Elasto-viscoplastic modelling of unsaturated soils under static and dynamic loading in 3D stress space

Abstract

Consolidation, dynamic analysis, and wave propagation are some of the topics in geomechanics in which a complete characterization of coupling of the solid skeleton deformation and fluid flows is necessary for an accurate evaluation of material response. Dynamic behavior of soils is widely investigated in the past decades; however, they have mainly concerned the behaviour of dry or fully saturated porous media. Considering a three-phase continuum system which accounts for the interactions between the phases is crucial for investigating dynamic behavior of real soils which are invariably in an unsaturated state. Deposits located near the surface of the earth with relatively low water content, highly plastic clays which undergo changing environment or loose silty sands which collapse under wetting process are examples of unsaturated soils and experience severe situations especially under dynamic conditions. This thesis presents an elasto-visco-plastic flow-deformation model for dynamic analysis of unsaturated soils including mechanical and hydraulic hysteresis. Governing equations of fluid and solid phases are derived based on theory of continuum mechanics considering phase interaction, and nonlinear deformation of solid skeleton subject to dynamic loading. A numerical scheme is developed using a robust Finite Element method as the global solution to solve various boundary value problems. For the local solution, a comprehensive bounding surface viscoplastic model is presented for unsaturated soils which accounts for suction hardening and rate effects and can simulate monotonic and cyclic loading paths. Consistency condition theory is used to describe the viscosity behaviour of the material. A unique relationship between stress, strain, and strain rate of the material is also defined to perfectly describe the effect of the strain rate hardening. Several examples are solved to validate the model and demonstrate the capability of the proposed framework for investigating behaviour of soils in complex hydro-mechanical conditions.