CONSTITUTIVE RELATIONS FOR SATURATED SAND UNDER UNDRAINED CYCLIC LOADING (PORE PRESSURE, PLASTICITY)

An elastic-plastic model is proposed to compute the pore pressure, stresses and strains in saturated loose and medium dense sand under undrained, general 3-D cyclic loading. The model uses an incremental theory of plasticity formulation which includes the concepts of isotropic and kinematic hardening and a field of elastic-plastic moduli. This field is defined in deviatoric stress space by the size and orientation of a series of nested yield surfaces. The behavior of the sand under loading is characterized by the translation of each yield surface and the decrease (degradation) of the associated moduli.; The degradation (state) parameter used is the normalized excess pore pressure, u*, which is calculated from a modified version of the total deviatoric strain trajectory, (lamda)'. This modified (lamda)' takes into account the physics of pore pressure buildup in sands and is based on a simplified 1-D pore pressure model for cyclic strain-controlled tests. The pore pressure increase in undrained strain-controlled tests was found to be a function of only the number of cycles and the amplitude of the applied cyclic strain. The parameters of the elastic-plastic model, including the initial positions and sizes of the yield surfaces, are easily obtained from a series of undrained cyclic axial triaxial strain-controlled tests. The motion of the yield surfaces in space can be visualized using a two-dimensional simplified deviatoric stress space.; The 1-D simplified pore pressure model was verified by correctly predicting the pore pressure buildup for Banding and Monterey #0 Sands under axial triaxial, torsional triaxial and direct simple shear loading. The elastic-plastic model parameters were determined from a series of axial triaxial tests on Banding sand. Additional soil test results were predicted using the elastic-plastic model, and were verified by actually running the tests on the same Banding sand. The cyclic tests run include axial triaxial with variable cyclic strain, solid cylinder axial torsional and a combined axial and torsional triaxial test.; A method to correct the measured pore pressures for the effect of membrane penetration during undrained cyclic triaxial tests is also presented.