Drift-filled hollows in Battersea; investigation of the structure and geology along the route of the Northern Line Extension, London

Drift-filled hollows (DFHs) are a major subsurface hazard for engineering in London. They are characterized by a steeply inclined cone-shaped hollow into (sometimes through) the London Clay Formation, filled with unconsolidated fine- to coarse-grained drift and often covered by terrace gravels, making them difficult to identify at the surface. Their origin remains uncertain but most probably formed towards the end of glacial epochs by meltwater scouring, perhaps of collapsed pingos. Usually associated with tributaries to the Thames, DFHs are particularly prevalent in the Battersea area, through which the Northern Line Extension (NLE) is to be built. This study uses 283 public borehole records and site reports to build a 3D geological ground model of two known DFHs in the Battersea area to develop a more complete understanding of their origin. We show that DFHs are probably older than previously assumed, dating from the end-Anglian (Marine Isotope Stage (MIS) 12), c. 300 kyr ago, before the deposition of the River Terrace Deposits. The two DFHs modelled fall into distinct types: a small shallow DFH that is probably a purely scour feature in origin, and a larger, deeper DFH that probably formed by the scouring of a perhaps fault-controlled pingo. It is unclear whether the faults controlled pingo formation passively by acting as a conduit for water, or in a more active sense by driving ground movements. Both DFHs represent a significant hazard for the NLE and require more detailed investigations to properly constrain their extent.