A Multidisciplinary assessment of subsurface risk associated with drift-filled hollows

Context
Drift-filled hollows (DFH), or unexpectedly thick zones of weak, permeable soil and sediment surrounded by more
competent ground, present significant, poorly understood risk to construction projects in London and elsewhere. Around 40
DFHs have been identified across central London (Banks et al., 2014). Risk associated with them is growing as subsurface development including tunnels and deep basements is increasing (HS2, CrossRail2, Thames Tideway). Additional risk may
be created if subsurface works induce new instability. Not addressing this could lead to failure of nationally-significant
infrastructure, excessive unplanned costs, damage to groundwater resources and loss of life (Ellison et al., 2004; Toms et
al., 2016). Recently, there has been renewed interest in DFH following projects which have experienced problems (Queen
Elizabeth Park and Battersea). To date, research has been somewhat piecemeal and there is an opportunity for significant
novel insight to be gained through a dedicated PhD student alongside a network of interested parties.
Aim
This project will adopt a multidisciplinary approach to understand geometry and characteristics of DFH and surrounding
geological structures. Along with samples of geophysical data and further samples provided by industry partners an
assessment of past, present and potential future stability of a range of DFH will be made. Linked to this will be development
of a toolkit for use by the site investigation sector to enable assessment of new sites effectively in terms of the quality of
information and cost.
State of knowledge
DFH are closed depressions formed into the surface of the local rockhead infilled with unconsolidated sediments and soils
(Berry, 1979; Hutchinson 1980; Ellison et al., 2004; Toms et al. 2016). The hollows vary in depth, some are shallow and
others penetrate to the Chalk aquifer underlying London (Fig. 1). Larger hollows often have steep margins, possibly
associated with geological faults. Little is known about what underlies DFH, though some have intrusions of broken chalk
pushed upwards by an as-yet unidentified process. Several subsidence-related hypotheses have been proposed to explain
DFH including ground ice, river scour and sinkholes.
Methodology
The project will examine a DFH at Ashford Hill, Hampshire. Historical borehole records exist and the site has undergone a
pilot investigation using geophysical methods (Raines et al. 2015). Work will then be undertaken with the British Geological
Society (BGS) and other partners to extend this geophysical dataset and enable a 3D ground model to be developed. Sites
in London where material is held by the BGS and site investigation companies (part of the supervisor's network) will also be
studied. A range of methods will be applied:
Geotechnical characteristics and properties - determine sediment origin, behaviour and potential failure surfaces.
Environmental chemistry - carbon and oxygen isotopes supported by major element geochemistry to establish the role
groundwater has in activity and origin.
Plant macrofossils, molluscs and palynology - determine site history during infilling.
Radiocarbon dating - establish the age of the features (funding from the QRA/NERC will be sought).
Impact
Improving understanding of DFH will significantly reduce the risk of unexpected ground conditions for major infrastructure
projects. Research will also contribute to management of groundwater resources. Additionally, the toolkit of guideline for
site investigation will be of considerable use to industry and academia