IMPLEMENTATION AND VALIDATION OF A HYDRO-MECHANICAL ELASTOPLASTIC CONSTITUTIVE MODEL FOR FULLY COUPLED ANALYSIS OF UNSATURATED SOILS

Abstract

Unsaturated soils are commonly encountered in natural soil deposits above the ground water table and in civil infrastructure construction as compacted soils. In earthquake prone areas, problems arise from dynamic loading of unsaturated soils. These problems have received increasing attention in geotechnical and geo-environmental engineering research in recent years. Many geohazards such as liquefaction, slope failures, and embankment collapse are triggered when unsaturated soils are subjected to dynamic loading. Although extensive work has been done to study the liquefaction behavior of unsaturated sands in the laboratory, relatively few numerical studies have been carried out to investigate the liquefaction potential of unsaturated sands under different degrees of saturation, relative densities, and initial effective stresses. Thus far, most of the developments in the numerical modeling of the dynamic response of unsaturated soils have occurred in relation to monotonic loading of unsaturated soils and the important effects such as elastoplasticity, hydro-mechanical coupling, and hydraulic hysteresis are rarely taken into account in the constitutive models used in these analysis procedures. In this dissertation, numerical investigation of the capability of a coupled hydro-mechanical elastoplastic constitutive model for unsaturated sands and silts (CM4USS) to predict the liquefaction potential of unsaturated sands at different degrees of saturation and confining pressures is first carried out. A design chart that can be used to evaluate the liquefaction potential of unsaturated sands is developed through numerical investigation on a series of undrained, stress-controlled, cyclic triaxial tests for Toyoura and Nevada sands under different degrees of saturation, relative densities and initial effective confining pressures. Then a hysteretic model for soil water characteristic curves (SWCCs) is implemented into U_DYSAC2, a fully coupled fluid flow-solid deformation finite element computer code. An unsaturated soil embankment subjected to base shaking is analyzed and the results obtained using the code with non-hysteretic (drying bound) and hysteretic SWCCs are compared to each other. CM4USS is then implemented into U_DYSAC2 and several numerical examples are used to verify the implementation. These examples demonstrate that the modified U_DYSAC2 is capable of predicting the dynamic behavior of unsaturated soils. Finally, the effects of the degree of saturation and relative density on the liquefaction potential of level ground unsaturated Nevada sand deposits subjected to base shaking is studied using the modified code. The simulation results are consistent with those predicted at a single element level, yet they provide valuable insight into the behavior of unsaturated sands in boundary value problems.