chalk and gravel aquifers. The site is within an Inner Source Protection Zone (SPZ1) defined by a travel time of 50 days or less for a pollutant to reach an abstraction point. Based on information supplied by the Environment Agency, there are two groundwater abstraction locations less than 10 m from the scheme Water Bearing Strata Groundwater was struck within the upper River Gravel, and again within the underlying chalk. The maximum depth of strike recorded within the chalk was at 24m bgl (43mAOD). Within the TWA boreholes, three standpipe piezometers were installed, the ground water levels in which have been recorded during three monitoring visits. The installation details and groundwater levels are given in Table 5 It should be noted that both 50 mm diameter standpipes exhibited significant levels of silting, reducing the effective response zone by up to m. At attempt to purge the material was made on 25 th August However, permanent obstructions were encountered at 7.7 m bgl in BHTWA 02 and 16.7 m bgl in BHTWA 03. It was noted during these works that the water extracted from the 50 mm diameter standpipe in BHTWA 02 contained no chalk fines, and was consistent with water extracted from the 19 mm diameter standpipe with a response zone in the superficial deposits. Monitoring Date 07/04/ /04/ /08/2011 Table 5: TWA Borehole Ground Water Monitoring Exploratory GWL GWL Standpipe Strata Hole No. (m BGL) (m OD) Diameter (mm) TWA 02 Depth to Base of Installation (m bgl) Original Installation Depth to Base (m bgl) Chalk Superficial Deposits TWA Chalk TWA Chalk Superficial Deposits TWA Chalk TWA Chalk Superficial Deposits TWA Chalk Mouchel GEO-EWK-RPT-AJL November

29 Croxley Rail Link Geotechnical Interpretative Memo Job_Name4 Table 5 shows that ground water is at approximately 53.4 m AOD in both the chalk and overlying superficial deposits, suggesting a hydraulic continuity between the two strata Groundwater Impact The existence of the SPZ1 will require construction work to be carried out in such a way as to prevent contamination of the groundwater. The relatively shallow depth of groundwater may impact excavations, leading to a need to dewater. Mouchel GEO-EWK-RPT-AJL November

30 Croxley Rail Link Geotechnical Interpretative Memo Job_Name4 4 Land Contamination Risks 4.1 Conceptual Model CLR11 Model Procedures for the Management of Land Contamination, , states that for contamination to potentially cause human health issues a source, pathway and receptor are required. A source of soil contamination exists within the made ground present at the site i.e. associated with the former railway line and embankment fill. A pathway exists via direct dermal contact and ingestion, inhalation and ingestion of airborne dust. However, given the proposed end use, long term exposure to human receptors is absent. As such long term chronic risk to human health in relation to the in-situ soils at the site is considered minimal. Whilst construction workers and maintenance staff may be exposed to contaminated soils at the site, the exposure frequencies and durations are low. The risks associated with short term exposure of these end users to potentially contaminated soils are managed with appropriate health, safety and welfare measures. To mitigate the risk to site workers the HSE publication HSG 66 (Risk to Construction Workers) 12 should be followed, and suitable Personal Protective Equipment (PPE) (including gloves) should be provided to minimise contact with any potentially contaminated material. Smoking, eating or drinking on site shall be prohibited outside clean restricted mess areas where appropriate welfare facilities must be provided. 11 Environment Agency, CLR11, Model Procedures for the Management of Contaminated Land, Health and Safety Executive, HSG66, Protection of Workers and the General Public during the Development of Contaminated Land, 1991 Mouchel GEO-EWK-RPT-AJL November 2011

31 Croxley Rail Link Geotechnical Interpretative Memo Job_Name4 4.2 Waste and Sustainability Considerations To allow the construction of the proposed new rail alignment, two lengths of existing embankment (near Ascot Road Bridge) will be excavated and replaced with new embankments. This and piling operations associated with the viaduct will generate soil arisings. Consideration will be given, wherever possible, to the reuse of the soils within the scheme design. However, given the composition of the existing earthworks reuse may not be feasible. If material is suitable for reuse it will be incorporated into the embankment widening of the existing disused lined at either or both of the proposed new stations (one including a car park facility) as part of general levelling works. 4.3 Summary of Chemical Laboratory Testing Chemical laboratory testing of two soil samples recovered from exploratory holes BHTWA02 and BHTWA03 as part of the Soil Mechanics ground investigation of 2011 Error! Bookmark not defined.. Representative samples were retrieved, which were labelled and appropriately stored within cool boxes with ice packs on site before transportation to Soil Mechanics in house ESG Scientifics Laboratory. Soil Mechanics and its in house laboratory is an approved vendor under the Mouchel QMS and is a UKAS accredited laboratory with MCERTS testing capability. The testing was scheduled by Mouchel, and included metals, Speciated polycyclic aromatic hydrocarbons (PAH by GC/MS), total petroleum hydrocarbons (TPH by GC/FID) and Gasoline Range Organics (BTEX and Aliphatic Carbon Ranges). The full list of determinants and chemical analysis results is presented in the CLEA Soil Screening for a Commercial End Use in Appendix B. Mouchel GEO-EWK-RPT-AJL November

32 Croxley Rail Link Geotechnical Interpretative Memo Job_Name4 4.4 Potential Reuse of Soils To assess the suitability for the potential for reuse of soils in terms of potential risks to human health, a generic screening of the results of the chemical laboratory testing has been completed for a commercial/industrial end use. Given the limited number of sampling points, a statistical based risk assessment is not appropriate. The chemical laboratory test results have been screened using 1% SOM values as a conservative approach. The tabled results of the screening are presented in Appendix B. With the exception of Benzo (a) pyrene, (BaP) within one sample, all other determinants were below the selected screening value. A concentration of mg/kg for BaP was identified in sample ES2 from exploratory hole BHTWA02 at 1.0m bgl, which is approximately two times the concentration of the screening value (14mg/kg). Despite this elevated concentration of BaP within one soil sample and based on the assumption that any reuse on site will be within embankments (and as such exposure frequencies and duration will be low) or beneath hardstanding beneath the stations/car parking facility, this initial screen suggests risks to human health will be acceptable. 4.5 Waste Assessment Under the European Waste Catalogue, the made ground at the site is classed as soils and stones containing dangerous substances. The results of the chemical laboratory testing were assessed to identify any potential hazardous properties as identified in the Hazardous Waste Directive Annex III, as presented in Appendix C. The soils tested are classified as non hazardous with concentrations of pyrene, and benzene, toluene, ethylbenzene and xylene (BTEX) highlighted as a potential H3 hazard with risk phrase R10, 11, 12, 15 and 17. This indicates the material is potentially flammable. Whilst the Waste Directive recommends the material may require testing to assess flammability using EC Test Method A10 for flammability (solids) for Hazard H3A (iii), with the concentrations encountered within the soils on site, the risk of flammability of these soils is considered to be low. Mouchel GEO-EWK-RPT-AJL November

33 Croxley Rail Link Geotechnical Interpretative Memo Job_Name4 4.6 Recommendations It is recognised that a limited number of samples were tested as part of this ground investigation and a detailed suite of chemical laboratory testing will be required as part of the detailed design for the proposed scheme to confirm the reuse of the soils at the site where potential for arisings are identified. This should be completed concurrently with geotechnical testing to assess compaction potential and hence suitability for reuse. Mouchel GEO-EWK-RPT-AJL November

34 Croxley Rail Link Geotechnical Interpretative Memo Job_Name4 Appendix A Extracts from Soil Mechanics Ground Investigation Factual Report Mouchel GEO-EWK-RPT-AJL November 2011

35

36 Borehole Log Drilled Logged Checked RJS Depth JJ MKR/KW Start 07/03/2011 End 16/03/2011 Samples and Tests Type & No B 1 B 2 ES 1 ES 2 Equipment, Methods and Remarks Hand dug inspection pit to 1.20m then cable tool boring to 20.00m then rotary coring to 40.50m. * Records Strata Depth from to Diameter 0.00m 1.20m 500mm 1.20m 19.00m 150mm 19.00m 40.00m 120mm Firm dark brown slightly sandy slightly gravelly CLAY. Sand is fine to coarse. Gravel is subangular to subrounded fine to coarse of flint. (TOPSOIL) Dark brownish grey slightly clayey sandy GRAVEL. Sand is fine to coarse. Gravel is angular to subangular fine to coarse of flint, concrete, macadam and brick. (MADE GROUND) Casing Depth 19.60m Ground Level Coordinates National Grid Chainage Date Time Description Depth,Level Casing Water (Thickness) 0.20 (1.55) Legend mod E N Backfill/ Instruments B 3 Dark greyish brown clayey sandy GRAVEL. Sand is fine to coarse. Gravel is subangular to subrounded fine to coarse of flint and brick. (MADE GROUND) 1.75 (0.85) D 4 SPT C B 6 W 7 D 8 SPT C B 10 * N=39 (8,11/8,10,10,11) N=50 (5,10/9,9,15,17) dry 3.10 Very soft orangish brown slightly sandy gravelly CLAY. Sand is fine to coarse. Gravel is subangular to subrounded fine to coarse of flint. (ALLUVIUM) Dense orangish brown slightly silty slightly sandy GRAVEL. Sand is medium to coarse. Gravel is subangular to subrounded mainly medium to coarse of flint. (RIVER GRAVELS) (2.00) D SPT C B 13 N=10 (3,4/4,3,1,2) Loose to medium dense orangish brown clayey sandy GRAVEL. Sand is fine to coarse. Gravel is subangular to rounded fine to coarse of flint. (RIVER GRAVELS) 4.75 (1.50) D SPT C B 16 D 17 N=22 (2,3/5,4,7,6) Medium dense orangish brown slightly sandy GRAVEL. Sand is medium to coarse. Gravel is subangular to rounded fine to coarse of flint. (RIVER GRAVELS) (0.95) SP D 18 SPT C B 20 N=23 (7,10/9,6,4,4) Extremely weak low density creamish yellow CHALK recovered as silty sandy GRAVEL. Gravel is subrounded. Frequent subangular fine to coarse flint gravel. (WHITE CHALK SUBGROUP) (1.30) 8.25 D SPT S D 22 D 23 B 24 N=4 (1,1/1,1,1,1) Extremely weak low density brownish cream CHALK recovered as slightly sandy slightly gravelly SILT. Gravel is subrounded. Occasional subangular fine flint gravel. (WHITE CHALK SUBGROUP) D 25 Weak low density white CHALK recovered as silty GRAVEL. Gravel is subrounded. Frequent subangular coarse flint gravel. (2.00) Depth Type & No Records Date Casing Time Water Stratum continues to m Groundwater Entries No. Struck Post strike behaviour Depth sealed (m) (m) Rose to 2.80 m after 20 minutes. Rose to 2.50 m after 20 minutes Depth Related Remarks * From to (m) Concrete obstructions Water added. Chiselling Depths (m) Time Tools used Notes: For explanation of symbols and abbreviations see key sheet. All depths and reduced levels in metres. Stratum thickness given in brackets in depth column. (c) ESGL Scale 1: /07/ :04:21 Project Project No. Carried out for Croxley Rail Link TWA Phase Ground Investigation G Mouchel Borehole BHTWA02 Sheet 1 of 5