Characteristics Of Anisotropic Mechanical Properties Of Jointed Rock Mass Using PFC And Engineering Applications

Jointed rock mass is inhomogeneous and anisotropic. Accurate and detailed characterization for rock mass can influence the design of mining layouts and safety of mines. Rock mass is a geologic body composing by the discontinuities which have a critical influence on the deformational behavior of blocky rock systems. The mechanical behavior of this material depends principally on the state of intact rock whose mechanical properties could be determined by laboratory tests and existing discontinuities containing bedding planes, faults, joints and other structural features. The distributions and strength of these discontinuities are both the key influencing factors for characterizing the discontinuous and anisotropic materials. Studies on mechanical properties of jointed rock mass still need to be done to characterize the anisotropic deformation behavior.The paper aims to assess the anisotropic parameters of jointed rock mass and discuss the application in rock engineering. The discrete fracture network (DFN) model was first established using PFC2D code and the size effect of mechanical properties was investigated. Then the represented elementary volume (REV) of jointed rock mass was studied and the anisotropic parameters were obtained.On the basis the anisotropic theory of elasticity for damage, an anisotropic model was established using Comsol Multiphysics code, and the influence of anisotropy on mechanical behavior of rock mass was studied. The results will gain an insight into the influences of discontinuities on the mechanical behavior of rock mass and offer some scientific evidence for the design of mining layouts or support requirements. Centered on the anisotropic parameters of jointed rock mass and the application, the main works and achievements in this thesis are as follows:1.On the basis of linear elastic theory and displacements equivalence, a computational model considering joints distribution and mechanical properties was established. The equivalent elastic moduli and equivalent Poisson's ratio of the stratified rock perpendicular and parallel to the loading direction were discussed.The relations of material properties and geometry parameters with the equivalent elastic moduli and equivalent Poisson's ratio in two directions were studied.2. The stratified rock samples with different dip angles were studied under uniaxial compression, Brazilian tensile and direct shear test. The elastic modulus, strength and failure patterns were studied. Then, the stiffness and strength parameters of joint elements were analyzed using PFC2D code. Then, the numerical tests were carried out on rock samples with varied dip angles compared with the experimental results. Finally, the mechanical properties were compared and verified compared with the failure patterns and strength distributions with different dip angles.3. The automatic generation algorithm named MC_JOINT of random joints network based on the Monte Carlo method was proposed using the programming language (FISH) embedded within PFC2D. The information and statistical distributions of certain parameters structural planes were then obtained by means of scanning and recognizing rock masses with ShapeMetriX3D measuring technology in this paper. This algorithm could represent the structural planes compared with the geological surveys. In simulating the compression test of a jointed rock sample, the mechanical behavior and crack propagation were investigated. The results reveal that the failure mode and crack propagation of the jointed rock are dominated by the distribution of joints in addition to the intact rock properties. The simulation result shows the feasibility of the joints generating method being applied to the aspects of jointed rock mass.4. The size effect and the corresponding representative elementary volume (REV) were studied. The uniaxial compression test was first performed during the compression process in laboratory. Results from numerical simulations using the particle flow code in two dimensions (PFC2D) were presented, and compared with experimental measurements. Then, the mechanical properties of complex rocks with discrete fracture network (DFN) were studied and scale effects on the elasticity and strength were then investigated.Then, the REV size of the considered rock mass was obtained. Finally, the anisotropic parameters of REV and principal values of mechanical properties were investigated by numerical tests, which will provide methods and parameters for anisotropy study in the jointed rock mass.5. The influence of anisotropy on rock engineering was discussed based on typical rock engineering with randomly distributed joints using the anisotropic parameters. On the basis of elastic damage theory, a numerical finite element approach to the mathematical model was developed to describe the anisotropic property of stress tensor of jointed rock mass. Then the rock stability of were studied using the Comsol Multiphysics code. The rock mass stability was performed numerically to investigate the influence of joints orientation on the anisotropic properties of elasticity of the surrounding rock mass around roadways in underground mining. The Hoffman criterion was used to assess the damage zone and the model for engineering stability analysis considering anisotropic properties was finally established.