Constitutive modelling of bonded geomaterials subject to the mechanical and moisture degradation

Abstract

An effective stress based constitutive model is proposed for the fully coupled flow-deformation analysis of unsaturated bonded geomaterials. Using the multiphase mixture theory, the governing equations are derived based on the equations of equilibrium and the conservation equations of mass and momentum. The coupling between solid and fluid phases is enforced according to the effective stress concept taking suction dependency and volume change of the effective stress parameter into account. The hydraulic hysteresis is accounted for through the effective stress parameter and the soil water characteristic curve.

A bounding surface plasticity model is proposed to predict the nonlinear response of unsaturated bonded geomaterials subjected to complex hydro-mechanical loadings. Particular attention is given to suction dependency of hardening modulus, bond’s stiffness and strength degradation, collapse phenomenon upon wetting, and accumulation of plastic deformation due to cyclic loading. The effects of both stress magnitude and the accumulation of plastic deformation on the degradation of bond stiffness and strength are taken into account. In the model, the plasticity is decomposed into stress driven and suction hardening/softening components which enables capturing wetting induce collapse in unsaturated soils in a numerically robust manner. A non-associated flow rule is adopted to generalise application of the model to a wide range of geomaterials. A generalised mapping rule is also proposed to detect the correct location of the image point in the 3D principal stress space.

The constitutive model is then implemented into a finite difference code. The modified Euler scheme with an automatic sub-stepping feature is utilized for the explicit integration of the model. The problems associated with the effects of overshooting in the solution of boundary value problems are treated numerically. An explicit drift correction scheme is also applied to prevent the violation of the yield surface condition.

The key features of the proposed model are validated by comparing the numerical results with experimental data for various drained and undrained tests on unsaturated bonded geomaterials. The simulation results of several boundary value problems are also presented to demonstrate the capability of the model to capture essential characteristics of unsaturated bonded geomaterials under complex hydro-mechanical loading conditions.