This is an interpretative geotechnical report of the Jubilee Line Extension which describes the extension from Green Park to Stratford running through South and East London. A site examination was undertaken by Soil Mechanics Ltd in 1990 for London Underground which was done as per the 1987 specifications for Ground Examination published by the Department of Transport (Attewell, 1995, p. 39).
This included Package 2 of the proposed Jubilee Line Extension, involving 20 boreholes out of which 18 boreholes were drilled using Cable Percussive and Rotary method was used to drill the remaining 2 boreholes situated in the Bermondsey region of London. Drilling works finished on the 14th November 1990.
The main objective of the investigation was to examine the ground conditions of the soil above and beneath the site preceding the proposed tunneling in the Bermondsey region.
The site shows the acquired borehole information of Package 2 and takes after the proposed tunneling course of the extension works southeast of London Bridge Station in Bermondsey, eastwards towards Canada Water in Rotherhithe. The site range in Easting’s and Northing’s is linear, running from 533343, 179834 (BH401) in the east to 535222, 179445 (BH417) in the west. This principally takes after the River Thames at an interval of roughly 400m from the south bank over the linear run. The passage runs through the overland train line southeast of Tower Bridge Station towards South Bermondsey Station for around 600m before curving eastwards and east-north-eastwards underneath Jamaica Road to the recently proposed Bermondsey Tube Station. The tunnel route proceeds eastwards underneath Southwark Park and ends at the edge of Neptune Road and Moodkee Street on the ‘Canada Estate’. Over the site the street levels are moderately flat, going from 1.9mOD in the western, to 3.6mOD in the east.
The site is situated on various strong and drifts stores at ground level as indicated in BGS solid and drift 1:50,000 maps (256 – North London; 270 – South London). The geological units which are probably going to be experienced over the site ranges from shallow to solid geological units are portrayed as follows from the youngest to oldest.
As its name implies, Made Ground according to BGS are areas where the pre-existing ground surface have been massively altered by man-made activities such as excavation. The thickness of the made ground ranges from 0.30m to 5.00m consisting of a mix of concrete, rubble, brick and refuse intermingled with gravel and sands frequently within a matrix of silt or clay. Studying this man-made geological unit also helps to get information of the past land because it is often associated with unstable ground, unpredictable engineering conditions and contaminated materials (Ford,Kessler, Cooper, Price & Humpage, 2010). These deposits are as a result of the urbanization and industrial development (Burland et al. 2001, p62).
The deposits are around the River Thames, basically lying unconformably upon River Terrace Gravels (Ellison, 2004). The Alluvium is mainly comprised of silty clay and clayey silt, with beds of fine-to-coarse-grained sand with thickness of 1m to 15 m. Inter-bedded peat is known to happen eastwards of Southwark and Westminster, with the broadest deposits extending west to the Rotherhithe tunnel (Ellison, 2004).
The River Terrace Gravels are comprised of variable quantities of sand and rock, deposited in a braided river system roughly 5km wide over the River Thames floodplain (Ellison, 2004). The deposits in this area of London are illustrative of the Terrace Gravel Formation, lying unconformably upon the solid geology of the region.
The London Clay Formation is mainly argillaceous in its upper part, with the greater part of the development involving overconsolidated intensely bioturbated, fissured bluish-grey slightly calcareous, silty to extremely silty clay (Ellison, 2004) usually containing pyrite and carbonate solidifications (claystone) of ferroan calcite (Huggett, 1994 in Ellison, 2004). This upper part is frequently oxidized and weathered to a brown colouration colouration, while the basal unit is remarkably sandier and siltier than the upper horizons (Burland et al, 2001).
Beneath the London Clay, this stratum is basically less than 1m thick (Burland et al 2001), and generally consist of sands, rock and stone beds (Hight et al. 2004; Ellison, 2004).
The Lambeth Group is made up of the Reading formation, Woolwich formation and Upnor formation (Ellison & Woods, 2004) and is divided into different lithological units.
Recognized primarily inside cores in central and eastern London, this stratum comprises mainly of mottled clay, silty clay and sediments with hues including pale brown, pale grey-blue, dark brown, light green, red-brown and dark-red, based upon the oxidation condition of the constituents (Ellison, 2004). At the base of this unit overlaid sand and silt with minor tunneling and local ripple overlays are apparent (Hight et al, 2004).
When compared to the Upper Mottled Clay there is no much difference, this lower horizon also includes purple to the list of potential colourations (Ellison, 2004). Moreover, 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)
Circulated mostly in south London amongst Westminster and Bermondsey (Ellison et al,2004; Height et al, 2004), this strata have a maximum thickness of 3m, including grey shelly clay thinly inter-bedded with grey-brown and fine sand (Height et al, 2004). Generally, there is a feebly established shell bed up to 0.43m thick, while amongst Bermondsey and Lewisham a consistent grey limestone bed (the Paludina Limestone) can be distinguished, with a thickness of 0.1-1.83m (Ellison, 2004).
Principally lying comformably on the Lower Shelly Clay, it is basically comprised of thinly interbedded and laminated fine- to medium-grained sands, silts and clays with scattered shells (Ellison, 2004). There is deposit of sand of thicknesses up to 4m likely covered channels are known to occur especially around the Lambeth and Bermondsey regions and contain pale olive to pale brown medium-grained very much sorted sands (Hight et at, 2004).
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 with the Laminated Beds or the Upper Mottled Clay (Ellison, 2004). Dark grey to black clay is the dominant lithology with abundant shell fragments (Hight et al, 2004). A few beds are totally shaped of weakly cemented shells, while less regularly brownish-grey clay beds, slightly solidified with siderite (Ellison, 2004) are identifiable through this exceedingly variable stratum. An oyster-rich bed occurs close to the base (Hight et al, 2004).
The pebble bed is identifiable as a different substratum from the lower Glauconitic Sand in the London region (Ellison, 1991 in Hight et al, 2004). In this region, the strata have thickness about 3m and typically consist of well-rounded flint pebbles, basically less than 30mm in diameter, however can be recognized as huge as 200mm (Hight et al, 2004; Ellison, 2004).
Mainly 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).Carbonate concretions have developed either as hard sporadic masses or powdery patches up to 0.5m in diameter (Ellison,2004). Irregular 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).
Basically comprising of a coarsening-upwards series of fine-grained intensely bioturbated grey sands (Ellison, 2004), these beds have a maximum thickness of around 30m inside the London Basin (Royse et al, 2008). The lower beds are clayey and silty, while bioturbation structures are identifiable by wisps of dark grey clay and silty clay (Ellison, 2004).
Denoting the base of the Thanet Sand Formation, this bed is thought to be a basal conglomerate, comprising of irregular rounded black flint pebbles (Royse et al, 2008) set inside a dark greenish grey, clayey fine-to-coarse grained sandy matrix with glauconite pellets (Ellison, 2004).
Basically a fine-grained white Limestone, this formation mainly comprises of coccoliths made almost completely out of calcium carbonate as low magnesian calcite with sporadic flint bands (Ellison, 2004). In the London region the lower parts of this formation is mapped as the Lewes Chalk or the Lewes Nodular Chalk, which is best characterized as a hard to very hard nodular Chalk with interbedded soft to hard gritty Chalks and regular flint bands (Bristow et al, 1997).
The motivation behind ground examination was to assemble data about the physical and designing properties of soils and subsoils in the study territory to recognize whether the site is appropriate for the proposed work. The excavations started on 24th September, 1990 and completed on 14th November 1990 and a total sum of 20 boreholes finished over the site.
A sum total of twenty boreholes were drilled over the site to examine the nature and condition of the soil underneath the site. This program of works started on the 24th September, 1990 and finished on the fourteenth November, 1990. Drilling depths range from -24.93 mOD (27.60 mbgl) in BH 413P to -49.84 mOD (53.05 mbgl) in BH404T with the majority of excavations being completed to a depth of 30.00 to 40.00 mbgl. Chiseling of harder strata was attempted where necessary. Hand-dug starter pits were burrowed at all locations to a depth of between 0.60 mbgl (meters subterranean level) and 1.20 mbgl so as to check for utilities before boring was initiated.
Fourteen of these boreholes were finished utilizing Cable Percussive method in shell diameters between 150mm and 250mm relying upon the number of decrement required per borehole. A further two boreholes were excavated utilizing open hole drilling (404T, 407T) utilizing a tricone rock bit, followed by Rotary drilling to the base of the boreholes.
Also, two boreholes (410T and 415T) were started utilizing Cable Percussive techniques, followed by Rotary boring strategies to the base of the boreholes. In every one of the four boreholes Rotary penetrating utilized polymer mud flush and SK6L wireline 100mm coring tool. The last two boreholes (403P and 413P) were drilled utilizing Cable Percussive tool together with Self-Boring Pressuremeter (SBP) tests), utilized principally to bore and test the London Clay Formation and Upper Mottled Clays and where pertinent, the Thanet Sand Formation. A more detailed of each borehole is outlined out in Table 1
Trial pits also known as shallow boreholes were dug in order to check gas, water and electricity pipelines due to the level of urbanization of the site location before actual excavation works commenced. The sensitivity of the locations means there is high risk of damaging existing infrastructures. The depth of the trial pits ranges from 0.6m to 1.2m as shown in table 1 above. Ten hand-dug inspection trial pits were drilled to a depth of 1.2m below ground level and these trial pits were: BH401, BH402, BH405, BH406, BH408, BH412, BH413, BH414, BH416, BH418, and BH419.
After the drilling process, laboratory test were undertaken on specific samples so as to give in-depth understanding of the site. The tests performed were as follows;
Index Properties, to classify samples accordingly to the Liquid & Plastic Limits, Plasticity Index, Moisture content and Bulk density.
Undrained Triaxial Tests, to calculate the undrained shear strength (Cu) values of the samples.
Chemical Tests, testing the pH for soil and water for chemical contamination analysis (which was carried out on some water samples, undisturbed and disturbed samples).
In addition, some chemical testing was carried out on some samples from boreholes to determine pH, pAH, organic content, heavy metals such as arsenic, cadmium, chromium, copper, mercury, nickel, zinc, lead, coal tars, mineral oils, phenols, solvent extracts, sulphide and sulphates which may indicate contamination in the site. X-ray Diffraction Analysis was also carried out in order to ascertain the presence of minerals.
From the ground investigations, it can be affirmed that the stratigraphy of the site relatively varies from northwest to southeast that is, from London Clay Formations to Chalk Formation. Table 2 gives a vivid summary of the succession of soil underlying the site. As presented in Table 2, there is decrease in London Clay Formation thickness from boreholes 401 to 410. With the Chalk Formation depth, it appears from 36m beneath ground level from boreholes 407 to 418. Moreover, it can be seen that Woolwich and Reading Formation interbedded beneath the site as upper shelly / mottled clay and lower shelly/ mottled clay. Blackheath bed underlies in upper shelly clay. There is a Bullhead Bed between Thanet Bed and Chalk.
Water level monitoring was carried out with piezometers installed in each borehole to aid measurement of groundwater across the site. Recorded measurement for less than three months (between 1st October 1990 to 17th December 1990 with depths of 0.27mBGL (BH401) to 17.53mBGL (BH405) indicates inconsistency along the site. Although there was a piezometer installed in BH403P, no water level was observed whilst 18 boreholes had one piezometer installed in each. Moreover, boreholes 402,403,407T, 410T, 415T and 416 had two piezometers installed in them respectively (upper and lower piezometer). Piezometer readings were observed to form a distinct ground water surface in the Terrace gravels which can be concluded as a permeable bed with water level observed in 8 boreholes. The contamination data given indicates that from the depth ranges of 1.50m (BH419) to 18.30m (BH308), Sulphides and Chromium occur from 0.01 to 5. There was presence of two mainly aquifers zone, the first phreatic zone is located between the permeable Terrace Gravels and the impermeable London Clay Formation in the west of the site (boreholes 401 to 407) extending to the underlying Upper Mottled Clay and Upper Shelly Clay from the Lambeth Group (boreholes 408 to 416) in the east of the site. Its depth goes from 4m to 12m below the ground level. The second phreatic zone is can be found in the permeable Laminated Sand and Silt Strata, between the Upper Mottled Clay and the Lower Shelly Clay (boreholes 407 to 418), at the east of the site with varying depth of 9m to 17m below the ground level
Experienced mostly at all ground level within all excavations, the stratum thickness varies across the site, ranging from 0.30m (BH 416) to 5.00m (BH402). Consisting of topsoil (0.10 to 0.20m thick), tarmac (0.05 to 0.15m) and concrete (0.05 to 0.40m) 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. Made Ground within BH 403 was described as oily whilst being described as having a strong smell in BH414. Chiseling was required within six boreholes whilst SPT ‘N’ values for this stratum varies from 3 to 24 in accordance with the variability of the soils. 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 characterized using limited data. However, these results suggest that DS-1 grade concrete may be utilized at this level.
Alluvium in experienced in boreholes 401, 403, 404, 405, 406, 410, 411, 412, 413, 418, 415, 416 and 417, and it is situated beneath made ground. Its thickness varies from 0.50 m (borehole 401) to 3.60 m (borehole 405). This stratum can be defined as medium dense greyish yellow clayey fine to coarse sand with some sub-angular to sub-rounded fine to coarse flint gravel, with some presence of firm to stiff brown-green slightly sandy (fine to medium) clay, soft blue-grey very oxidized brown very silty lay with pockets (<20 mm) of red-brown very clayey silt, firm grey-green, becoming yellow brown-orange brown slightly sandy (fine) very silty clay with rare rootlets, yellow brown slightly silty fine to medium sand with occasional pockets (<40 mm) and loose to medium dense orange - brown silty and clayey fine to medium sand. There was presence of rare organic matter in boreholes 410, 411, 417. Based on the SPT results, 13 tests were made in total, 5 of which were made using Split Spoon Sampler (S) and 8 using Solid Cone Test (C). The N value in this stratum varies from 3 to 24, with an average of 13, so it can be considered as a medium dense according to density index of sands classification made by Craig (2004).
This stratum is situated below the Alluvium with thicknesses of 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 sub rounded flint gravel with occasional flint cobbles and pockets of brown silty clay. Chiseling was required at the base of the Terrace Gravels in BH409 (6.90-7.70mbgl for 2hours). SPT ‘N’ values range from 4 to 79, with the majority of values plotting between 11 and 47 as presented in figure 5. Based on this data a design line of N=23 is proposed for the Terrace Gravels, which shows a drained friction angle of 34° (Peck et al, 1974).
Due to the sandy and gravelly nature of this stratum, plasticity index, triaxial and moisture content tests were not undertaken since it is perceived that these soils are non-plastic.
Nevertheless, 15 water soluble sulphate and pH tests were performed on water samples obtained within the Terrace Gravels. The results of these show a pH range from 7 to 9.7 and a water soluble sulphate level of 0.08 to 0.31 g/l.
It was present in 12 boreholes 401, 402, 403, 403P, 404, 405, 406, 407, 408, 410, 412, 413 and it is located below the terrace gravel stratum. It is a combination of weathered (boreholes 401 and 412) and unweathered London Clay. Thicknesses of the London Clay range from 29.65m (BH401) to 0.30m (BH410T). The weathered London Clay can be described as stiff thinly laminated extremely close to fissure brown clay with rare bioturbation fissures randomly oriented whilst unweathered London Clay can be described as very stiff thinly laminated close to fissured grey-brown, grey, brown clay with occasional bioturbation, fissures are randomly orientated with occasional black mottling. The base of this stratum presents occasional pockets of light brown 20 silty fine sand (<20 mm), dark grey silt (<15 mm) and rare pyrite nodules. With depth, it showed some beds of grey silty fine sand, very stiff sandy clay, grey claystone strong. Near the contact with the Upper Shelly Clay, the colour changes to grey-green very silty clay with many shell fragments. 60 SPT tests were made in total, 1 of which was made using Split Spoon Sampler (S) and 59 using Solid Cone Test (C). The N value in this stratum varies from 12 to 82, with an average of 37. According to density index of sands classification made by Craig (2004), it can be considered as a dense soil. Looking at the STP results, it is clearly shown that N-value increases with depth.
With regards to the Index Test, 30 tests were completed and the results were: % LL from 50 to 88, % PL from 19 to 43, and % PI 29 to 58. Then, these results were plotted, as it can be seen from figure 7, the majority of the results are located in the range of high to very high plasticity clays. Just one isolated point is located below the A-line, so it was ignored in this case. The moisture content varies from 8 to 35%. , the natural moisture content is considerably close to the plastic limit, in some cases is less than the plastic limit, which indicates that the soil is in a plastic to semi-solid state. The bulk density of this stratum varies from 1.90 to 2.10 Mg/m3. They were made 46 triaxial tests to determine the undrained shear stress of this stratum and the results were from 46 to 394.
Typically, Lambeth Group is comprised of three subgroups namely Woolwich Formation (Upper Shelly Clay, Laminated Beds and Lower Shelly Clays), Reading Formation (Upper Mottled Clay and Lower Mottled Clay) and Upnor Formation (Pebble beds and Glauconitic Sands as shown in table 3. The clays of the Upper Mottled Clay have higher plasticity than those of the Lower Mottled Clay and there are 7 blows of SPT for Lower Mottled Clay and 31 for the former making Upper Mottled Clay a very hard stratum. The SPT values are 45 to 76 and indicate a large distribution in sample strengths and the design line for Glauconitic sand is N = 64 + 22.63z. The Lambeth Group is generally very plastic and has a consistency of between -7 to 185. In BH408 at -17.69m depth, CI is -7 and can be inferred to be in liquid state being extremely soft and has eligible shear strength. The average N60 values (which was calculated from the average N-value) is between 37 to 73 making Lambeth group to fall in the very stiff to hard range. The average N-value is 45 to 76 and the relative density is from dense to very dense. Considerable high values of Undrained Triaxial Test are also indicative of the strength of the Lambeth Group.
Thanet sand lies beneath the Glauconitic Sand (Lambeth Group) and is appeared in BH404T to BH419 with thickness ranging between 10.50m (BH 411) to 12.50m (BH 417). Thanet sand strata has very dense grey dotted black slightly fine to medium SAND with rare fine sub rounded rock gravels, which is slightly silty(<30mm) with grey clay. Plasticity Chart in figure 7 indicates clay of low plasticity from soft to weak with Moisture Content almost equal to Plastic Limit. SPT was not undertaken showing the strata are weak.
Chalk formation has maximum thickness of 8.60m occurring in boreholes 407T, 410T, 411, 415T and 418 below Bullhead Bed. The stratum was unproven, characterized by the presence of white slightly to moderate weathered CHALK, weak, fractures closely spaced, infilled up to 9 mm with comminuted chalk. Occasional rounded flint cobbles as angular fine gravel fragments. Chalk recovered as fine gravel sized white very weak with 31 much matrix of with sandy silt sized fragments. It was identified as chalk grade III and IV. Considering the SPT results in figure 5, they were made 7 tests in total using Split Spoon Sampler (S). The N value in this stratum varies from 61 to 85, with an average of 71. According to density index of sands classification by Craig (2004), this soil can be considered as a Very Dense Soil. SPT, index test and moisture content, triaxial test and bulk density test were not applied to Chalk samples.
Digimap.edina.ac.uk,. (2016). Digimap Home Page. Retrieved 6 December 2016, from http://digimap.edina.ac.uk
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