Study On Equivalent Orthotropic Mechanical Parameters And Yield Criterion Of Jointed Rock Mass And Its Engineering Application

Many joints with variant trace length distributed in jointed rock mass in a complex manner. The joints made the mechanical properties of rock mass nonlinear, discontinuous, heterogeneous, anisotropic, and the mechanical properties is variable under different geological conditions. The above characteristics of the rock mass mechanical properties make it difficult to obtain the proper mechanical parameters. In practice, the main factors that have great effect on rock mass deformation and failure are taken into account of to get the equivalent mechanical parameters by some scientific method.Take into account of the data of geologic investigation and joint statistic, results of test of in-situ rock mass and laboratory rock sample, technique of joint network simulation, the method for computing the equivalent deformation and shear yield parameters of macro rock mass is presented. The orthotropic yield criterion is obtained in this paper. In order to take the research results of the paper into use in numerical simulation of rock mass engineering, some promotion are made for FLAC^3D. The main research works and conclusions are summed up as followed.(1) The numerical simulation analysis method, the computation method for equivalent parameters of rock mass, the technique of joint network and the theory of anisotropic yield criterion are summarized in this paper. The analysis on development and main difficulties of the above are made. On the basis of the above summary and analysis, the research emphasis and study methods of the paper are presented.(2) Base on the analysis of position relations of finite plane area, limit line and spatial point, the joint network simulation method which the joints is simulated by finite tetragon plane is presented. The concept of joint area vector is introduced to define rock mass REV scale index. The rock mass REV scale index is the ratio of project area in three orthogonal planes and rock mass volume. The research results on REV scale indicate that the REV scale is about three to four times of the maximum expected value of trace length of rock mass joints sets.(3) Based on the feature of joint deformation, a new constitutive model for normal displacement of joint is presented which consider of initial stress state and joint compress strength. Equivalent strain of rock mass in every direction can be calculated according to theory of equivalent continuous strain. The elastic orthotropic deformation parameters are obtained by use of the equivalent strain in every direction and the elastic orthotropic constitutive model. In order to solve the conflict equations established through elastic orthotropic constitutive model, the interval enumeration method is developed on the basis of the theory of least squares method.(4) Damage mechanics is applied to evaluate the damage variable of rock mass with several crack groups. Considering the damage variable as weighting factor, the equivalent shear strength parameters (that is cohesion and friction factor) of any section are defined as the weighted average value of intact rock parameters and joint parameters. The trace curve surface of equivalent cohesion and friction factor are fitted into theorthotropic parameters so that the equation of ellipsoid surface can be used to describe the relations between the equivalent shear strength parameter value and the normal vector of section.(5) The equations of ellipsoid surface for equivalent cohesion and friction factor are introduced into the classical Mohr-Coulomb yield criterion so that the orthotropic yield criterion can be obtained. Theoretic analysis and results of example show that the orthotropic yield criterion is related to the three principal stresses and isotropic yield criterion is a special case of orthotropic yield criterion. The analysis also indicates that the maximum shear stress plane can not be the same of yield plane in orthotropic materials. Since no explicit form, the numerical method of orthotropic yield criterion is studied in this paper.(6) In order to simulate so many joints with variant trace length in rock mass, the classification method of the joints is studied according to the trace length. Every set of joint has its special treat method in numerical simulation of rock mass. Base on the above research, the main work and basic process for rock mass numerical simulation is summed up systematically.(7) Codes for the research results of this paper had been developed by using of Visual Foxpro software and FISH language of FLAC3D. Some interface program for drawing the plot of joint network and equivalent shear strength parameters is also developed.(8) The research results of this paper are applied to numerical simulation of high rock slope of permanent ship lock of Three Gorges Project. The analysis indicates that the theory and program of this paper is reasonable. Numerical simulation results comparison between orthotropic constitutive model and isotropic constitutive model shows that the results of orthotropic constitutive model have more accordance with the real mechanical behavior of rock mass. The in-situ measuring data shows that rock mass have obvious unloading weakened effect during the slope excavation. The unloading weakened degree index is presented by the ratio of first stress invariant of rock mass elements between excavated stress state and initial stress state. The plot of measuring unloading zones indicates that the index of unloading weakened degree in this paper is reasonable.