Multiphase Deformation Analysis of Elasto-viscoplastic Unsaturated Soil and Modeling of Bentonite

Abstract

The deformation behavior of unsaturated soil has been the subject of numerous experimental and theoretical investigations. However, this phenomenon is not fully understood. Problems, such as the adoption of the proper stress variables, reduction of suction inducing collapse, suction effect on soil stiffness, rate dependency and air trapped within the soil under rainfall infiltration still need additional studies. In the present studies, an elasto-viscoplastic model for unsaturated soil is used based on two stress variables: 1) the skeleton stress is adopted as the stress variable; 2) suction is incorporated into the constitutive model to describe the collapse behavior. In addition, to investigate the multiphase behavior of unsaturated soil, a three-phase coupled model has been proposed based on the Theory of Porous Media (TPM) and finite deformation theory. Van Genuchten type of equation is employed as a constitutive equation between the saturation and the suction. Three-dimensional multiphase simulations are carried out to reproduce the behavior of unsaturated soil during monotonic loading triaxial tests under drained and undrained conditions for water and air. Compared with experimental results and the simulated results, it is seen that the proposed formulation is very suitable to describe the mechanical behaviors of unsaturated soil. Cyclic behavior of unsaturated soil has attracted much attention during the past few years. An elasto-viscoplastic cyclic model for saturated soil is extended for modeling of unsaturated soil. Based on finite deformation theory, three-dimensional multiphase analyses for unsaturated soil under cyclic loading are presented. The simulations are verified with cyclic triaxial tests on unsaturated silty clay under undrained for water and air conditions. It shows that the proposed multiphase formulation can be used to simulate the behaviors of unsaturated soil under cyclic loading. The high expansiveness of bentonite is another significant problem in unsaturated soil mechanics. In this research, an elasto-viscoplastic model for unsaturated expansive soil has been developed. An evolutional equation is adopted for describing the absorption of water into interlayer of clay platelets. In addition, the internal compaction effect caused by swelling of clay unit is expressed with the expansion of overconsolidation boundary surface and static yield surface. Based on the model, one-dimensional finite element analysis is conducted to study the development of swelling pressure. Compared with experimental results and simulated results, it is found that the proposed model can reproduce the effects of dry density and initial water content on swelling behavior. Using the proposed swelling model, two-dimensional swelling behaviors of the waste barrier are simulated.