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Table 7.2 Tunnelling and deep excavation projects and references

7.4 FOUNDATIONS

7.4.1 Deep foundations

Within the Lambeth Group deposits the choice of whether to found on, or to penetrate, hard layers within a weaker medium is made more difficult by the impersistence, variability in thickness and strength, and unpredictability, of such layers. These layers may be in the form of shelly limestones (Lower and Upper Shelly Clays of the Woolwich Formation), calcretes (Upnor Formation and Lower Mottled Clays) or silica-cemented gravels and sands (e.g. Upnor Formation). A 1 m thick layer of limestone nodules (calcrete) within the Lambeth Group was successfully used as a founding medium for some 2000 kN capacity 600 mm x 16 m continuous flight auger piles on the South Quay Plaza (Phase 2), Isle of Dogs (Solera, 1998). Other piles were founded in underlying Lambeth Group sands. In central London, under-reamed piles have been successfully foundered in the Upper Mottled Clay, Reading Formation.

The de-stressing of the Lambeth Group clays following excavation can result in de-structuring, swelling, and softening. Thus the relationship between strength, bearing capacity, and depth is important in the design of foundations as is timely construction after pile boring, and control of surface water. Although in central London where the water table has been lowered, the Lambeth Group is generally considered to be under-drained by the Thanet Formation and Chalk. Water bearing sand units such as sand channels in the Laminated Beds and Upnor Formation gravel should be carefully monitored during the site investigation phase of projects involving deep pile foundations. If high water pressures are encountered then actions such as bentonite support introduced towards the base of the London Clay Formation, should be considered to prevent collapse of bored piles.

Canary Wharf was one of the largest developments in Europe. Started in the late 1980’s it has expanded across many of London’s 19th and early 20th century docks. Notable infrastructure developments have included the Docklands Light Railway (DLR) and a network of roads, whilst preserving much of the waterway system. Fill, Alluvium, Terrace Gravel and Lambeth Group strata underlie the centrally situated Canary Wharf site in the West India Docks area of the Isle of Dogs. Here, the Lambeth Group is approximately 12 m thick, with typically two-

Project name Reference Subject Thames Tunnel Skempton and Chrimes, 1994 Engineering and geology Rotherhithe Tunnel Tabor, 1909 Engineering Blackwall Tunnel O’Reilly, 1997 Construction, ‘scour hollows’ Limehouse Link – (A13), Canary Wharf to City of London

Stevenson and DeMoor, 1994 Design and performance

Victoria Line Follenfant et al., 1969 Jubilee Line Extension (JLE) Linney and Page, 1996

Burland et al., 2001, Batten et al., 1996

Engineering geology Construction

Docklands Light Railway (DLR) Lewisham Extension

Sugiyama et al., 1999 Tunnelling

Channel Tunnel Rail Link (CTRL)

Beckwith et al., 1996 Whittaker, 2004

Ground investigation Groundwater control

Crossrail Lehane et al., 1995 Lithological variability

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thirds of this consisting of Reading Formation deposits. The Lambeth Group strata are of uniform thickness across most of the site, but limestone and gravel layers are intermittent. As a result of the heterogeneity of the strata, it was found that SPT data best characterised the engineering properties (Troughton, 1992).. Considerable seepage from the gravel bed in the upper beds of the Upnor Formation, and the Thanet Formation beneath was managed by using bentonite and casing (Troughton, 1992). Major buildings were founded in the Thanet Formation, while smaller structures and roads, usually by driven piles, and cofferdams were founded within the Lambeth Group. Some driven piles reached refusal in the limestone/marl layers.

Large diameter bored piles were successfully used under dry conditions at the British Library site in Euston, London, where shaft adhesions of over 200 kPa were achieved in Lambeth Group clays and sands at between 9 and 13 m depth (O’Riordan, 1982). Design compressibility and permeability parameters for the clays and sandy clays of the Lambeth Group for the Royal Albert Dock Spine Road (RADSR) are given as mv = 0.15 m2/MN, cv = 10 m2/yr, and k = 1 x 10-8 m/s (Card and Carter, 1995). Other examples of pile design and tests are given in Hight et al. (2004).

7.4.2 Shallow Foundations

The lack of publications on problems associated with shallow foundations on the Lambeth Group indicates that their construction generally presents no major difficulties. Those settlements that have been documented indicate that they are about half those generally found for the London Clay Formation (Morton and Au, 1974). Nevertheless, shrinkage and swelling of high plasticity Reading Formation clays, and possible instability in the Upnor Formation gravel or loose sand beds in any unit, should be considered during the site investigation and design stages.

Mobilisation of pyritic material within the lignite layers of the Lambeth Group can give rise to sulphate-rich groundwater and consequent local damage to foundation concrete, after oxidation. Deposits with a large proportion of plant remains may provide poor and variable foundation conditions.

7.4.3 Summary of key issues for foundation design in the Lambeth Group

Deep foundations:

Control of groundwater (for example by site specific designed de-watering),

Founding-on, or penetrating, strong layers or lenses (i.e. shelly limestone within the

Lower and Upper Shelly Clays of the Woolwich Formation and calcrete in the Upnor

Formation and Lower Mottled Clay),

Sulphate attack of concrete foundation in the Woolwich Formation,

De-stressing, heave and softening of clays in excavation.

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Shallow foundations:

Shrink/swell in clays:

Related to

o Seasonal moisture content changes,

o Desiccation due to trees,

o Heave due to removal of trees,

o Swell due to leaking drains.

Softening of clays in the presence of water bearing sand beds.

7.5 EMBANKMENTS AND USE AS FILL MATERIALS

Due to the highly variable lateral and vertical nature and extent of the Lambeth Group lithologies and their relatively thin development, fill materials derived from these deposits are likely to be composed of more than one unit and lithology. Therefore, use of the Lambeth Group as an engineered fill will require a good knowledge of the lithologies present and available at a potential source area and their strength/compaction characteristics. During the construction of the Newbury bypass (A34) the Lambeth Group provided a good source of fill for embankments and landscaping when emplaced in the correct condition, but low plasticity clays and silty sands typically proved to be highly sensitive to changes in moisture content. Acceptable criteria for use as engineered fill should be ascertained during the planning, investigation and construction phases (Hight et al., 2004).

Data on embankments constructed from the Lambeth Group is sparse partly because it has a relatively small outcrop. A survey of the motorway networks by TRL (Perry, 1989) found failure of 7.6% of embankments constructed from Lambeth Group deposits (undertaken prior to revision of the stratigraphy, the survey reported ‘Reading Beds’ as ‘Eocene’, rather than Palaeocene in age). This was second only to the Gault Formation. Typically failure occurred on 1 in 2 slopes within 22 years of construction. Failure modes include not only slope failures but also tension and shrinkage cracks, excessive settlement, water seepage and erosion of the toe. The survey noted that drainage ditches on the slope itself contributed significantly to reducing the number of failures. The maximum allowable embankment slopes assessed during this survey are presented in Table 7.3.

Table 7.3. Maximum slope (vertical to horizontal) allowable for embankments constructed from the Reading Formation to reduce failure to below 1% within 22 years of construction (Perry, 1989).

Maximum slope

Slope height (m) 0 - 2.5 2.5 - 5.0 >5.0

Fine grained 1:3 1:4 1:4

Coarse grained 1:1.75 1:1.75 1:1.75