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

Banks, V.J.; Bricker, S.H.; Royse, K.R.; Collins, P.E.F.. 2015 Anomalous buried hollows in London: development of a hazard susceptibility map. Quarterly Journal of Engineering Geology and Hydrogeology, 48 (1). 55-70. 10.1144/qjegh2014-037

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Abstract/Summary

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. This paper presents a new British Geological Survey (BGS) geographic information system (GIS)-hosted hazard susceptibility map for naturally occurring, buried (sediment-filled) hollows in London that are commonly referred to as ‘drift-filled hollows’ or ‘rockhead anomalies’ in the literature. Here they are referred to as ‘buried hollows’. Characteristically, these anomalous features comprise zones of disturbed ground that are buried beneath and extend up into the superficial cover. The method behind the GIS map layer is described, along with its limitations and potential applications, including the implications for process understanding with respect to the buried hollows. A number of the buried hollows found in central London (Fig. 1) are associated with deep zones of disturbance in the bedrock geology (London Clay Formation, Lambeth Group and in a few cases the White Chalk Subgroup of the London Basin; see Tables 1 and 2 and Fig. 2). Usually they have asymmetric, funnel-shaped forms and are typically 15–25 m deep, but locally the zone of disturbance or ‘root’ of the features may be considerably deeper; for example, up to 33 m in Battersea and over 60 m in Blackwall (Ellison et al. 2004). They are normally buried beneath, and partially in-filled with, sands and sandy gravels of probable Devensian age. Sediment fill is of silt to boulder grade material, predominantly comprising flint gravel, but with bedrock (Chalk, Lambeth Group or London Clay) injections, diapirs or mélange recorded in some instances (Berry 1979; Lee & Aldiss 2011). An associated bulging of the underlying strata has also been observed at some locations (Berry 1979; Ellison et al. 2004). Further information on the characteristics of the documented buried hollows is summarized in the Appendix. The buried hollows can be subdivided between those that are underlain by London Clay and those with roots that extend below the London Clay (Appendix, Tables A1 and A2 respectively). However, absence of proven connectivity may be due to the nature of the site investigation and should not be taken as proof of absence of connectivity between the near surface and the Chalk.

Item Type: Publication - Article
Digital Object Identifier (DOI): 10.1144/qjegh2014-037
ISSN: 1470-9236
Date made live: 24 Mar 2015 13:51 +0 (UTC)
URI: http://nora.nerc.ac.uk/id/eprint/510428

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