On the basis of the practical deformation curve of the composite rock under a triaxial stress state and the detailed research about the Duncans constitutive model and the Salamons equivalent theory, we have put forward the constitutive model, which could describe the constitutive relation of the composite rock made up of many layers of rock possessing different thickness ratio under a triaxial stress state.
In such the projects as the civil engineering, the mining engineering, the water conservancy engineering, the railroad engineering and the building engineering, as we know, the pattern of the stress distribution in rock-mass must be researched by the analytical solution method or the semi-analytical solution method or the numerical solution method, so that we can discuss the steadiness of the rock-mass possessed complex constitution in the complex initial stress field. Moreover, the constitutive relation of the rock-mass under a three-dimensional initial stress field must be mentioned in these researches. Although many mechanics workers over the world have done a great deal of research work about the constitutive relation of rocks and soils under a triaxial stress state, the research about the nonlinear constitutive relation of the composite rock under a three-dimensional stress state has been rarely done up to now. Therefore, on the basis of the practical deformation curve of the composite rock under a triaxial stress state, in the light of the Duncans constitutive model and the Salamons equivalent theory we have put forward the constitutive model which could describe the constitutive relation of the composite rock made up of many layers of rocks possessing different thickness ratio under a triaxial stress state.
The stress-strain's relations of rocks and soils are usually nonlinear and their elastic coefficients are also not constants, so the variable-modulus model was raised which could directly imitate the practical stress-strain's relative curve without such the parameter as the clear yield condition. The Duncan's constitutive model is one of the E— ? variable-modulus models expressed by the elastic modulus E and the Possion ratio ?. Even though the Duncan's constitutive model is very simple and direct and it has not the artificial yield point of rocks and soils and it's parameter are easily obtained by the triaxial compressive experiments, the Duncan's constitutive model only deals with the stress-strain's relation of the linear deformation process and the strain hardening process of rocks and soils under a triaxial stress state and cannot reflect the stress-strain's relation of the initial convergence process and the strain softening process of rocks and soils under a triaxial stress state, and it cannot describe the volumetric expansive phenomenon of rocks and soils under a triaxial compressive stress state. So it's necessary to modify the Duncan's constitutive model in order to obtain the most exact computing result suited to the practical deformation characteristic curve of rock-mass under a complex triaxial stress state and the engineering site well.
