DEWATERING STRATEGIES FOR LONDON | BTS MEETING
the inflow under the secant pile. Figure 4 shows the project during excavation with the internal pumped wells clearly visible.
PRESSURE RELIEF STRATEGY IN LONDON CLAY London Clay is relatively stable and typically does not require dewatering but sometimes pressure relief wells are needed in the strata below to control uplift pressure. This was the case at the auxiliary shaft at Limmo on the Crossrail project. Here, a 27m-diameter shaft to 40m depth was
excavated in London Clay to 5m above the Lambeth Group sands. The strategy was to install nine passive relief wells which leak excess pressure from the underlying sand strata into the excavation, with sump pumping used to remove the inflow. This strategy can be effective for excavations in London Clay with high pressure below.
DEWATERING STRATEGY IN LAMBETH GROUP The Lambeth Group is predominantly cohesive, and
at some locations is entirely cohesive, but contains occasional granular horizons or channel sands. The water-bearing granular horizons in the Lambeth Group range from laminations to channels as little as a couple of meters wide, up to 300m or more in some places, and can vary in thickness from a few millimetres to over 10m. Figure 5 shows an in-shaft dewatering system used
for the Tideway shaft at Heathwall. This dewatering system was installed after partial excavation of the shaft to control seepage flows in a Lambeth Group sand horizon in order to facilitate excavation and SCL support for the shaft.
SURFACE DEWATERING VERSUS IN-TUNNEL / IN-SHAFT DEWATERING Dewatering systems installed from ground surface have the following characteristics: ● Often limited access particularly in congested areas. ● It may be challenging to intercept sand channels or intermittent horizons.
● It is inefficient to install deep wells from the surface with short screen-length.
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Figure 5:
In-shaft wellpoints system in Tideway shaft at Heathwall
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