Recent work in the Thames basin and East Anglia has shown that the reactivation of buried structures is likely to have influenced patterns of thickness and fracturing in the Chalk. Although not previously recognised, these findings are consistent with the relatively shallow depth of faulted geological basement beneath the London and East Anglian region. Borehole geophysical logs allow recognition of formational units within the Chalk that are characterised by contrasting physical properties related to their original environment of deposition (for example, hardness, nodularity, clay content). These logs have been used to trace out the subsurface development of chalk units across southern England, including East Anglia where the Chalk is still poorly understood. The results of this work have been compared to data showing evidence for the presence of deeply buried structures, such as regional gravity and aeromagnetic anomaly maps. Changes in thickness of the Chalk can be seen to occur sharply across the Glinton Thrust, and more generally across the faulted eastern margin of the Midlands microcraton, north-east of London. Unusually thick chalk in parts of East Anglia may point to interaction of geological structure and ocean circulation influencing plankton productivity in the seas that formed the Chalk. Boreholes straddling some inferred fault lineaments in southern East Anglia appear to show changes in fracture frequency, with potentially important implications for understanding how groundwater flows through the Chalk.
For more information contact Mark Woods.
In the design of major construction works, the better the ground conditions are understood, the more control there is on the assessment of risks for construction, contract and personnel, and ultimately on final costs. In parts of London, faulting introduces lateral heterogeneity to the local ground conditions, especially where construction works intercept the Palaeogene Lambeth Group. Although faults are probably widespread and numerous in the London area, few are shown on the geological maps of the area. This can make ground investigation data difficult to understand.
This paper discusses a successful resolution of this problem at a site in east central London, where a new underground railway station is planned. A 3D geological model was used to provide an understanding of the local geological structure in faulted Lambeth Group strata, which had not been possible by other commonly used methods.
The model was constructed in GSI3D using about 145 borehole records, including many legacy records, some of significant age. GSI3D modelling provides the means to evaluate such records in their 3D real-world context and so to use them (or not) with more confidence than is otherwise possible. Initially, a geological model was constructed as if no faults were present. This initial model was then interpreted to show the presence of at least seven faults, with downthrows ranging from about 1 m to about 12 m. Close to the line of the tunnels, the position of most of the faults can be constrained to within an envelope less than 20 m wide.
For more information contact Ricky Terrington.
The Cotswold Hills, in which lie the headwaters of the River Thames, are renowned by geologists for their complexity. The Jurassic sequence present in the Cotswolds is formed of highly variable geological deposits which are both faulted and highly fractured. As a result it is difficult to assess the hydrology and groundwater flow mechanisms, making the management of water resources harder.
To enhance our understanding of the catchment hydrogeology a 3D geological framework model has been developed by BGS. Hydrogeological data such as groundwater levels have been integrated with the geology model to examine local hydrogeological detail against the background geological setting. With an improved understanding of groundwater flow in the Cotswolds a predictive groundwater process model is being developed to assess the impact of climate change on groundwater resources and river flows.
For more information contact Steph Bricker.
This paper looks at the geology of the Chalk beneath east London and the Thames Gateway and discusses its key features that affect engineering geology. In particular, the variable stratigraphy preserved beneath the sub-Palaeogene erosion surface, the evidence for syn-depositional tectonics in the Chalk and the recognition of tectonic fractures and strata-bound fracture systems are emphasised.
The contrasting physical properties of chalk and flint are discussed and the depth of weathering in the subcrop and outcrop are compared and contrasted. The information gained from separate ground investigations is combined to suggest that there are regions in east London where better quality chalk and less permeable ground are present between regions of poorer quality chalk with higher permeability, closely related to zones of faulting.
The initial screening tool (IST) has been developed to enable planners to assess the potential risk to ground and surface water due to remobilisation of contaminants by new developments. The IST is a GIS application that builds upon previous contaminated land screening tools developed by BGS through the inclusion of 3D geological data and an enhanced scoring methodology resulting in the ability to track individual pollutant linkages from a source of contamination, along multiple possible pathways to potentially susceptible receptors. The key outputs have been the pilot GIS application for the 2012 Olympic Park Site (developed using ArcGIS 9.3.1 and Visual Basic for Applications) and a publication in Computers & Geosciences.
For more information contact Andy Marchant.