Engineers Australia

Evaluating effective stress parameters and undrained shear strengths of soft-firm clays from CPTu and DMT

P.W. Mayne


Results from piezocone penetration (CPTu) and flat dilatometer tests (DMT) can be used to evaluate the stratigraphy, soil types, and a suite of engineering parameters that are needed for geotechnical analysis and design, especially for finite element methods (FEM). Of particular interest herein is the utilization of in situ test data for assessing the effective stress strength envelope (c’ and f’) in soft to firm clays, as well as undrained shear strengths (su), since many FEM codes have built-in constitutive soil models that are based on critical-state soil mechanics and require effective stress parameters as input. An existing undrained limit plasticity solution for evaluating f’ in clays from CPTu is reviewed and then extended to the DMT via a link established through spherical cavity expansion theory. Laboratory and field results on soft Bothkennar clay at the British national test site and additional CPTu data in clays are used to illustrate the methodologies.