Superficial Hollows and Rockhead anomalies in the London Basin, UK: origins, distribution and risk implications for subsurface infrastructure and water resources

Recent findings in London show that the subsurface is much more complex than expected,with a number of apparently anomalous features that present a direct hazard to infrastructure development and a risk to ground water management. Of these features, one of the least understood are the large superficial hollows which occur in the rockhead—in much of the London Basin, this is the top of the London Clay Formation—and which are infilled by a range of Quaternary deposits, principally alluvial sands and gravels deposited by the River Thames and its tributaries. The hollows range in size and shape. Several are a few hundred metres across and can be up to 40–50 m deep, though determining their exact form is problematic. The soil and sediment infill of the hollows differs substantially from the surrounding ground in terms of strength and drainage, as well as some differences in chemistry. This presents a real hazard to infrastructure as there is a potential for vertical and horizontal movement, flooding, as well as increasing the risk of contamination of the deeper aquifer. In the paper, the locations and characteristics of known hollows and deformed strata are reviewed and evidence for how they formed is reassessed, systematically considering different hypotheses (scour, ground ice, karst subsidence, seismo-tectonic). From this we consider the implications for continued development of subsurface infrastructure development, and for water resources.