Studies On Elastic-plastic Behavior Of Cohensionless Soil During Load-decreasing Process

Constitutive relationship of soil plays a key role in soil mechanics. Ubiquitous volume-retraction during load-decreasing process is an important factor leading to liquefaction and failure of saturated sand, which could not be reasonably explained by existing elastic-plastic constitutive models of soil. Principal stress in the direction of zero principal strain has significant effect on strength and deformation of soil in plane strain state. In order to predict the principal stress in the direction of zero principal strain correctly, it is necessary to understand elastic-plastic behavior of soil during load-decreasing process. An elastic-plastic constitutive model proposed embodying yield characteristic during load-decreasing process is of special academic and practical value. Elastic-plastic behavior of cohensionless soil during load decreasing and reloading processes was studied and the main issues were summarized as follows.1. Based on experimental and theoretical studies, mechanism of elastic-plastic behavior of cohensionless soil has been explored. It was revealed that the reversible deformation is not elastic on occasion. Volume-retraction during load-decreasing process is a kind of plastic volume deformation due to shear stress decreasing, which represents yielding during load-decreasing process on a macro-scale. Yielding during load-decreasing process as characteristic of elastic-plastic behavior is introduced by the micro-structure change of cohensionless soil. Due to reverse friction, in load-decreasing process there is an initializing elastic region, which was decided by volume strain change in conventional triaxial compression tests and proved by both contrast tests and numerical simulations. A method has been proposed to obtain elastic moduli and device strain into elastic and plastic parts considering yield characteristic during load-decreasing process.2. Based on Tsinghua Elastic-Plastic Model of Soil, an elastic-plastic constitutive model embodying yield characteristic during load-decreasing process has been developed taking account of anisotropy induced by stress and effect of deformation history. In the model, yield surface and isotropic hardening axis rotate during load-decreasing process and the express of hardening mechanism adopts alterable equivalent stress point as initial state. In addition, yield surface and hardening parameters during primary loading, load-decreasing and reloading processes were obtained from experimental data respectively. The model is proved to capture elastic-plastic behavior which includes hysteresis loop of stress recycle and dilatancy during loading process and volume-retraction during load-decreasing process. It is an extension of the existing Tsinghua Elastic-Plastic Model of Soil to load decreasing and reloading processes.3. Elastic-plastic behavior and evolution of principal stress in the direction of zero principal strain during loading and load-decreasing process were studied in plane strain state. Principal stress evolution in the direction of zero principal strain is entirely determined by constitutive relationship of soil, which may be major, intermediate or minor principal stress in different conditions. The model embodying elastic-plastic behavior during load-decreasing process was applied to predict reasonably principal stress in the direction of zero principal strain compared with experiments. Through analyzing elastic-plastic behavior, some conclusions could be obtained as below. In order to ensure deformation and stress boundary conditions in plane strain state, yielding during load-decreasing process should be embodied. Principal stress in the direction of zero principal strain changes from minor via intermediate to major principal stress and tends to state of limit equilibrium eventually. The conclusions above provide a deep insight into elastic-plastic behavior of soil and plane strain condition.