Implicit scheme for integrating constitutive model of unsaturated soils with coupling hydraulic and mechanical behavior

A constitutive model of unsaturated soils with coupling capillary hysteresis and skeleton deformation is developed and implemented in a fully coupled transient hydro-mechanical finite-element model (computer code U-DYSAC2). The obtained results are compared with experimental results, showing that the proposed constitutive model can simulate the main mechanical and hydraulic behavior of unsaturated soils in a unified framework. The non-linearity of the soil-water characteristic relation is treated in a similar way of elastoplasticity. Two constitutive relations are integrated by an implicit return-mapping scheme similar to that developed for saturated soils. A consistent tangential modulus is derived to preserve the asymptotic rate of the quadratic convergence of Newton's iteration. Combined with the integration of the constitutive model, a complete finite-element formulation of coupling hydro-mechanical problems for unsaturated soils is presented. A number of practical problems with different given initial and boundary conditions are analyzed to illustrate the performance and capabilities of the finite-element model.