Undrained shear strength

Transcription

1 1.0 SUMMARY This Geotechnical Report relates to a Site Investigation undertaken by Messrs Soil Mechanics Limited on behalf of London Underground Limited involving Package 4 of the proposed Jubilee Line Extension in the Bermondsey area of London. A total of twenty (20 no.) boreholes (14 Cable Percussive, 2 Cable Percussive followed by rotary drilling, 2 open hole drilled then rotary drilled, 2 Cable Percussive and Self-Boring Pressuremeter tests) were completed across the site in order to determine the nature and consistency of the soils beneath the site. This programme of works commenced on the 24 th September, 1990 and ended on the 14 th November, Drilling depths range from mod (27.60 mbgl) in BH 413P to mod (53.05 mbgl) in BH404T, with the majority of excavations being completed to a depth of to mbgl. The soils encountered include Made Ground, Alluvium and Terrace Gravels (Superficial deposits) overlying solid geology of the London Clay Formation, Lambeth Group, Thanet Sand Formation and Upper Chalk Formation. Piezometers were subsequently installed following excavations, and monitoring completed. Following this drilling programme laboratory tests were undertaken on selected samples from a range of stratigraphic units. Properties of these soils are as follows: Bulk unit weight Undrained shear strength Effective Cohesion Angle of shearing resistance Coefficient of permeability γ c u c' φ' k (kn/m³) kn/m² kn/m² ( ) (m/s) MADE GROUND 15 to to to 35 1x10-2 to 1x10-6 ALLUVIUM 16 to to to 29 1x10-4 to 1x10-6 TERRACE GRAVELS 19 to 20 N/A x10-3 to 5x10-6 LONDON CLAY 18 to 20.5 SPT'N' x to to 28 1x10-7 to 1x10-9 BLACKHEATH BEDS 19 to to to 25 1x10-5 to 1x10-7 UPPER SHELLY CLAY 17 to to to to 32 1x10-6 to 1x10-8 UPPER MOTTLED CLAY 19 to to to to 32 1x10-6 to 1x10-8 LAMINATED BEDS 20 to 22 LOWER SHELLY CLAY 20 to 22 LOWER MOTTLED CLAY 19 to to 150 (silt) 100 to 300 (clay) 0 to (Clays) 28 to 42 (Sands) 1x10-4 to 1x to 200 (silt) 200 to 310 (clay) 0 to to 32 1x10-6 to 1x to 100 (silt) 100 to 300 (clay) 0 to to 32 1x10-6 to 1x10-8 PEBBLE BED 19 to 21.5 N/A 0 37 to 44 1x10-3 to 1x10-6 GLAUCONITIC SAND 20 to to 120 (fine soils) 0 to to 44 (granular soils) 1x10-3 to 1x10-6 THANET BED 20 to 22 N/A x10-2 to 1x10-5 BULLHEAD BED 20 to 22 N/A x10-2 to 1x10-5 UPPER CHALK 20 to 22 N/A x10-1 to 1x10-2 Two phreatic surfaces at approximately mod and mod are identified from piezometer data, indicating pore pressures within the soils at or about hydrostatic pressures. 1

2 CONTENTS: 1.0 SUMMARY INTRODUCTION SITE DETAILS TOPOGRAPHY GEOLOGY Made Ground Alluvium (includes Peat) River Terrace Gravels London Clay Blackheath Beds Reading Formation Lambeth Group Upper Mottled Clay Lower Mottled Clay Woolwich Formation Lambeth Group Upper Shelly Clay Laminated Beds Lower Shelly Clay Upnor Formation Lambeth Group Pebble Bed Glauconitic Sand Thanet Sand Formation Thanet Beds Bullhead Bed Upper Chalk Formation GROUND INVESTIGATION EXCAVATIONS IN-SITU TESTING INSTALLATIONS SAMPLING LABORATORY TESTING GROUNDWATER MONITORING GROUND CONDITIONS STRATIGRAPHY GROUNDWATER CONDITIONS GEOTECHNICAL CHARACTERISATION Made Ground

3 5.3.2 Alluvium Terrace Gravel London Clay Blackheath Beds Upper Shelly Clay Upper Mottled Clay Laminated Beds Lower Shelly Clay Lower Mottled Clay Pebble Bed Glauconitic Sand Thanet Bed Bullhead Bed Upper Chalk CONCLUSIONS REFERENCES FIGURES: 1. SITE LOCATION PLAN 2. BOREHOLE LOCATION PLAN 3. GEOLOGICAL MAP 4. CROSS SECTION BH401-BH CROSS SECTION BH409-BH PORE PRESSURE VS REDUCED LEVEL 7. SPT VS REDUCED LEVEL 8. MOISTURE CONTENT VS REDUCED LEVEL 9. PLASTICITY CHART 10. UNDRAINED SHEAR STRENGTH VS REDUCED LEVEL 11. SULPHATE & ph DATA APPENDICES: 1. BOREHOLE LOGS 2. LABORATORY TEST RESULTS 3. PIEZOMETER READINGS 3

4 2.0 INTRODUCTION A Site Investigation was undertaken by Messrs Soil Mechanics Limited in carried out in accordance with the 1987 specifications for Ground Investigation published by the Department of Transport (Attewell, 1995, p. 39). These works were completed on behalf of London Underground Limited and involved Package 4 of the proposed Jubilee Line Extension, comprising of 20 boreholes (18 Cable Percussive, 2 Rotary) sited in the Bermondsey area of London. Drilling works were completed on the 16 th November, The purpose of the investigation was to determine the ground conditions prior to the proposed tunnelling in the Bermondsey area. The following factual and interpretative report describes the works undertaken and presents copies of the test data obtained. 3.0 SITE DETAILS 3.1 TOPOGRAPHY The site is represented by the obtained borehole data of Package 4, and follows the proposed tunnelling route of the extension works southeast of London Bridge Station in Bermondsey, eastwards towards Canada Water in Rotherhithe. In view of the proposed tunnelling programme, the site extents (in Easting s and Northing s) are considered to be linear, running from , (BH401) in the east, to , (BH417) in the west (Figs. 1 & 2). This generally follows the River Thames at a distance of approximately 400m from the south bank across the linear run. In plan view, the tunnel is understood to follow the overland train line southeast of Tower Bridge Station towards South Bermondsey Station for approximately 600m before curving eastwards and east-north-eastwards beneath Jamaica Road to the newly proposed Bermondsey Tube Station. Following on from Bermondsey Tube Station the proposed tunnel route continues eastwards beneath Southwark Park. The extent of this site area terminates at the corner of Neptune Street and Moodkee Street on the Canada Estate. Across the site the street levels are relatively flat, ranging from 1.9mOD in the western extent, to 3.6mOD in the east (EDINA, 2010). 3.2 GEOLOGY The site is considered to be located on a number of solid and drift deposits at or near ground level (Fig. 3) identified in BGS solid and drift 1:50,000 maps (256 - North London; South London). For simplicity, the geological units which are likely to be encountered at depth across the site, from superficial to solid geological units are described below in order of youngest to oldest. 4

5 3.2.1 Made Ground Often very variable in thickness, these deposits are representative of the urbanisation and industrial development (Burland et al. 2001, p62). This stratum can be considered as highly variable in nature, consisting of a mix of concrete, rubble, brick and refuse intermingled with gravel and sands frequently within a matrix of silt or clay Alluvium (includes Peat) This stratum is localised around the River Thames, generally resting unconformably upon River Terrace Gravels (Ellison, 2004). The Alluvium consists predominantly of silty clay and clayey silt, with locally developed beds of fine-to coarse-grained sand. Interbedded peat is known to occur eastwards of Southwark and Westminster, with the most extensive deposits extending west to the Rotherhithe tunnel (Ellison, 2004) River Terrace Gravels These deposits consist of variable proportions of sand and gravel, having been deposited in a braided river system approximately 5km wide across the River Thames floodplain (Ellison, 2004). The appended geological map (Fig. 3) suggests that deposits in this region of London are representative of the Taplow Gravel Formation, resting unconformably upon the solid geology of the area London Clay The London Clay Formation is predominantly argillaceous in its upper part, with the majority of the formation comprising overconsolidated heavily bioturbated, fissured bluish-grey slightly calcareous, silty to very silty clay (Ellison, 2004) often containing pyrite and carbonate concretions (claystone) of ferroan calcite (Huggett, 1994 in Ellison, 2004). This upper part is often oxidised and weathered to a brown colouration, whilst the basal unit is notably sandier and siltier than the upper horizons (Burland et al, 2001) Blackheath Beds Underlying the London Clay, this stratum is generally less than 1m thick (Burland et al, 2001), and generally comprises of sands, gravels and pebble beds (Hight et al. 2004; Ellison, 2004) Reading Formation Lambeth Group Upper Mottled Clay Identified principally within cores in central and eastern London, this stratum consists predominantly of mottled clay, silty clay and silts with colours including pale brown, pale grey-blue, dark brown, pale green, red-brown and crimson, based upon the oxidation state of the constituents (Ellison, 2004). At the base of this unit laminated sand and/or silt with minor burrowing and local ripple laminations are evident (Hight et al, 2004). 5

6 Lower Mottled Clay Although similar to the Upper Mottled Clay, this lower horizon also includes purple to the list of potential colourations (Ellison, 2004). Furthermore, it is also noted to contain carbonate nodules up to 0.5m in diameter, particularly in the top parts in east London these appear to have coalesced to form a limestone up to 1.6m thick (Hight et al, 2004). Minor amounts of irregularly cemented calcareous clayey sands are recorded in east London, becoming increasingly dominant further eastwards (Hight et al, 2004) Woolwich Formation Lambeth Group Upper Shelly Clay Distributed mainly in south London between Westminster and Bermondsey (Ellison et al, 2004; Height et al, 2004), this strata is generally a maximum thickness of 3m, comprising grey shelly clay thinly interbedded with grey-brown silt and fine sand (Height et al, 2004). Locally, there is a weakly cemented shell bed up to 0.43m thick, whilst between Bermondsey and Lewisham a continuous grey limestone bed (the Paludina Limestone) can be identified, with a thickness of m (Ellison, 2004) Laminated Beds Generally resting conformably on the Lower Shelly Clay, this stratum generally comprises of thinly interbedded and laminated fine- to medium-grained sands, silts and clays with scattered shells (Ellison, 2004). Localised bodies of sand of thicknesses up to 4m probable buried channels are known to occur particularly around the Lambeth and Bermondsey areas, and typically comprise of pale olive to pale brown medium-grained well sorted sands (Hight et at, 2004) Lower Shelly Clay Generally thickening from central London towards the south-east, this rests disconformably on the Lower Mottled Clay of the Reading Formation, whilst the top of the unit is generally sharp or transitional with the Laminated Beds or the Upper Mottled Clay (Ellison, 2004). The predominant lithology is that of dark grey to black clay, with abundant shell fragments (Hight et al, 2004). Some beds are almost entirely formed of weakly cemented shells, whilst less often brownish grey clay beds, slightly cemented with siderite (Ellison, 2004) are identifiable through this highly variable stratum. An oyster-rich bed can occur locally near the base (Hight et al, 2004) Upnor Formation Lambeth Group Pebble Bed This unit can only be identifiable as a separate substratum from the lower Glauconitic Sand unit in the London area (Ellison, 1991 in Hight et al, 2004).In this area this strata is up to 3m thick, generally comprising of well-rounded flint pebbles, generally less than 30mm in diameter, but have been identified as large as 200mm (Hight et al, 2004; Ellison, 2004). 6

7 Glauconitic Sand Dominated by fine- to medium-grained sand and clayey sands with variable amounts of glauconite grains of fine to medium sands grey to greenish grey when fresh, weathering to pale grey-brown and yellow brown (Hight et al, 2004). Localised carbonate concretions have developed either as hard irregular masses or powdery patches up to 0.5m in diameter (Ellison, 2004). Sporadic beds of well-rounded flint pebbles (Hight et al, 2004) occur throughout this unit, as do clay dominated units of up to 0.3m thickness (Ellison, 2004). It is important to note that the relationship between the Lambeth Group Formations is most complex in London s central and south-east extents, (Ellison, 2004), with interbedding between the Reading and Woolwich Formations common, and the Upnor Formation less so Thanet Sand Formation Thanet Beds Consisting of a generally coarsening-upwards sequence of fine-grained heavily bioturbated grey sands (Ellison, 2004), these beds reach a maximum thickness of approximately 30m within the London Basin (Royse et al, 2008). The lower beds are noticeably clayey and silty, whilst bioturbation structures are identifiable by wisps of dark grey clay and silty clay (Ellison, 2004) Bullhead Bed Marking the bottom of the Thanet Sand Formation, this bed is considered to be a basal conglomerate, consisting of sporadic rounded black flint pebbles (Royse et al, 2008) set within a dark greenish grey, clayey fine- to coarse-grained sandy matrix with glauconite pellets (Ellison, 2004) Upper Chalk Formation Essentially a very fine-grained white Limestone, this formation consists predominantly of coccoliths composed almost entirely of calcium carbonate in the form of low magnesian calcite, with sporadic to occasional flint bands (Ellison, 2004). In the London area the lower parts of this formation is mapped as the Lewes Chalk, or the Lewes Nodular Chalk, which is best defined as a hard to very hard nodular Chalk with interbedded soft to hard gritty Chalks and regular flint bands (Bristow et al, 1997). 4.0 GROUND INVESTIGATION The following works were undertaken in order to assess the soils and subsoils beneath the proposed Package 4 Jubilee Line extension. These works were undertaken by Messrs Soil Mechanics Ltd, commencing on the 24 th September, 1990, and ending on the 17 th December,

8 4.1 EXCAVATIONS A total of twenty (20 no.) boreholes were completed across the site in order to determine the nature and consistency of the soils beneath the site. This programme of works commenced on the 24 th September, 1990 and ended on the 14 th November, Drilling depths range from mod (27.60 mbgl) in BH 413P to mod (53.05 mbgl) in BH404T, with the majority of excavations being completed to a depth of to mbgl. Chiselling of harder strata horizons was undertaken where necessary. Hand-dug starter pits were completed at all locations to a depth of between 0.60 mbgl (metres below ground level) and 1.20 mbgl in order to check for utilities before drilling was commenced. Fourteen (14 no.) of these boreholes were completed using Cable Percussive techniques in shell diameters of between 150mm and 250mm depending on the number of reductions required per borehole (depth and geology dependant). A further two (2 no.) boreholes were initially excavated using open hole drilling (404T, 407T) using a tricone rock bit, followed by Rotary drilling to the base of the boreholes. Similarly, two (2 no.) boreholes (410T and 415T) were commenced using Cable Percussive methods, followed by Rotary drilling methods to the base of the boreholes. In all four boreholes Rotary drilling employed polymer mud flush and SK6L wireline 100mm coring equipment. The remaining two (2no.) boreholes (403P and 413P) were drilled using Cable Percussive equipment with the inclusion, at times, of Self-Boring Pressuremeter (SBP) tests), used primarily to drill and test the London Clay Formation and Upper Mottled Clays and, where applicable, the Thanet Sand Formation. A more in-depth summary of each borehole is outlined in Table 1 below. 8

9 BH Drilling Method Eastings Northings Hand Dug Pit Depth (mbgl) Date Start Reduced Level (mod) Date Finish Reduced Level (mod) Depth (m) 401 Cable Percussive /09/ /09/ Cable Percussive /09/ /10/ Cable Percussive /10/ /11/ P Cable Percussive/SBP /10/ /10/ T Open Hole/Rotary /09/ /10/ Cable Percussive /10/ /10/ Cable Percussive /10/ /10/ T Open Hole/Rotary /10/ /10/ Cable Percussive /10/ /10/ Cable Percussive /10/ /10/ Cable 410T Percussive/Rotary /10/ /10/ Cable Percussive /10/ /10/ Cable Percussive /10/ /10/ P Cable Percussive/SBP /10/ /11/ Cable Percussive /10/ /10/ Cable 415T Percussive/Rotary /10/ /10/ Cable Percussive /10/ /10/ Cable Percussive /10/ /10/ Cable Percussive /10/ /10/ Cable Percussive /10/ /10/ Table 1 Borehole data including location, start and finish dates, depths and inspection pit information. 4.2 IN-SITU TESTING A number of tests were undertaken within the boreholes during excavation. These included the following: Standard Penetration Tests (SPT), with Split spoon testing completed within clay and silt horizons and Cone Penetration Testing (CPT) were completed within sand and gravel horizons during Cable Percussive excavation. Both methods are essentially the same, although split spoon testing allows a sample to be retrieved, whilst CPT methods involve a solid cone penetrating the strata, and therefore no sample in obtained. These tests involve an initial number of seating blows to a depth of 150mm from the test start datum, with the number of blows required to drive the split spoon or cone a further 300mm counted, giving an N-value. Down-hole falling head permeability tests were undertaken during the drilling operations within a number of the boreholes (403, 404T, 409, P, 414, 416, 417, 418, 419). 9

10 Self-boring Pressuremeter (SBP) tests were conducted within boreholes 403P and 413P. These tests enable a variety of total and effective stresses to be determined in-situ for clays and silts, including undrained shear strength (c u ) and pore water pressures (Clarke, 1990). In sands, this test method can determine the angle of shearing resistance, angle of dilation, and the secant and tangential shear modulus (Hughes et al, 1977). 4.3 INSTALLATIONS The majority of the boreholes excavated during this investigation were installed with one piezometer and sand filter in order to enable groundwater measurements and fluctuations across the line of section. Exceptions to this include six (6 no.) boreholes (402, 403, 407T, 410T, 415T and 416) which were dual-installed with an upper and lower piezometer, and BH403P, which was not installed. Following installation or completion all boreholes were backfilled with bentonite. 4.4 SAMPLING In excess of 1200 samples have been collected for testing and characterisation. These involved different sampling methodology involving the following: Undisturbed Samples within clayey and silty horizons Obtained predominantly using 100mm diameter driven tubes. Some samples obtained using pushed piston methods. Small disturbed samples Obtained from SPT tests in clayey and silty horizons, or taken at intervals determined by the drilling methodology. Bulk disturbed samples Obtained following CPT testing on sand and gravel horizons. Core samples extracted from rotary cored boreholes (404T, 407T, 410T and 415T) Water samples Extracted from a number of boreholes on occasions where groundwater levels were noted. 4.5 LABORATORY TESTING A number of tests were conducted on a selection of the samples extracted from the borehole programme in order to give a good and detailed spread of the site data. Laboratory testing was undertaken by Messrs Soil Mechanics Ltd, with the following tests undertaken: Index Properties testing on clayey and silty samples in order to identify the Liquid and Plastic Limits of the samples, their Plasticity Index, and the natural moisture contents. Undrained triaxial tests were undertaken on undisturbed clay and silt samples in order to calculate the undrained shear strength (c u ) values of a selection of samples. Where deemed appropriate, the laboratory cut three (3no.) 38mm diameter undisturbed samples from the original 100mm diameter samples in order to subject the soil to different cell pressures. Soil and water sulphate tests, including ph testing, were undertaken on a number of water, undisturbed and disturbed samples. 10

11 The results of all the laboratory tests are located in Figures 7 to GROUNDWATER MONITORING Monitoring of all the installed piezometers was undertaken in the period of the 4 th October 1990 to the 17 th December All installations were monitored for the depth of the groundwater on several occasions over this period with the exception of BH403. The results from groundwater monitoring have been plotted on the cross-sections (Figures 4 & 5), with pore pressures calculated and plotted in Figure GROUND CONDITIONS 5.1 STRATIGRAPHY Stratigraphic logging of the boreholes revealed the following soil succession underlying the site: FORMATION STRATA REDUCED LEVEL (mod) THICKNESS (m) TOP BOTTOM MIN MAX N/A MADE GROUND ALLUVIUM ALLUVIUM PEAT RIVER THAMES TERRACE GRAVEL TERRACE GRAVEL LONDON CLAY FORMATION LONDON CLAY (WEATHERED) -4.21(-3.96) (-7.29) 0.30 (0.50) (0.55) HARWICH FORMATION BLACKHEATH BEDS UPPER WOOLWICH FORMATION UPPER SHELLY CLAY UPPER READING FORMATION UPPER MOTTLED CLAY LOWER WOOLWICH FORMATION LAMINATED BEDS LOWER SHELLY CLAY LOWER READING FORMATION LOWER MOTTLED CLAY UPNOR FORMATION THANET SAND FORMATION PEBBLE BED GLAUCONITIC SAND THANET BEDS BULLHEAD BED UPPER CHALK FORMATION UPPER CHALK (NP) 2.65 (NP) 8.6 (NP) Table 2 Stratigraphic data identifying the order of the soils from shallowest to deepest for package 4 of the Jubilee Line. Note NP means that the base of the strata was not proven. Cross-sections through the site are located in Figures 4 and 5, with lines of sections shown in Figure 2. Both Table 2 and the Figures 4 and 5 indicate that the Woolwich and Reading Formations are interbedded beneath the site area. There also appears to be a north-northwest dip of the solid geological succession from consideration of boreholes 401 to 408, where the cross-section is 11

12 from northwest to southeast. This is less evident in the remaining boreholes because the corresponding cross-section is orientated west to east. 5.2 GROUNDWATER CONDITIONS Groundwater levels monitored and measured within the piezometers were observed to form two distinct phreatic surfaces present (Figure 6). An upper surface at approximately mod is identified, whilst a second, lower phreatic surface at approximately mod is also observed. This upper surface is considered to correspond with the permeable Terrace Gravels overlying the relatively impermeable London Clay Formation in the northwest and west of the section (BH 401 to BH 408), and continues in the eastern section of the site with the Terrace Gravels overlying the less permeable beds of the Upper Shelly Clay and Upper Mottled Clay (BH 410T to BH419). The lower groundwater surface is considered to relate to the pore pressures within the Lambeth Group and the Thanet Sands. The water levels measured within the piezometers plot about the hydrostatic lines drawn through mod and mod, and it is therefore considered that pore pressures within the strata at both shallow and deep levels closely match hydrostatic pore pressures, taking into consideration minor fluctuations between boreholes for example, BH 401 indicated groundwater at 0 mbgl. Furthermore, it is not considered that there is any under-drainage of soils beneath the site. It should be noted that piezometers which has been installed within clay horizons may take weeks, month or longer for groundwater conditions to stabilise after the drilling programme. The monitoring round extends for no more than three months after initial installations, and it is therefore recommended that further monitoring be undertaken to identify any recent changes to the groundwater conditions. The findings may or may not indicate the pore pressures to plot closer to the hydrostatic lines at mod and mod, or may indicate that the phreatic surfaces have moved. 5.3 GEOTECHNICAL CHARACTERISATION Whilst down-hole permeability tests and SBP tests were undertaken during the drilling programme, this information has not been made available for this report. Therefore, these will not be discussed in the following sections Made Ground Encountered at ground level within all excavations, this stratum was very of very variable thickness across the site, ranging from 0.30m (BH 416) to 5.00m (BH402). This comprised of topsoil (0.10 to 0.20m thick), tarmac (0.05 to 0.15m), or concrete (0.05 to 0.40m), or a combination of these, overlying soft to firm brown grey green and black silty sandy Clay with abundant fine to coarse angular to rounded brick, concrete, ash, tile mortar, flint coal and chalk gravels and rare cobbles of tile, chalk, concrete and timber. Although contamination 12

13 testing is beyond the remit of this report, it should be noted that Made Ground within BH 403 was described as oily, whilst being described as having a pungent odour in BH414. Chiselling was required within six boreholes (Figures 4 and 5), whilst SPT N values for this stratum vary from 3 to 24, in line with the variability of the soils. Only one index properties test was completed, with a result indicating the following: LL (%) PL (%) PI (%) w (%) This indicates that the tested soil plots below the A-line and is therefore a silt of intermediate plasticity (Figure 9). Two sulphate and ph tests were also completed indicating a ph of 7.5 to 8.0, a soil sulphate content of 0.05 % and a water sulphate content of 0.09 g/l. It cannot be recommended that such a variable stratum be characterised using limited data. However, these results suggest that DS-1 grade concrete may be employed at this level. Further research suggests that the following soil properties may be considered for Made Ground (After Burland et al, 2001): Bulk unit weight γ (kn/m³) 15 to 19 Undrained shear strength c u kn/m² 15 to 70 Effective Cohesion c' kn/m² 0 Angle of shearing resistance φ' ( ) 22 to 35 Coefficient of permeability k (m/s) 1x10-2 to 1x Alluvium Encountered in all but six boreholes (402, 403P, 407T, 408, 414 and 419) situated beneath the Made Ground, this stratum is of variable thickness across the site, ranging from 0.50m (BH401) to 3.60m (BH405). This can be described generally as a soft to firm yellow orange brown and green slightly sandy (fine to medium) occasionally very sandy silty to very silty CLAY with occasional fine to coarse angular to sub-rounded flint gravel with rare rootlets. Some beds are also observed to be predominantly sandy, comprised of loose to medium dense greyish yellow orange and brown clayey and silty SAND with occasional fine to medium angular to sub-rounded flint gravel and occasional pockets (<20mm) of brown very clayey silt. One instance of Peat was identified within a bed of Alluvium in BH405 at mod, 0.50m thick. This was describes as firm black SILT with abundant fragments of organic material (decaying wood), within the borehole log. SPT N values vary between 3 and 23, highlighting a similar variability to that of Made Ground. Based upon this a design line of N=8 is proposed for the Alluvium, taking into consideration the sand and gravel fractions of this stratum. This relates to a drained friction 13

14 angle of 29 (Peck et al, 1974). Three index tests were completed on the Alluvium, with the findings as follows: LL (%) PL (%) PI (%) w (%) 24 to to 28 9 to to 24 This outlines the variability of this stratum, with clays plotting as clays with low to high plasticity, and the silt fraction plotting within the intermediate plasticity in Figure 9. Two of the three moisture contents plot to the left (less than) the plastic limit. This would suggest that these soils are often in a semi-solid state. One bulk density test on the clay fraction of this Alluvium was carried out, with a result of 2.01 Mg/m³. Two ph and sulphate tests were completed, identifying a ph range of 7.1 to 7.8, a soil sulphate of 0.05% and a water sulphate concentration of 0.12g/l. These results both plot within the DS-1 class concrete range (Figure 11). Further research suggests that the following properties can be considered for Alluvium at this site: Bulk unit weight γ (kn/m³) 16 to 20 Undrained shear strength c u kn/m² 25 to 50 Effective Cohesion c' kn/m² 0 Angle of shearing resistance φ' ( ) 22 to 29 Coefficient of permeability k (m/s) 1x10-4 to 1x10-6 All data in the above table refers to Burland et al (2001) with the exception of the coefficient of permeability, which relates the observed soils to BS 8004: 1986 (in Craig, 2004) Terrace Gravel Encountered in all excavations, this stratum was identified immediately beneath the Alluvium if Alluvium was encountered otherwise located beneath Made Ground. Thicknesses of this stratum range between 2.70m (BH411) and 6.90m (407T), and is generally described as a medium dense to dense orange brown very sandy medium to coarse sub-angular to subrounded flint GRAVEL with occasional flint cobbles and pockets of brown silty clay. Chiselling was required at the base of the Terrace Gravels in BH409 ( mbgl for 2 hours). SPT N values range from 4 to 79, with the majority of values plotting between 11 and 47. Based on this data a design line of N=23 is proposed for the Terrace Gravels, which indicates a drained friction angle of 34 (Peck et al, 1974). Owing to the sandy and gravelly nature of this stratum, plasticity index, triaxial and moisture content tests were not undertaken, as it is considered that these soils are non-plastic. However, 15 water soluble sulphate and ph tests were carried out on water samples obtained within the Terrace Gravels. The results of these indicate a ph range of 7 to 9.7, and a water 14

15 soluble sulphate level of 0.08 to 0.31 g/l. These results plot within the boundaries of DS-1 class. Further research indicates the following table of soil properties for the Terrace Gravels (after Burland et al, 2001, with the exception of φ, which is calculated from Peck et al, 1974): Bulk unit weight γ (kn/m³) 19 to 20 Undrained shear strength c u kn/m² N/A Effective Cohesion c' kn/m² 0 Angle of shearing resistance φ' ( ) 34 Coefficient of permeability k (m/s) 5x10-3 to 5x London Clay Observed in twelve of the twenty boreholes ( , 410T, 412 and 413P) beneath the Terrace Gravels, this stratum is of a variable thickness owing to the inferred dip of the geological strata to the north-northwest, and was, therefore, not identified east of BH413P. Thicknesses of the London Clay range from 29.65m (BH401 combination of weathered and unweathered strata) to 0.30m (BH410T), with the top of the formation noticeably weathered in BH401 (0.55m thick) and BH412 (0.50m) This stratum is described generally as a stiff to very stiff thinly laminated very closely to closely fissured dark grey and grey-brown CLAY occasionally bioturbated with occasional pockets and partings (<2mm) of light brown grey silty fine sand and rare strong claystone gravels and cobbles, pyrite nodules and shell fragments. Fissures are randomly orientated clean planar to undulose smooth and occasionally polished. The upper weathered section was similar to the above description, with the exception of being brown in colour. The base of this stratum is noticeably siltier with a proportion of sand, and can be described as stiff to very stiff thinly laminated very closely to closely fissured very silty slightly sandy CLAY occasionally bioturbated with occasional pockets (<20mm) and discontinuous partings (<10mm) of light brown grey silty fine to medium sand and occasional pyrite and lignite nodules. Fissures are randomly orientated clean planar to undulose smooth to rough. The presence of some polished fissure surfaces suggests that these could be representative of shear surfaces. Chiselling was required in BH401 (16.50mbgl to 17.10mbgl for 1 ¼ hours; 22.50mbgl to 23.40mbgl for 1 ½ hours), corresponding with irregular claystone layers within the London Clay. Sixty-four (64 no.) SPT tests were conducted throughout the London Clay sequence, with N values varying from 12 to 99 (extrapolated values to 1500). 15

16 Plasticity Index testing was completed on thirty (30 no.) samples, with the results as follows: LL (%) PL (%) PI (%) w (%) 50 to to to 58 8 to 35 The vast majority (75%) of the samples plot within the very high plasticity clay section of the Plasticity Chart (Fig. 9), with approximately 20% plotting in the high plasticity clays. One sample (BH401 at 33.20mbgl) plots as a silt of intermediate to high plasticity, and another (BH403 at 21.20mbgl) plots as a silt of very high plasticity. Both of these samples are from the London Clay s basal beds, and therefore match the borehole records. For London Clay with a Plasticity Index (PI) of 21% to 58% Stroud and Butler (1975) report case histories where c u /N (kn/m²) ranges between 4.3 and 4.8. A design line based upon c u /N =4.6 (kn/m²) provides a relatively good fit to the data, and it is therefore recommended that this be used in the design. The natural moisture contents plot (Fig. 8) suggests that the majority of the samples have moisture contents at or less than the plastic limit, indicating that the London Clay is likely in a plastic to semi-solid state. Bulk density tests on the London clay show a range of values from 1.90 to 2.10 Mg/m³. Triaxial tests were conducted on twenty-eight (28no.) samples, with undrained shear strength (c u ) values ranging from 66 kn/m² to 394 kn/m². These results show a relatively good correlation (Fig. 10) with the design line of 4.6N identified from SPT testing (Fig. 7), although samples at depths greater than mod appear to be under quantified compared to the design line. Ten (10 no.) ph and sulphate tests were completed, identifying a ph range of 6.9 to 8.6, a soil sulphate content between 0.02% and 0.45% and a water sulphate concentration of 0.05g/l to 0.92g/l. The results of these tests suggest that DS-2 grade concrete should be used within this stratum. The presence of pyrite within the London Clay would also suggest that localised areas with significantly lower ph values than have been identified in the tests are likely to be present. Further research indicates the following table of soil properties for the London Clay: Bulk unit weight γ (kn/m³) 18 to 20.5 Undrained shear strength c u kn/m² SPT N x 4.6 Effective Cohesion c' kn/m² 0-12 Angle of shearing resistance φ' ( ) Coefficient of permeability k (m/s) 1x10-7 to 1x10-9 All data based upon information from Buriton et al (2001) with the exception of c u which has been calculated using Stroud and Butler (1975), and the permeability (k) which has been calculated from Craig (2004), considering that fissuring of the London Clay will significantly influence the soils permeability. 16

17 5.3.5 Blackheath Beds Identified within the majority of the excavations underlying the London Clay or, in its absence, the Terrace Gravels, this stratum is of a fairly constant thickness, with depths of between 0.20m (BH413P) and 0.70m (BH411). It was not identified within BH401 due to the base of the London Clay not being reached, and in BH410T a void was located where this unit would have been expected (Figure 5). Elsewhere, this unit was not identified because the geological succession is considered to dip gently to the north-northwest, based on crosssectional data. This stratum can be described as a stiff to very stiff grey brown black slightly sandy very silty CLAY with occasional to abundant medium to coarse sub-rounded to rounded flint gravels and rare flint cobbles and rare to occasional shell fragments with rare to occasional pockets (<40mm) of green grey glauconitic silty sand. Chiselling of this stratum was required within four boreholes (BH mbgl to 35.60mbgl for approximately 1 ½ hours; BH mbgl to 25.80mbgl for approximately 1 hour; BH403P 26.10mbgl to 26.50mbgl for 1 hour; BH mbgl to 21.40mbgl for 3 hours). A SBP test within BH403P at 31.25mbgl was noted to have burst at 2200kPa within this strata. Two SPT N tests were undertaken, with values of 39 and 214 (extrapolated). It is not considered that enough data has been obtained in order to give a design value of any accuracy for this stratum. Owing to the clayey nature of this unit, three index tests were completed, with the results as follows: LL (%) PL (%) PI (%) w (%) 63 to to to to 25 Two of these results plot as clays with a high to very high plasticity (Figure 9), whilst one sample (BH412 at 8.15mbgl) is regarded as non-plastic. One of the samples (BH402 at 34.00mbgl) plots to the left of its plastic limit, suggesting that some parts of this stratum are in a semi-solid state. Seven bulk density tests were completed, with results varying from 1.93 to 2.01 Mg/m³. Furthermore, two samples were selected for triaxial testing, with c u results varying from 111 (-4.48 mod) to 235 kn/m² (-5.61 mod). Only one ph and soil sulphate test was conducted within this strata, with values of 6.3 and 0.31 % respectively, which plots within the DS-2 class. 17

18 Based upon the engineering description of the Blackheath Beds beneath the site area the following table outlines the design properties of this stratum: Bulk unit weight γ (kn/m³) 19 to 20 Undrained shear strength c u kn/m² 75 to 300 Effective Cohesion c' kn/m² 0 Angle of shearing resistance φ' ( ) 20 to 25 Coefficient of permeability k (m/s) 1x10-5 to 1x10-7 All data has been estimated using tables and data made available by Waltham, (2002) with the exception of permeability, which has been estimated using tables in Craig (2004) Upper Shelly Clay Observed within thirteen (13 no.) of the boreholes (BH , BH P) beneath the Blackheath Beds or, where this is not present, Terrace Gravels, this stratum is of a reasonably constant thickness, ranging from 0.55m (BH412) to 2.50m (BH408), and is cut out of the line of section due to the gently dipping nature of the geology, with the exception of BH403P, where it is not within the sequence. This is considered to be due to the interbedded nature of the Woolwich and Reading Beds of the Lambeth Group. This material comprises of very stiff locally thinly laminated closely fissured dark grey and grey-green very silty CLAY with abundant shells, occasional pockets and partings of greygreen sandy (fine) silt and occasional sub-rounded flint gravels, with thin (<90mm) impersistent strong to very strong grey shelly Limestone, recovered as fine to coarse subangular gravel. Fissures are random to sub-vertical clean and planar. Chiselling was required in three boreholes BH402 through the Blackheath Beds through to the Upper Mottled Clay (35.40mbgl to 36.50m for 7 hours); BH403 through the Blackheath Beds to the Upper Mottled Clay (25.20mbgl to 26.70mbgl for 3 hours); BH405 (22.10mbgl to within the Upper Mottled Clay at 25.20mbgl for 5 ½ hours). These episodes of chiselling are considered to correlate with the Limestone and flint gravel bands identified within the boreholes Twelve (12 no.) SPT tests were carried out on the Upper Shelly Clay, with N values varying from 32 to 273 (extrapolated). Plasticity Index tests were completed on five samples with the results as follows: LL (%) PL (%) PI (%) w (%) 29 to to to to 30 These samples plot predominantly as clays of low to intermediate plasticity, with the exception of BH407T (13.34mbgl) which corresponds to a clay of high plasticity, with a significantly higher liquid limit than other samples of this stratum. The natural moisture contents plot (Fig. 8) suggests that the majority of the samples have moisture contents at or 18

19 less than the plastic limit, indicating that the Upper Shelly Clay is likely in a plastic to semisolid state. For a PI of 14% to 36% Stroud and Butler report c u /N (kn/m 2 ) case histories of between 4.3 and 6. However, their chart indicates that Woolwich and Reading Clays plot significantly lower than their best fit line, at between 3 and 3.5. A design line based upon c u /N =3 (kn/m²) provides a relatively good fit for the data. Bulk density tests on the Upper Shelly Clay show a range of values from 1.71Mg/m 3 to 2.18Mg/m 3. Triaxial tests were competed on four samples, with an Undrained Shear Strength (c u ) varying from 26 kn/m 2 up to 132 kn/m 2. From consideration of the triaxial plot (Fig. 10) and the borehole data, the lower c u value is considered to be due to the sample (-8.09 mod in BH408) being composed predominantly of Silt, compared to the other samples being of clay. One ph and soil sulphate test were completed by the laboratory, with results of 6.3 and 0.03% respectively. These plot within the DS-1 grade boundaries (Fig. 11). Further research indicates the following table of soil properties for the Upper Shelly Clay: Bulk unit weight γ (kn/m³) 17 to 22 Undrained shear strength c u kn/m² 25 to 400 Effective Cohesion c' kn/m² 0-15 Angle of shearing resistance φ' ( ) Coefficient of permeability k (m/s) 1x10-6 to 1x10-8 Effective cohesion, angle of shearing resistance and coefficient of permeability based upon information from Buriton et al (2001) Upper Mottled Clay Identified within all boreholes with the exception of BH402 which terminated above the presumed stratum, this is situated beneath the Upper Shelly Clay or, where this is not present, the Terrace Gravels or the Blackheath Beds (BH403P). This stratum is of variable thickness across the site, identified at between 1.60m (BH419) and 7.65m (BH403). This stratum is considered to be variable in its stratigraphy, comprising predominantly of stiff to very stiff extremely closely fissured green grey blue red brown mottled CLAY occasionally very silty and sandy with rare to occasional angular to sub-rounded flint gravel and occasional pockets of light brown silty sand. Fissures are random to sub-vertical planar smooth and occasionally polished with rare slickenlines. Rare moderately strong to strong limestone bands and occasional beds of very dense brown blue green and grey silty fine to medium SAND are also present. The presence of slickenlines and polished surfaces on some fissures suggests that some may represent relict shear surfaces. Chiselling was required in five of the boreholes on more than one occasion. This information is identified within Figures 4 and 5 and the borehole logs. Episodes of chiselling are 19

20 considered to be representative of sections where the Upper Mottled Clay could be described as hard. Forty-nine (49 no.) SPT tests were undertaken on the Upper Mottled Clay, with N values varying from 15 to 333 (extrapolated). Plasticity Index tests were completed on twenty-four (24 no.) samples with the results as follows: LL (%) PL (%) PI (%) w (%) 20 to to 31 9 to to 35 These samples plot from low plasticity clays through to very high plasticity clays predominantly centred around the high to very high plasticity clay sections. Two samples BH411 at 14.30mbgl and BH405 at 25.50mbgl plot within the intermediate and high plasticity silts, respectively. The results indicate the variability of this stratum. The natural moisture contents plot indicates that all samples have moisture contents that plot at or higher than their liquid limits; clays and silts within this stratum can therefore be considered as plastic. For a PI of 9% to 50% Stroud and Butler report c u /N (kn/m 2 ) case histories of between 4.3 and 7. However, their chart indicates that Woolwich and Reading Clays plot significantly lower than their best fit line, at between 3 and 3.5. A design line based upon c u /N =3 (kn/m²) provides the best fit for the Upper Mottled Clays clay fractions. Bulk density tests on the Upper Mottled Clay show a range of values from 1.88Mg/m 3 to 2.15Mg/m 3. Triaxial tests were completed on twenty-two (22 no.) samples, with Undrained Shear Strength (c u ) varying from 22 kn/m 2 up to 277 kn/m 2. From consideration of the triaxial plot (Fig. 10) and the borehole data, the lower c u value is considered to be due a number of the tested samples having polished fissures. In these instances it may have been the residual strength of these samples which was being measured. Whilst the design line fits the less fissured Clays, it is not considered to represent a good fit of the data for the strata with polished fissures. Nine ph tests were completed, varying from 6.4 to 8.4. Five soil sulphate tests and six water sulphate tests were also completed, with values varying from 0.01% to 0.42% and 0.06 g/l to 0.46 g/l respectively. These values suggest that DS-2 grade concrete be used within this strata, if necessary. Further research indicates the following table of soil properties for the Upper Mottled Clay: 20

21 Bulk unit weight γ (kn/m³) 19 to 21.5 Undrained shear strength c u kn/m² 25 to 400 Effective Cohesion c' kn/m² 0-15 Angle of shearing resistance φ' ( ) Coefficient of permeability k (m/s) 1x10-6 to 1x10-8 Effective cohesion, angle of shearing resistance, coefficient of permeability and the upper level of undrained shear strength are based upon information from Buriton et al (2001) Laminated Beds Observed within all boreholes with the exception of BH401 and BH402, where drilling ended before the stratum was reaches, the Laminated Beds are located beneath the Upper Mottled Clay in all instances, with thicknesses varying from 1.10m (BH405) to 2.98m (BH418). The Laminated Beds can be generally described as a very stiff thinly to thickly laminated grey occasionally brown silty to very silty CLAY with occasional partings (<6mm) of brown silt and sand and rare carbonaceous pockets, interlaminated with very dense thinly to thickly laminated grey silty fine to medium SAND with occasional bands of grey very silty sandy clay, and very stiff thinly laminated grey very clayey sandy SILT with rare shell fragments. Despite its very stiff and very dense attributes chiselling was only required once during the drilling programme, in BH409 (15.50mbgl to 15.80mbgl for 1 ½ hours). This is presumed to be due to the very stiff nature of the very silty Clay in this instance. A void was noticed within BH404T from mod to mod, which may relate to groundwater flow within the Laminated Beds. Nineteen (19 no.) SPT tests were completed within this stratum; with N value results ranging from 25 to 300 (extrapolated), with the majority ranging between 25 and 82. The four values greater than this were observed within very dense Sand (188, 210 and 300) and very stiff silt (110). Plasticity Index tests were completed on fifteen (15 no.) samples with the results as follows: LL (%) PL (%) PI (%) w (%) 27 to to 35 5 to to 35 These samples plot from low plasticity clays through to very high plasticity clays predominantly centred around the intermediate to high plasticity clay sections. One sample (407T at 16.73mbgl) plots as a low plasticity silt, with the sample itself described as a very sandy silt. The stratigraphic variability of the Laminated Beds is evident in these results. The majority of these samples natural moisture contents plot within or on the plasticity limit, which suggests that this stratum is in a plastic state. For a PI of 5% to 62% Stroud and Butler report c u /N (kn/m 2 ) case histories of between 4.3 and 7. However, their chart indicates that Woolwich and Reading Clays plot significantly 21

22 lower than their best fit line, at between 3 and 3.5. A design line based upon c u /N =3.5 (kn/m²) provides a reasonable fit for the clay fractions of the Laminated Beds. Bulk density tests on the Laminated Beds show values varying from 2.00Mg/m 3 to 2.20Mg/m 3. Triaxial tests were completed on eleven samples, with Undrained Shear Strength (c u ) varying from 102 kn/m 2 to 314 kn/m 2. From consideration of the triaxial plot (Fig. 10) and the borehole data, the lower c u value is considered to be due a number of the tested samples containing significant amounts of silt, or comprising predominantly of silt. Whilst the design line is considered to fit around the clay data, it is not considered to represent a good fit of the data for other soil types within the Laminated Beds. Considering the SPT data for the sand horizons of the Laminated Beds in relation to Peck et al (1974) a drained angle of shearing resistance for the sands within the Laminated Beds is considered to be of 38 to 42. Six ph tests were completed, with results varying from 6.2 to 8.6. Five soil sulphate tests and four water sulphate tests were also completed, with values varying from 0.01% to 0.51% and 0.30 g/l to 0.63 g/l respectively. These values suggest that DS-2 grade concrete be used within this stratum, although one sample plotted narrowly inside the DS-3 class for soil sulphate content. Further research indicates the following table of soil properties for the Upper Mottled Clay: Bulk unit weight γ (kn/m³) 20 to 22 Undrained shear strength c u kn/m² 100 to 150 (silt) 100 to 300 (clay) Effective Cohesion c' kn/m² 0-10 Angle of shearing resistance φ' ( ) 28 (Clays) 38 to 42 (Sands) Coefficient of permeability k (m/s) 1x10-4 to 1x10-7 Effective cohesion, angle of shearing resistance of clays and the coefficient of permeability are based upon information from Buriton et al (2001) Lower Shelly Clay Identified within all boreholes with the exception of BH401 and BH402 which terminated above the presumed stratum, the Lower Shelly Clay is located beneath the Laminated Beds. This stratum is of a variable thickness across the site, ranging from 0.33m (BH404T) to 4.57m (BH407T). This stratum is considered generally comprise of very stiff extremely to very closely fissured thinly to thickly laminated dark grey mottled green yellow brown and purple very silty CLAY locally calcareous cemented with occasional to abundant shell fragments and occasional partings (<6mm) and pockets (13mm) of light grey silt and fine sand and rare fine to medium sub-angular to sub-rounded flint gravel. Fissures are predominantly horizontal to sub-horizontal planar to undulating smooth and slightly polished. 22