BTS MEETING | DEWATERING STRATEGIES FOR LONDON
Right, figure 3: Television Centre Redevelopment: section diagram
mOD 10
Recharge well both as means of disposal and to maintain groundwater levels
Sheet piles: not full perimeter due to listed retained facade
Made Ground 5 Alluvium
Existing basement broken out and lowered
Abstraction well
0
RTD
London Clay -5
influence, high flow, and high well-yields. However,
flows are dependant on the jointing, and where there is limited or no jointing there may be minimal flow.
Below, figure 4: Thames Tideway East DEPCS CSO during the excavation
CUT-OFF STRATEGY IN RIVER TERRACE DEPOSITS (UPPER AQUIFER) This strategy was used in the construction of Heathrow Terminal 5 in 2002, and is based on cutting off the groundwater to control inflow to the construction area. The geological sequence at Heathrow is illustrated in figure 2 and as can be seen the excavation in River Terrace Deposits is dry. This was achieved by the presence of a historic slurry wall which was subsequently extended
around most of the area of the excavation to provide a cut-off through the River Terrace Deposits into the London Clay. The trapped groundwater within the cut-off was removed by pumping prior to excavation.
ABSTRACTION AND RECHARGE STRATEGY IN RIVER TERRACE DEPOSITS Where a full cut-off is not feasible, abstraction, sometimes with recharge to the same aquifer, may be an alternative solution. This was the case for the redevelopment of Television Centre in White City, London. This had a listed retained façade and the existing basement was to be deepened. There was limited sewer capacity for discharge and creating a complete cut-off around the structure was not possible. The strategy adopted was to install a sheet pile cut-off around part of the basement and to use internal abstraction wells and external recharge wells (figure 3). Recharge is an increasingly important aspect
of groundwater control schemes because it is an alternative means of disposal, avoiding the cost of discharge to a sewer or the consents required for discharge to surface waters. In the right circumstances, it can provide mitigation for surface settlement or adverse environmental impact.
DEWATERING STRATEGY WHERE THE LONDON CLAY IS NOT PRESENT At the Tideway CSO at Deptford Church Street, the London Clay and Lambeth Group had been removed by erosion, and excavation was through the River Terrace Deposits to just above the Thanet Sand. A secant pile retraining wall was installed into the Thanet Sand with dewatering from the Thanet Sand to control
14 | November 2021
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