Experimental mapping of elastoplastic surfaces for sand using undrained perturbations

Elastoplastic models are commonly used in modern geotechnical practice to numerically predict displacements, stresses, and pore pressures in large construction projects. These elastoplastic models use presumed functional forms for yield and plastic potential functions that are rarely obtained from experimental measurements. This research describes a simple experimental technique that can be used to obtain the slopes of the plastic potential and yield functions during shear based on the deformation theory of plasticity. The method imposes small perturbations in the direction of the stress increment by closing the drainage valve, thereby abruptly switching from drained to undrained loading conditions during plastic loading. Elastoplastic moduli are obtained immediately before and after the perturbations from the measured deviatoric stress, mean effective stress, deviatoric strains, and volumetric strains for the stress paths immediately before and immediately after closing the drain valve. During drained shear, samples were sheared while the mean effective stress was maintained constant. Combining tests performed at several confining stresses, the proposed method was able to map conventional isotropic yield and plastic potential surfaces and predict their evolution for a wide range of stresses. The proposed technique can also be used for kinematic yield surfaces and to develop new and more accurate elastoplastic constitutive models.