Deformation simulations of sand under Bi-directional shear loading

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A well-developed sand plasticity model is further revised and validated based on bi-directional simple shear test data on medium to high relative density sands, which includes circular, oval and figure-8 type stress paths at different cyclic stress ratio (CSR) levels. The sand properties revealed from non-proportional loading tests, such as extended contractive behavior for medium to dense sands, flow rule dependency on plastic strain increment direction, stress increment direction dependency of plastic moduli, as well as the so-called non-coaxial property are incorporated into the new model development. Specifically, the recently proposed strain increment direction dependent flow rule is formulated and validated using bi-directional simple shear test results; and the relatively simple plastic moduli revised for the non-proportional loading are validated.After the model parameters are calibrated using a circular stress path, predictions are made for oval and figure-8 type stress paths at two cyclic stress levels. The modeling results show that the computed deformations up to a large strain state (i.e., 6% double amplitude strain) match fairly well with the test results; and the predicted pore-water pressure generation (or the effective stress change) followed the trend of test measurements including the repeating contractive and dilative cycles near the failure surface. A fabric dilatancy concept-based stiffness reduction approach is adopted that is active for the simulations of oval and figure-8 type stress paths.By comparing with test results, the model predictions demonstrate the capability of the present model in simulating the complex deformation magnitudes and patterns for sands under bi-directional loadings.