Constitutive and Strength Theory

The strength and the constitutive relation of soil are the basic mechanics properties of soil, which are closely, linked each other. Hundreds of constitutive models have been proposed in recent years. In these models, the hyperbolic model(Duncan & Chang，1970) and modified Cam-Clay model(Roscoe, Burland,1968) are applied Widely. Some models proposed by Chinese scholars also has the suitable application (Yin Z, 1988; Shen ZJ, 1987).

Stress condition determines the strength of soil. The relationship between shear strength and normal stress at given plane was established by Coulomb. Then the theory of Mohr Circle was introduced, which can consider the effect of the maximum and minimum principal stress. In order to reflect the effect of intermediate principal stress, it extended to the failure surface stress space and many yield criterion, such as von mises yield criterion、maillard criteria、mastuck criterion were put forward. Another important factor which impacts the strength of soil is the pore water pressure. The effective stress method and the total stress method were introduced in order to consider this factor. In addition, the soil strength has also been influenced by soil structure,over-consolidation ratio,void ratio,water content,property of load,loading history and the rate of loading and other factors.

Both the strength and deformation, as for soft clay, will change over time. This is the rheology of soil. After the rheological theory was applied in soil mechanics, the reason of unstable long-term ground deformation and the sliding after long-term using of the slope are both revealed (Lin HD& Wang CC, 1998; Zhan ML, 1993). Admittedly, the complexity of the constitutive equation has brought difficulties in application. The disturbance of the construction and the climate changing, for example, will lead to the changing of Rheological Parameters of soil. So, regardless of the construction and the situation of specific engineering is undesirable. Therefore, how to simplify the complex theoretical model for the actual project conveniently and quickly is an urgent project.

The macroscopic law of strength and deformation of soil is directly related to the microstructure. Through the studies on the microstructure, the mechanism of the macroscopic law can be clearer understood, and thus the macroscopic law can be studied more accurately. There are progresses in this area not only at home but abroad. But because of the constraints of equipments, it is not universal. The integration of the macro and the micro of soil may lead to a new level in the study of physical characteristics.