Study On Properties Of Strength And Deformation Of Soil Based On Physical Characteristics

Soft clay widely distributed in the southeast coast of China and some inland cities along the river (such as Wuhan, Kunming, etc.). A large number of projects are carried in these cities. Although existing soil mechanics theories have some guidance for practice, actually, it is found that the effect is limited. Many theories about the strength and deformation of soil are developed based on the theory for continuum materials. Appling to the non-continuous and strongly-structured soil materials, they can not solve the practical engineering problems in a real sense. Most of the parameters in soil constitutive models have quite large difference, which will cause prodigious instability on the stress-strain result. Obviously, there are significant fault in the engineering practice. This thesis, considering the characteristics of non-continuous and strongly-structured soil materials, we explore a new method to describe soil behaviors by combining analytical tests of mesoscopic structure of soils with macro experiments of mechanical properties of soils under the condition of modern testing technologies such as the Shear Wave Velocity Test of Bender Element. At the same time, we establish a new stress state equation of soils, put forward a new strength criterion of soils and build a new method of predicting the compression characteristics of in situ structural soils. Finally we set up a new soil physical-mechanical constitutive model. The new theory are examined and verified by utilizing numerical analysis. This thesis has important theoretical significance and application value on clarifying the stress state, strength and deformation of structured soils in the engineering practice.The main aspects and achievements of this thesis are as follows:1、According to the discontinuity characteristics of soils, a new stress condition in soil masses is proposed by introducing the physical index:porosity. Comparative analysis indicates the inadequate of the existing strength theory such as Mohr Coulomb theory. Based on the mechanism analysis of the strength properties, a series of drained triaxial compression tests under different conditions were performed to quantitatively study the influence of the initial void ratio and plasticity index on the shear strength of remolded saturated clays. A new synthesized physical parameter ω, which simultaneously represent both the type and the condition of remolded saturated clays, is defined based on the test results in this work. The functional relationships among the parameters φd and peak deviatoric stress in Mohr-Coulomb equation and the parameter ω are established to develop a modified Mohr-Coulomb equation by considering physical properties of soil.2、Based on the one-dimensional disturbed state concept compression model of structured soils proposed by Liu and Carter, a new simplified parameter, relative void ratio Ev, representing the influence of variation of void ratio caused by sampling disturbance, is proposed. Through the consolidation tests of Hangzhou soft clay carried out with different initial vibration, quantitative relationships among field yield stress, filed compression destructuring index and relative void ratio are established. A new method considering the influences of changes of relative void ratio both on the field destrucring index and field yield stress are proposed.3、A theoretical analysis was conducted to determine the egg-shaped yield surface parameters of soil material. The relationships between parameters a, b, and d of the egg-shaped yield function and the initial plastic modulus are established. The soil shear wave velocity, a meso-scale parameter, was also introduced to establish the relationships among the parameters. Parameter a was determined based on the consistency of the value between shape parameter a and the tangent function of the internal friction angle. Through a series of laboratory tests, regression analysis was performed by using the egg-shaped yield function, which quantitatively indicates the correlations between the initial shear wave velocity, peak internal friction angle, and egg-shaped yield surface parameters.4、The structural yield stress p0is introduced into the formula of initial elasticity modulus to replace the atmospheric pressure pa. A new formula considering the impact of the structural characteristics of soils is proposed and the relationships between the initial shear wave velocity and the parameters in the formula are established. Based on the framework of the plasticity incremental theory, this thesis take the egg load function as the loading surface and the function of plastic work as the hardening parameter, use the associated flow rule, establish an egg-shaped elastic-plastic model of structural soil. The model parameters are determined through a series of laboratory tests. The soil shear wave velocity, a meso-scale parameter, was also introduced to establish the relationships among the parameters.5. Introduce the research status of elastic-plastic finite element analysis and establish the finite element program of egg-shaped elastic-plastic model by using the modified Newton-Raphson constant stifness iteration method combined with the Implicit return constitutive integration algorithm and the theory of displacement control method. Numerical simulation of stress-strain relationship under conditions of triaxial tests are carried out, which, to verify the reasonableness of the constitutive relations. Furthermore, a numerical example about the passive earth pressure of retaining structure is carried out. It is found that the new finite element analysis results are stable and reliable according to the comparative analysis.