Interpretation of free-fall piezocone tests in clay using a simplified cylindrical cavity expansion solution

This paper presents the derivation of a simplified closed-form expression for the interpretation of the undrained shear strength of clay soil from free-fall piezocone tests measurements, based on an undrained cylindrical cavity expansion solution. The cavity expansion solution employs an elastic-viscoplastic constitutive model to describe the rate-dependent behavior of clay upon loading. The derived expression correlates the measured tip resistance and pore pressure with the undrained shear strength by using a cone factor and a power function to account for strain rate effects. Unlike existing formulas that entail calibration of empirical parameters, the proposed expression requires as input only the constitutive model parameters and the cone geometry, which all have direct physical meaning. The cavity expansion solution, on which the interpretation expression is based on, is validated via comparison of its results against published studies, and accordingly the accuracy of the proposed expression is benchmarked against centrifuge test measurements. We show that, despite the simplifications introduced in the derivation, the proposed expression is capable of reproducing results of centrifuge tests with reasonable accuracy.