Anomalous buried hollows in London: development of a hazard susceptibility map

Abstract

Engineering works carried out in central London over many decades have revealed a number of buried hollows that exhibit curious characteristics. Some extend deep into the bedrock geology and are in-filled with disturbed superficial deposits and reworked bedrock. Others are contained within the superficial deposits. They can be up to 500 m wide and more than 60 m in depth. As the infill material often has different behavioural characteristics from the surrounding deposits failure to identify them during an initial site investigation can prove costly. This paper considers their common characteristics and describes the method used to develop a buried hollow hazard susceptibility map. This map provides planners with a broader awareness of the potential location of difficult ground conditions associated with them, thereby reducing the potential for unforeseen ground conditions through effective site investigation design. The paper continues with a discussion of some of the likely processes associated with their formation, which are attributed to cryogenic processes, and concludes with potential future research directions.

Buried Late Weichselian thermokarst landscape discovered in the Czech Republic, central Europe

“…Similar thermokarst processes and landforms were observed, for instance, on Bylot Island in northern Canada (Godin & Fortier ) and are suggested to be the driving mechanism for the formation of non‐ramparted, up to 15 m deep, elongated depressions (‘hollows’) described from the former periglacial zone of England (Berry ; Banks et al . ).…”

“…; Van Huissteden & Kasse ), northern Poland (Dylik ), southern England (Berry ; Banks et al . ), and northern France (Van Vliet‐Lanoë et al . ; Bertran et al .…”

“…This could be due to their low potential for preservation in the fossil record (French 2017) once the permafrost had thawed, such thawing reducing the terrain to the level of former depression floor (Ballantyne 2018). In Europe, nonramparted thermokarst depressions filled by lacustrine deposits have so far been described from several sites in the Netherlands and eastern Germany (Bohncke et al 1993;Van Huissteden & Kasse 2001), northern Poland (Dylik 1963), southern England (Berry 1979;Banks et al 2015), and northern France (Van Vliet-Lano€ e et al 2017; Bertran et al 2018). In contrast to N and NW Europe, knowledge on thermokarst processes and their importance in the genesis of the Late Pleistocene landscapes of central Europe remains poor (Czudek 1986(Czudek , 2005; Vandenberghe 2001;Z ak et al 2012).…”

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

“…The origin of DFHs is uncertain; proposed mechanisms (reviewed by Banks et al 2015) include fluvial scour (Berry 1979) and erosion of former pingos (Hutchinson 1980), which have been identified elsewhere in formerly periglacial areas of the British Isles. Pingos form in permafrost conditions either as open systems with expanding ice cores or due to upward movement of groundwater from taliks through artesian pressure (Mackay 1979) during periods of interglacial warming (Berry 1979), which in London corresponds most recently with the end of the Last Glacial Maximum [MIS 2] or Younger Dryas [MIS 1] (MIS is defined as 'Marine Isotope Stage', a set of standard subdivisions of the Quarternary based on marine oxygen isotopes).…”

Sinkholes and the Engineering and Environmental Impacts of Karst: Proceedings of the Fourteenth Multidisciplinary Conference

“…Buried hollows encountered in the Chalk can be a hazard for construction in the London region where they comprise closed depressions that are associated with ground deformation and may be infilled with softened and/ or an unpredictable range of sediments (e.g., Banks et al, 2015). Some of the buried hollows in London extend deep into the bedrock geology and are in-filled with disturbed superficial deposits and reworked bedrock.…”