Study On Yielding Characteristics And Aniso--tropic Yield Surface Equation Of Soft Clay

The study on stress-strain characteristics and constitutive model for natural soft clays subjected to static and dynamitic load is one of the fundamental issue in the field of soil mechanics. Structure and plastic anisotropy are the essential nature of naturally deposited soft clays, which have considerable influence on the strength and stress-strain response of natural clays. Moreover, principal stress rotation in the ground is the typical stress path under excavation, wave and vehicular loading. However the experimental and theoretical research in this aspect is less, and need enrich eagerly. Based on the former researchers’achievement, by experimental investigation and physical interpretation and theoretical formulation, the dissertation was focused on the study on the deformation behavior and strength character of natural Hangzhou soft clay undergoing principal stress axes rotation, combined with the structural yield characteristic and plastic anisotropy. A three-dimensional anisotropic bounding surface eqution for structured clay has been elaborated, which can describe simultaneously the effects of principal stress rotation.The main aspects and achievements of this thesis are as follows:1. Experimental investigations on yield characteristics of natural structural soft clay were performed systematically to study the influence of structure and anisotropy on deformation and strength behavior. The test items covered one-dimensional consolidation tests and undrained triaxial tests under isotropic modes and a series of drained stress probe tests. The test results show that the properties of natural Hangzhou soft clay are characterized by structural yielding and anisotropic plasticity. The initial state bounding surface of Hangzhou soft clay can be described by an inclined ellipse on the p’-q plane, and the limit state surface is not symmetrical as well with respect to the Ko line.2. Consolidated undrained tests on remolded and intact soft clay specimens were carried out using a hollow cylinder apparatus. Tests were conducted by maintaining a fixed principal stress direction with angleα from the vertical direction together with two different fixed value of intermediate principal stress coefficient b. It is observed that both the friction angle and normalized strength decrease with an increase inα, then a small growth. However, the variance is not significant for different remolded clay species, so the stress-induced anisotropy caused by changes of principal stress direction have a smaller effect on strength. Moreover, it is revealed that inherent anisotropy have a great influence on the stress-strain characteristics and pore pressure magnitude, and almost no effect on the pore pressure development mode.The result show that the anisotropy under rotation of principal stress axis have a great effect on yielding characteristics and strength, so the establishment of the anisotropic yield surface equation need considered inherent anisotropy.A three-dimensional anisotropic bounding surface model for natural soft clay was constituted, which can describe the loss of structure and stress anisotropy and the effects of principal stress rotation. Based on the experimental study on yield characteristics and the effects of principal stress rotation, the anisotropic bounding surface equation with a modified shape function of Lode angle was established. Validity of the model is verified by groups of test results for Hangzhou soft clay.