Experimental study and constitutive modelling of the time-dependent stress-strain behavior of soils

Abstract

The time-dependent stress-strain behavior of soft soils is of great concern to geotechnical engineers for relevant projects. This thesis mainly focuses on both experimental and constitutive modelling studies of the time-dependent stress-strain behaviors of soils and applications. A new simplified Hypothesis B method for calculating consolidation settlement of a single soil layer and double soil layers with creep is also proposed and verified. The time-dependent behavior of remolded Hong Kong Marine Deposits (HKMD) is investigated by Multi-Stage Loading (MSL) oedometer test. The compressibility properties, the nonlinear creep during loading stages and nonlinear swelling during unloading stages are observed and analyzed. It is found that the nonlinear function, proposed by Yin (1999), is suitable for describing both the nonlinear creep and nonlinear swelling behaviors with a long term period. For the constitutive modelling of the time-dependent stress-strain behavior, the Elastic Visco-Plastic model considering Swelling (EVPS model) is improved to consider the nonlinear creep and swelling. The EVPS model is generalized into three-dimensional condition and implemented into finite element software. Then, the implemented EVPS model is verified by comparing the simulation results and laboratory test results or measured settlement data of Hong Kong International Airport in the field, published in previous literatures. The time-dependent stress-strain behavior of a plasticine material is investigated using MSL oedometer tests, the isotropic creep (1-D stress) tests (in both loading and unloading stages) and Constant Rate of Strain (CRSN) triaxial tests. The creep and the strain rate effects of the material are observed and analyzed based on the test data. It is found that Yin and Graham's logarithmic function can fit the creep data for the whole period including the initial period of time, and the strain rate effects are obvious on the shear strength of the plasticine material. A new simplified method based on Hypothesis B is proposed for calculating consolidation settlement of one single soil layer with creep under different stress-strain states. This new simplified method is verified by comparing the calculated results with the fully coupled finite element simulations using an Elastic Visco-Plastic (EVP) model and measured data from laboratory physical model tests in previous studies. It is found that relative error values of the new simplified Hypothesis B method are in the range of 0.37% to 8.42% with a correction factor {220}=0.8 used in the creep part. The new simplified Hypothesis B method is generalized to calculate the consolidation settlement of double soil layers for different stress-strain states under instant loading. Zhu and Yin method and US Navy method are adopted to obtain the average degree of consolidation for double soil layers. The calculated settlement of the new simplified Hypothesis B method and the results of finite element (FE) simulation are compared to evaluate the accuracy. It is found that the results from the new simplified Hypothesis B method adopting Zhu and Yin method are generally in good agreement with results from FE simulation, which verifies that relative error of Zhu and Yin method is within 11% for all calculated conditions of double layer soil profiles. As a result, Zhu and Yin method is recommended to obtain the average degree of consolidation of double soil layers. There are serious settlement problems for three reclamation projects of Binhai Garden in Guangzhou. The undisturbed clayey soil specimens, taken from the in-situ site of Binhai Garden, were conducted for determining the parameter values of compressibility and creep. Then, the new simplified Hypothesis B method using Zhu and Yin method and finite element simulation using a soft soil creep model and the improved EVPS model are used to predict the consolidation settlement. The construction period is considered in the calculation, which extends the applicable conditions of this new simplified Hypothesis B method. In the last chapter, main findings and conclusions of the study have summarized and listed.Recommendations for further are also presented.