DEWATERING STRATEGIES FOR LONDON | BTS MEETING


The London Basin is formed from chalk at the base with a succession of soils above. The chalk is a major water supply aquifer and is recharged at outcrops to the north and south of London. Between 1830 and about 1960, the lower aquifer was over exploited for water supply and this resulted in a big drop in the chalk’s water level. Since then, abstraction has reduced, groundwater levels have risen and are now controlled by managed abstraction for water supply and by construction dewatering. A simplified geological profile in London is shown in figure 1. The River Terrace Deposits (RTD) above the London


Clay form the Upper Aquifer. Below the London Clay, the Lambeth Group channel sands form the intermediate aquifer. The channel sands are frequently bounded above and below by clay with limited distant lateral recharge. This implies the potential for a large distance of influence and relatively limited inflow. The Thanet Sand forms part of the lower aquifer and


is overlain by Lambeth Group clays with a recharge boundary below from the chalk. This means the distance of influence may be limited by recharge from the chalk, and well yields are limited by the low permeability of the Thanet Sand. The chalk below is the major part of the lower aquifer. Groundwater flow in chalk is predominantly via joints.


Low k Med to high k


Alluvium RTD


Left, figure 1: Simplified geological profile in London


Impermeable


London Clay


Low k Low to med k Med to high k


Lambeth Group


Thanet Sand


Chalk


Pumping from the chalk underdrains the Thanet Sand. The chalk aquifer permeability typically reduces with depth. This implies the potential for a large distance of


Below, figure 2: Heathrow Terminal 5 Project


SGWL Terrace Gravels London Clay


November 2021


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