|Novellino, A, Terrington, R, Christodoulou, V, Smith, H and Bateson, L. 2019. Ground Motion and Stratum Thickness Comparison in Tower Hamlets, London. British Geological Survey Internal Report, OR/19/043.|
Urban areas are covered with a multitude of different types of artificially modified ground (AMG) which vary in character and geometry (Bridge et al, 2005, Bridge et al., 2010, Price et al., 2012, Burke et al., 2014). AMG have been mapped and studied extensively by the BGS, particularly as they impact on areas where there tends to be a large human population. Understanding the geometries and character of the AMG, and the interaction with underlying bedrock units improves the way in which the land is utilised for further development and how hazards, such as subsidence and uplift, are mitigated. Vertical motion is a major geological hazard that affects the stability of foundations and deep basements of buildings with the Association of British Insurers estimating that the average cost of shrink-swell-related subsidence to the insurance industry stands at over £400 million a year. In London, the London Clay has long been known as a major contributor to subsidence and uplift due its inherent characteristics for shrinking and swelling (Jones, 2011) which is affected by groundwater levels. The water table in London has risen up to 15 m since 2000 despite the London Licensing Strategy encouraging abstraction in areas of the aquifer where the pressure head is in the London Clay (Environment Agency – EA, 2018).
This study aims to identify any lithological control on the ground deformation detected by Interferometric Synthetic Aperture Radar (InSAR) and related to groundwater level changes.
The thickness and geometries of AMG and underground deposits derived from the London and Thames Valley 3D geological model have been considered as lithological parameters.
The Area of Interest (AoI) for this study is the London Borough of Tower Hamlets (Figure 1) because:
- An AMG thickness map was constructed there just prior to this study thanks to the availability of 6,353 boreholes in the 19 77 km2 occupied by the AoI.
- The London and Thames Valley London Lithoframe 50 model covers this area so the underlying modelled unit thicknesses and geometries could be considered (Burke et al, 2014). The London Basin 1:50 000 resolution 3D geological model covers a total area of 4,800 km2 in southeast England, from easting 450 000 to 570 000 and from northing 160 000 to 200 000 (Figure 1).
- InSAR ground motion data available back to 1992 has already shown that, historically, this area strongly undergoes ground elevation changes on short temporal scales (Cigna et al., 2015).
The datasets used to extract the thickness information for the AMG and the underlying units are described in Artificially Modified Ground and London and the Thames Valley Geological Model, respectively. The methodology adopted for the spatio-temporal comparison between the thicknesses and InSAR motions is shown in Methodology. Results details the results obtained considering both the average and the pattern of InSAR motion. Section 4 represents the discussion and conclusions of this work based on its main findings and limitations.
Artificially modified ground
Anthropogenic deposits are the material accumulations formed by human action, which along with human reshaping of the landscape through excavation and transportation of material forms part of AMG, deeply affecting the urban development of Tower Hamlets and the entirety of City of London (Terrington et al, 2018). Ford et al (2014) used a morphogenetic approach to classifying AMG into five mapped categories based upon morphological relationships:
- Made Ground: areas where material is known to have been placed by humans onto the pre-existing natural land surface, including engineered fill such as road, rail and canal embankments and dumps of dredged materials from natural river channels (e.g. Mudchute Park, Isle of Dogs).
- Worked Ground: areas where the pre-existing land surface is known to have been excavated by humans. In the study area it is dominated by excavations for the Docklands in Tower Hamlets, but also includes cuttings for the metro system and for ornamental lakes in Victoria Park;
- Infilled Ground: areas where the pre-existing land surface has been excavated and subsequently partially or wholly backfilled by humans. In the study area it is dominated by the infill of parts of the Docklands excavations in Tower Hamlets at Wapping, Canary Wharf and Isle of Dogs;
- Disturbed Ground: areas of surface or near-surface mineral workings where ill-defined excavations, areas of subsidence caused by workings, and spoil are complexly related. This is mainly associated with brickearth workings in the study area, but these deposits have been commonly buried by subsequent development and are now shown as Made Ground;
- Landscaped Ground: areas where the pre-existing land surface has been extensively remodelled but where it is impracticable to delineate separate areas of Made Ground, Worked Ground or Disturbed Ground. Landscaped Ground is not explicitly shown on published 1:50 000 scale geological maps of the area, with the exception of small areas of industrial development in Tower Hamlets, but is likely to be more extensive in areas where Made Ground is not observed.
Recent progress made by BGS and others around the world in this field has meant that AMG is increasingly mapped and modelled, and is now regarded by many as an important deposit or excavation likened to natural geological processes (Bridge et al., 2005; Bridge et al., 2010; Burke et al., 2014; Price et al., 2012; Zalasiewicz et al., 2011). Boreholes are an important resource for mapping the geometry and character of AMG, as these records preserve former landscape evolution inferring the thickness change from previous land levels and the start heights of boreholes (Terrington et al, 2018), and help indicate current thicknesses of AMG using logged core (Terrington et al, 2015).
The data, methods and processes used to calculate the thickness of the AMG are a continuation of those used in the Terrington et al work of 2018. This involved the following steps:
- Deriving the maximum thickness of each borehole log that has recorded AMG in the BGS Borehole Geology and Geotechnical databases.
- For those borehole logs without AMG recorded, the start height (height at which drilling was commenced and a measured ground level) was used as a proxy for land surface elevation change against a modern Digital Terrain Model (DTM) from which a pseudo thickness value could be calculated. For some areas negative values occurred for the thickness, which is where the modern DTM would show Worked Ground when measuring against the historical start height of a borehole. For those areas showing positive thickness values, this indicates areas of Made Ground or potentially even Worked and Made Ground.
- The results of the above were used to calculate a thickness map using ArcGIS using both Inverse Distance Weighting and Kriging functions and assess which is most suited to give ‘reasonable’ values.
Around 54.8 million m3 of AMG characterize the AoI with the spatial distribution of the deposit controlled by the proximity to the River Thames and variation in underlying geology with the highest values usually in the southern part of the AoI (Terrington et al., 2018).
AMG distribution in Tower Hamlets has a large variety of historical landuse and building types spanning from buildings of exceptional national interest (e.g. the Tower of London, Tower Bridge and Christ Church Spitalfields) and recent commercial/residential infrastructures following the closure of London’s docks in the 1960s and regeneration of dormant land began in earnest in the 1980s.
London and the Thames Valley geological model
The geological model for the units underlying the AMG was constructed using the GSI3D software and methodology (Kessler & Mathers 2004, Kessler et al. 2009). The superficial units were calculated in GSI3D, while the bedrock units were calculated in GOCAD using the Structural Modelling workflow as these were faulted structures. This model is intended for use at scales around 1:50 000, together with the corresponding DiGMapGB-50 geological map data. This model is not recommended for site specific studies or use, but gives a wider city to regional scale appreciation of geological structure and geomorphology (Figure 3).
In total, 64 superficial and artificial geological units were modelled (including mass movement deposits) for the London and Thames Valley Model from the surface to a maximum depth of several hundred meters (Figure 3). AMG was mapped in the model in 2D, but was excluded from the model calculation because there was insufficient data to constrain the base of these deposits (the Z elevation) and so produce a calculated volume. Hence AMG was calculated separately (Artificially modified ground). In total, 7174 borehole logs were considered, comprising both confidential and open access borehole data, plus geotechnical boreholes that were absent from the BGS Single Onshore Borehole Index (SOBI) and 922 cross-sections were constructed across the area of varying lengths and detail.
In the Borough of Tower Hamlets, the following superficial and bedrock units are present and were used in the comparison of the AMG thickness against the InSAR derived ground motion data (Table 1). Conventionally, superficial deposits are the youngest geological deposits formed during the most recent period of geological time, the Quaternary, which extends back about 2.6 million years from the present. They rest on older deposits or rocks referred to as bedrock.
|Inferred Age||Lexicon code||Full Name – category||Lithology|
|Holocene||ALV||Alluvium – superficial||Fluvial deposits of modern flood plains, consisting of clay, silt, sand and peat|
|Late Anglian – Devensian glacigenic and river terraces||RTDU||River Terrace Deposits (undifferentiated) – superficial||Sand and gravel deposits directly beneath alluvium|
|Late Anglian – Devensian glacigenic and river terraces||LASI||Langley Silt Member – superficial||Varies from silt to clay, usually yellow brown and massively bedded|
|Late Anglian – Devensian glacigenic and river terraces||KPGR||Kempton Park Gravel Member – superficial||Sand and gravel, with local lenses of silt, clay or peat|
|Late Anglian – Devensian glacigenic and river terraces||TPGR||Taplow Gravel Member – superficial||Sand and gravel, locally with lenses of silt, clay or peat|
|Late Anglian – Devensian glacigenic and river terraces||HAGR||Hackney Gravel Member – superficial||Sand and gravel, locally with lenses of silt, clay or peat|
|Eocene||LC||London Clay Formation – bedrock||Bioturbated or poorly laminated, blue-grey or grey-brown, slightly calcareous, silty to very silty clay|
|Eocene||LMBE||Lambeth Group – bedrock||Vertically and laterally variable sequences mainly of clay, some silty or sandy, with some sands and gravels, minor limestones and lignites and occasional sandstone and conglomerate|
|Eocene||TAB||Thanet Formation – bedrock||Glauconite-coated, nodular flint at base, overlain by pale yellow-brown, fine-grained sand that can be clayey and glauconitic. Rare calcareous or siliceous sandstones|
The alluvial deposits dominate the surface the near subsurface in the south of the AoI, and range between 1 and 9 m in thickness. Terrace gravels and Langley Silt member dominate the remainder of the northern half of the AoI. Both the London Clay Formation and Lambeth Group are thinning to outcrop in the south of the area, and becoming thicker and deeper to the north. The London Clay Formation averages 14 m in thickness, and at its thickest point in Tower Hamlets it is 30–35 m. The Lambeth Group averages 16.5 m in thickness, and at its thickest point is ~45 m (Figure 4). HAGR, RTDU and TPGR have the most heterogeneity in thickness.
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