Drift-filled hollows in the London basin: characteristics, origins and assessment of risk

Abstract

Anomalous geology creates sub-surface risk to engineering projects. Within the London Basin, anomalous geological depressions, named drift-filled hollows (DFHs) have been identified and inducing sub-surface risk for the last century. DFHs are depressions identified at an anomalous depth to the local bedrock, infilled with largely unconsolidated sediment and occasionally local bedrock. The features range in size from 5-500 m wide and up to 75 m deep, narrowing with depth. Not visible from ground level, DFHs are often missed during site investigations which induces sub-surface risk on construction projects. Not addressing the features if and once encountered could lead to failure through differential subsidence, tunnel face collapse, impact on nationally-significant infrastructure, excessive and unplanned costs, damage to groundwater resources and loss of life. This thesis compiles information to create the largest DFH dataset of 89 features and their known characteristics. A multi-disciplinary approach is employed to further understanding of the physical characteristics of DFHs as well as to use an evidence-based approach to determine their origin and processes which have shaped their current form. The results illustrate the high level of variability between features and within a single feature. Data quality restrictions and methodological limitations are discussed in detail throughout. These limitations are then amplified with highly variable sediments, such as the infill of DFHs. Further outcomes of the research include furthering the understanding of London’s geology, not solely through identifying further anomalies, but through mapping the sub-surface of two differing areas and showing notable differences in depth to bedrock and presence and absence of strata within relatively short distances. A guide for the site investigation sector has also been created to readily identify and characterise DFHs and so reduce risk. All of the outcomes reduce sub-surface risk through an increased knowledge of the features and London’s geology as a whole. Finally, the enlarged dataset and the recognition of the high level of variability within a single feature and between features has enabled the existing and potential process hypotheses to be analysed and a new formation hypothesis is proposed for features with a diapir being formed through an increase of water and gas pressure.