Dewatering the Thanet Beds in SE London: three case histories

The construction of the Jubilee Line Extension in southeast London required the construction of a number of deep excavations extending into the Thanet Beds. These Lower Tertiary strata are predominantly silty fine sands, and in order for the excavations to be carried out in the dry, dewatering was necessary. Three examples of excavations in the Thanet Beds are described in this paper, in each of which dewatering was attempted by pumping from the underlying Chalk to induce drainage of the Thanets. Although the three sites were only approximately 1 km apart, the performance of the dewatering schemes was different at each site. One excavation proved impossible to dewater by this method, whilst the other two, although successfully completed, demonstrate very clearly the extreme variations in the permeability of the Chalk and the difficulties that may arise therefrom.