The role of faulting in London: How tectonic-scale processes influence site-scale geotechnical behaviour
Dr Richard Ghail, Reader in Planetary & Engineering Geology, Royal Holloway
Dr James Lawrence, Senior Lecturer in Engineering Geology, Imperial College London
EPSRC (CDT in Sustainable Civil Engineering)
Major faulting in London has only recently become recognised as a serious issue for geotechnical engineering projects here, since it increases uncertainty and detrimentally affects local mechanical and hydrogeological rock mass behaviour. It is imperative that these faults are themselves investigated and understood to better assess their impact on the engineering behaviour of near-surface lithologies that are interacted with by London’s infrastructure.
Unexpected ground conditions, detrimental to geotechnical engineering, have been identified across London, and appear to spatially coincide with major faults, implying a causal link. These major geological structures are poorly-understood and are currently not predicted by the current London Basin model, which itself predates plate tectonic theory and the recognition of major inversion in southern Britain.
Figure 1 – Interpretation by Morgan et al. (in review) of the known Wimbledon-Streatham-Greenwich fault system as a series of en échelon Riedel shear inherited from an underlying Variscan strike-slip fault.
The project aims to improve our understanding of faulting in London, and their impact on local geotechnical behaviour. To achieve this, Tom is linking tectonic-scale processes to engineering observations at the site-scale through three objectives:
1. The origin of faulting in the London Basin
The major faults from London are interpreted to have been inherited from structures in the underlying basement ~300m below the surface. These reactivated during the Late Cretaceous-Palaeogene and propagated into the overlying sediments (fig. 1). As multiple fault sets are present in the basement, their upward propagation has compartmentalised London into a series of discrete blocks.
2. Fault location and architecture in London
Extensive urbanisation and the presence of highly homogenous lithologies in the near-surface inhibits conventional structural mapping techniques typically employed by geologists. Additionally, the lack of historic hydrocarbon exploration in the region limits the availability of deep-penetrating data, such as seismic surveys.
To overcome this, Tom is creating a ground model of London’s subsurface to identify the distribution of faults and their structural architecture. The model is constructed by interpreting and digitising the large volume of shallow ground investigation boreholes available in London. Figure 2 demonstrates how data extracted from the ground model has been used to preliminarily indicate the presence of two faults in East London. The model will be progressively developed and constrained as additional data is input, with industry encouraged to share data from their sites.
Figure 2 – Rockhead map developed from a ground model of the Blackwall-Greenwich Peninsula area in East London, with both topographic and lithological variations used to infer two faults. This has been adapted from Morgan et al. (2019.b) and will be further constrained as additional data is input into the model.
3. The impact of faulting on local engineering behaviour
In addition to the uncertainty caused by their unknown distribution in London, faults will also mechanically weaken and hydrologically disrupt the rock mass locally. So far, work has been done on the mechanical implications for faulted hard rocks in southwest England (fig. 3) and in Pembrokeshire. Difficulties associated with the lack of exposure, the applicability of non-destructive testing, and the availability of geotechnical data representative of faulting has limited work undertaken on the softer sediments present in the London Basin. However, upcoming data analysis and fieldwork are being planned to overcome this.
Figure 3 – Schematic simplification of a fault core and the surrounding damage zone for a fault in southwest England. Morgan et al. (2018) identified the mechanical degradation caused by faulting using GSI. This was applied to a hypothetical scenario to demonstrate the localised detrimental impact on rock mass strength.
Fundamentally, the geotechnical community needs to move away from the simplistic ‘layer-cake’ interpretation for London and recognise that major structures have compartmentalised and disrupted the subsurface that we engineer in.
Tom’s research has identified the mechanisms behind major faults in London, with mapping being used to validate this and identify their near-surface locations. The implications of identifying a fault on-site in London are being investigated, in order to help industry predict and manage their presence.
- Morgan et al. (2018) Fault damage zones: Implications for geotechnical engineering near faulting. 15th Young Geotechnical Engineers Symposium, pp. 33-34. University of Surrey.
- Morgan et al. (in review) Structural inheritance in the London Basin: The role of reactivated basement faulting. QJEGH Special Publication: ‘London Basin Forum’
- Cosgrove & Morgan (in draft) A review of the deformation history of southern England, and its potential implications for basement structure behaviour beneath the London Basin. QJEGH Special Publication: ‘London Basin Forum’
- Morgan et al. (2019.a) The London Basin is not a basin: Rethinking a region through multiple lines of evidence. EGU General Assembly 2019. Vienna, Austria.
- Morgan et al. (2019.b) Developing a regional ground model of London to minimise site-scale uncertainty. 'Developing the Ground Model': 2nd EGGS Annual Conference. Cambridge: Quarterly Journal of Engineering Geology and Hydrogeology.
- Morgan et al. (2018) Faulting in London: Current research on the impact and origin of faulting in the capital. 1st Annual Early Careers Evening of the Engineering Group. Geological Society of London.
Youtube recording of talk: https://youtu.be/ncEWltZ-jE8?t=2523