The role of chalk in the development of buried (“drift-filled”) hollows

Drift-filled’ hollows are an increasingly recognised hazard in the London Basin. This largely reflects improvements in data quality relating to ground investigations for recent and ongoing development, together with improved access to this data in digital format. Potential problems linked to them include differential subsidence, tunnel face collapse, unexpected water ingress into excavations and possible contamination of groundwater. The hollows occur as large closed depressions in the rockhead – typically London Clay Formation, though sometimes penetrating down to the Chalk. They are infilled by a range of Quaternary (superficial) deposits including river gravel and sand, lacustrine silt and clay, and some organics. Where sample quality has been good, this infill often shows signs of post-depositional, and possibly syn-sedimentary disruption. The hollows’ distribution shows a rough pattern associated with valley floor geomorphology, current or former artesian conditions, thickness of the London Clay Formation, and presence of clay-rich units within the Lambeth Group (Banks et al. 2014). There may also be an association with geological structures. Consideration of the available deeper evidence shows that some, at least, of the hollows are associated with significant disturbance of the underlying ‘solid’ strata, including upwards folding and diapir-like intrusion of disaggregated chalk, together with vertical mixing of reworked Cretaceous, Tertiary and Quaternary materials. The genesis of the hollows is still under debate. This paper will propose a formation hypothesis, with a particular focus on the role of the Chalk. The break up and subsequent mobilisation of chalk will be examined and its possible contribution to the evolution of the hollows, and potential ongoing hazards, will be considered. Ongoing research will use the evidence within this paper to evaluate competing existing and forthcoming hypotheses.