On the seasonality of soil strength in CPTu investigation

Abstract

Increasing extremes in atmospheric drought associated with climate change raise growing concerns on the potential impact of climatic actions on the geotechnical stability of regional dykes in the Netherlands. Accurate quantification of seasonal soil strength variations under in-situ conditions is essential to assess this impact. Cone penetration testing (CPTu) is widely adopted to assess material strength, however, the influence of seasonality on CPTu data is largely overlooked in current practice. This contribution presents the results of CPTu investigations repeated at one regional dyke location under variable atmospheric conditions, ranging from winter wetness to severe summer drought. The observed differences in resistance and pore pressure between the CPTu data collected over different seasons highlight the impact of drought on soil strength. It is shown that conventional interpretation tool for saturated soils cannot be used straightforwardly to analyse the data. In the variably saturated zone, clear derivation of strength is hindered by current knowledge gaps. Possible interpretation of CPTu data for constant water content shear strength is discussed for both the saturated and the unsaturated zones in view of dyke stability assessment, with the aid of expected soil physical behaviour and parallel data from in situ hydro-mechanical monitoring.