Effective stress strength of clays evaluated from piezocone and flat dilatometer tests

Abstract

During geotechnical site investigations involving clay deposits, results from piezocone penetration tests (CPTu) and flat plate dilatometer tests (DMT) are traditionally interpreted using a total stress analysis, and consequently the evaluation focuses on the undrained shear strength (su). In actuality, the fundamental strength of clays is really governed by effective stress conditions, specifically the effective stress friction angle (f') which is required for stress path analyses, critical state soil mechanics (CSSM), and the prediction of pore pressures during construction using finite element analyses. This research program offers the interpretation of f' from in-situ CPTu data for clays and clayey silts of normally consolidated to lightly-overconsolidated to intact overconsolidated and fissured overconsolidated clays. Towards this purpose, an existing effective stress limit plasticity developed in Norway is modified, applied, and statistically verified by examining several databases (lab chamber tests, centrifuge models, and 110+ field case records) involving clay soils subjected to both CPTu and triaxial compression tests. This research program also establishes a nexus between CPTu and DMT in soft to firm clays through spherical cavity expansion (SCE) theory, thus provides a means for evaluating f' in clays using DMT data. Two databases are prepared: (a) results from 49 clay sites to validate the CPT-DMT link; and (b) results from another 46 clay deposits with companion DMT-TC information. Finally, an effort is undertaken to assess the undrained rigidity index (IR) of soft to firm clays from DMT data which can evaluate su, yield stress, and coefficient of consolidation based on a hybrid SCE-CSSM analytical approach.