State-dependent induced dilatancy angle and hyperplasticity model for granular soils

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This study presents a novel constitutive model for granular soils within the framework of hyperplasticity theory. Based on fundamentally distinct mechanisms, two types of volume variations exist. The first type includes variations arising from changes in stress, accompanied by energy dissipation. The other type of variations is purely kinematic and can be measured using the induced dilatancy angle, which is transformed into a state-dependent form. The state-dependent induced dilatancy angle decreases with decreasing material density, which enables the simulation of the softening phenomena and weakens dilatancy. After attaining the critical state, the incremental behavior reverts to an isotropic state. The compression and unloading curves considering grain crushing are introduced, and the corresponding free energy functions are derived under varying densities. In the proposed model, the different mechanical properties of loose sand and dense sand are derived from the state dependency of the kinematic constraint. Toyoura sand simulations reveal that the proposed model is able to accurately describe the stress-dilatancy relation of varying initial densities and confining pressures under both drained and undrained conditions. Triaxial compression tests on Leighton Buzzard sand and Rumei rockfills with different densities and confining pressures also demonstrate the validity of the model.