Evaluation and prediction of anticipated depths of weathering (engineering rockhead) as a function of geomorphology in areas of chalk outcrop in southern England and northern France

Conceptual models of weathering in areas of chalk outcrop in southern England indicate that the depth of weathering and engineering rockhead are variable with respect to geomorphology. This variation is typically demonstrated by the transition from interfluve to valley with the depth of weathering changing as result of elevation and aspect. Valley axes are shown to have the deepest weathering, interfluves the shallowest and valley slopes a transitional depth with slope gradients on north-west facing slopes generally greater than south-east facing slopes. The main processes considered to have formed the weathering profiles in the Chalk of southern England are periglacial processes associated with seasonal freeze-thaw and mass movement in the last ice-age. In this study, analysis of historical site investigation data, including geophysical surveys, has allowed these conceptual models to be reviewed. This analysis has suggested that two physical transitions occur within the near-surface chalk rock mass that relate to geomorphology. The first, and deepest, transition is considered to represent an opening of discontinuities in the rock mass as a consequence of relaxation in the near surface due to unloading. The second transition is considered to represent an increase in discontinuity frequency in the rock mass due to formation of new discontinuities which progressively intensifies towards the surface. The base of the second transition is generally considered to be engineering rockhead for most engineering situations. Using the data reviewed, a model was developed to estimate the approximate position of these transitions based on relative ground surface elevation within a geomorphological domain. This model may be used to estimate the depth of engineering rockhead based on topographical data, such as a digital terrain model (DTM), in the absence of site investigation data for a site. This has applications for preliminary design where piled foundations or shallow tunnelling are being considered, and may also have hydrogeological applications for conceptualisation of aquifer properties.