The population of London is around seven million; the infrastructure to support this makes London one of the most intensively investigated areas in the UK, where countless boreholes have been drilled for ground investigation and water supplies.
Yet subterranean construction work in London continues to reveal the presence of 'anomalous' ground conditions. These occurrences have been discovered in isolation, with no further work to explain them, yet such conditions can prove costly if not predicted by initial site surveys.
Thus there is a need to establish a refined geological framework for London, within which these 'anomalies' can be explained.
The Lambeth Group is a complex assemblage of clay, silt, sand, gravel and lignite, some of which may be shelly, deposited during the Palaeocene in a subtropical climate.
This complexity is due to the range of depositional environments including nearshore marine, estuarine, lagoon and broad floodplains, which moved in response to changing sea level.
During an extended period of low sea level, subtropical weathering altered the deposits to form duricrusts including calcretes, silcretes and ferricretes.
Although relatively thin (often between 10 and 20 m thick), the Lambeth Group's lithological variability and position in London, affects the investigation and construction of deep excavations and tunnels.
The variability of the Lambeth Group, can give rise to difficult ground conditions for civil engineering works.
The detailed distribution of the component units is so poorly understood, that these sediments are amongst the most difficult to engineer in the United Kingdom.
The Lambeth Group appears to be most complex under central London, where it occurs widely at shallow depth.
Understanding these deposits is crucial for major civil engineering projects, particularly those involving tunnelling, such as the Jubilee Line Extension, the Channel Tunnel Rail Link (CTRL) and the development of the Crossrail network.
Therefore there is a need for a better understanding of the group's heterogeneity, physical characteristics and distribution.
Researchers at BGS are currently using innovative 3D modelling techniques to understand the distribution of these sediments better throughout central London.
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