UC San Diego

A semi-empirical elasto-plastic model with a hyperbolic stress-strain curve was developed with the goal of predicting the seismic compression of unsaturated sands in the funicular regime during undrained cyclic shearing. Using a flow rule derived from energy considerations, the evolution in plastic volumetric strain (seismic compression) is predicted from the plastic shear strains of the hysteretic hyperbolic stress-strain curve. The plastic volumetric strains are used to predict the changes in degree of saturation from phase relationships and changes in pore air pressure from Boyle’s and Henry’s laws. The degree of saturation is used to estimate changes in matric suction from the transient scanning paths of the soil-water retention curve. Changes in small-strain shear modulus estimated from changes in mean effective stress computed from constant total stress and changes in pore air pressure, degree of saturation and matric suction, in turn affect the hyperbolic stress-strain curve’s shape. The model was validated using experimental data from undrained cyclic simple shear tests on unsaturated sand specimens with different initial degrees of saturation in the funicular regime. While the model captured trends in hydromechanical variables as a function shearing cycles well, it underestimated the measured volumetric strain and degree of saturation changes at large numbers of cycles in some tests. A linear decreasing trend between seismic compression and initial degree of saturation was predicted from the model while a nonlinear increasing-decreasing trend was observed in the experiments, which may be due to not considering post-shearing reconsolidation or experimental variations in cyclic shear strain amplitudes.