Understanding the geological model in a chalk environment: an example from the High Speed Two (HS2) railway

Chalk is known to have a potential risk for natural cavities, thus developing an accurate geological model and documenting the location and nature of these geohazards is important to infrastructure projects. Building the geological model requires investigation of the geomorphology, engineering geology, geological history and behaviour of geological materials (soil and rock mass). The identification of dissolution features is based on geomorphological mapping, LiDAR surveys, intrusive ground investigation and surface geophysics, with the aim of understanding the geological environment and the associated risk. Peter Fookes’ geological model approach is illustrated with a case study from High Speed Two (HS2) railway line in the North Chilterns Area section. This section includes cuttings, embankments and viaducts that will be designed and constructed in chalk formations in an area where the risk of dissolution features is moderate to high. These features appear either as infilled features near the base of cuttings and embankments, or as void or infilled features, that are present immediately below the toe of foundation piles or along their shaft. The exposed ground conditions during construction of the cutting are presented and compared with the predicted geological model. In brief, the model could not predict accurately the shape of the surface between the chalk and the clay infill material, and the excavation revealed a geometry for the infilled dissolution features in the chalk with irregular patterns that would have not been possible to be predict accurately.