Experimental and computational modeling of unsaturated soil behavior under true triaxial stress states

Previous elasto-plastic critical state-based constitutive models for describing the stress-strain behavior of an unsaturated soil are presented and numerically investigated. The basic framework of the existing models is an extension of the classical Modified Cam-Clay model, now defined in terms of four state variables: net mean stress, p = (sigma 1 + sigma2 + sigma3)/3 - u a; deviatoric stress, q = (sigma1 - sigma 3); matric suction, s = (ua - uw); and specific volume, v = 1 + e. Step-by-step procedures are outlined for integration of the constitutive relations in solving the incremental stress-strain behavior. Computational constitutive drivers are then implemented to allow for the numerical simulation of conventional triaxial compression (CTC) and triaxial compression (TC) tests. Simulations are obtained from both explicit and implicit, integration techniques. The computational algorithms support numerical analyses in a deviatoric (p : q : theta : s) stress plane, where theta (Lode-angle) is a third stress invariant, and by using a mixed control (stress- and strain-controlled) constitutive driver in conjunction with a Generalized Cam-Clay model within a constant-suction scheme. Additional integration levels are to be devised to accommodate the nonlinear nature of the function accounting for matric suction effects.; A servo-controlled true triaxial (cubical) testing device is modified to test 10.0 cm (4.0 in) per side, cubical recompacted specimens of unsaturated soil under varying matric suction conditions. Recompacted silty sand ( SM) specimens of Piedmont residual soil are tested following a multi-stage testing scheme by controlling the major, intermediate, and minor principal stresses. Matric suction states in the specimens are induced and maintained constant during true triaxial testing by using the axis-translation technique. Results from a series of drained, stress/suction-controlled CTC and TC tests are used for validation of the constitutive models proposed for an unsaturated soil under axisymmetric loading conditions, and the model enhancements proposed in the present work. In all cases, the incipient critical state condition was defined at approximately 12% of total shear strain. The agreement between observed and computed results in a recompacted silty sand (SM) is encouraging, suggesting that these models can be incorporated within FEM-based analyses of boundary value problems involving soils with significant changes in the negative pore-water pressure due to varying environmental conditions.; True triaxial testing data from a series of drained, stress/suction-controlled triaxial compression (TC), triaxial extension (TE ), and simple shear (SS) tests, conducted in the cubical testing setup, are used to evaluate the nature of major, minor, and intermediate principal strain response of 10.0 cm (4.0 in) per side, cubical recompacted silty sand (SM) specimens under varying matric suction conditions, and for shear stress paths not achievable in a conventional cylindrical apparatus. The influence of matric suction on the shape, size and position of the failure envelopes in the octahedral (sigma1 - ua : sigma2 - ua : sigma3 ua) and principal (sigma 1 - ua : 2 (sigma2 - ua) : 2 (sigma3 - ua)) stress planes is also analyzed from the true triaxial testing data.