Study On Rate Dependent Constitutive Model For Unsaturated Soils

Soft clay is widely distributed in various regions of our country.Its poor engineering mechanical properties bring many difficulties and challenges to geotechnical workers.The existing knowledge of the properties of unsaturated soil is very limited due to its complicacy.Present study of the properties of clay mainly contains two aspects:experiment and theory.Generally speaking,experimental research is the foundation;however,it is the key to propose an appropriate model from the theoretical point of view.The existing research shows that soil is obvious rate dependent,whose preconsolidation pressure and undrained shear strength both increase with the increase of strain rate.In this paper,based on the existing elastoplastic model,a rate-dependent elasto-viscoplastic constitutive model for unsaturated soils is presented,in which the effect of strain rate on the hardening behavior of soil is taken into account.The main work can be summarized as follows:(1)Based on the elastoplastic model,an elastic viscoplasticity model in one-dimensional state of unsaturated soils is proposed.The relationship between the strain rate and the preconsolidation pressure for unsaturated soil is provided based on the research of saturated soil.The coupled elasto-viscoplastic hardening equation of unsaturated soil is obtained by introducing the strain rate effect to hardening equation of unsaturated soil,and the coefficient of strain rate increment in hardening increment equation is derived.Combined with the liquid yield equation and LC yield equation,a one-dimensional elastic viscosity increment constitutive model,which can describe the loading rate effect and creep behavior,has been proposed.(2)The model is extended from one dimension to three dimension by introducing the extended Cam Clay model.Because the model considering the effect of shear stress,it can reflect the shear behavior of soils under different stress paths.(3)A fortran program is presented to simulate soil behaviour under different conditions including different stress paths and initial conditions.The simulation results are then compared with the existed experimental data.It is shown that the model can capture the basic behavior of soils,which verify the reliability of the model.