Investigation On Marco-meso Mechanical Properties And Energy Evolution Of Granite Damaged By Deep Cyclic Loadings

The mining environment with three"high"and one"disturbance"-high ground stress,high temperature,high osmotic pressure,and strong mining disturbances brings great challenges to the safe production of deep mine.With the extremely complex geological stress,the mechanical properties and failure characteristics of deep ground ore chamber and ore column are significantly different from those in the superficial part.Under the high ground stress and mining cycle disturbance,the development of meso-damage in the ore column is intensified.As a result,the bearing capacity of the ore column is weakened,which in turn makes the instability of mining field.Therefore,taking the stress environment of room-and-pillar mining in deep metal mines as the background,the theoretical analysis,laboratory tests,numerical simulations,and other technical means are used in the research.The macro-micro damage deformation characteristics and energy evolution law of deep granite are obtained by the triaxial cyclic pre-damaged test,damaged granite uniaxial recompression test.An improved constitutive model of rock fatigue damage is established and verified.The damage model and mechanism of granite under deep cyclic load are analyzed,the main conclusions of the research are obtained as follows:（1）The pre-damaged tests of granite under triaxial cyclic loading are designed and carried out.The law of rock energy evolution is obtained by analyzing the deformation and energy characteristics.The results show that when the number of cycles is the same,the deformation modulus of granite is negatively correlated with the confining pressure.When the confining pressure is constant,the dissipated energy and energy dissipation ratio of rock both decrease slowly with the increase of cycle numbers.（2）The nuclear magnetic resonance tests are performed before and after the pre-damaged tests of triaxial cyclic loading,which can obtain the meso characteristic parameters of granite.And the fractal relationship between granite porosity and T₂ spectrum is quantitatively analyzed.When the number of cycles is constant,with the confining pressure increasing from 10 to 120 MPa,the porosity increment first increases rapidly,and then increases slowly.The T₂ spectrum of granite both before and after the triaxial cyclic loading have obvious fractal characteristics.When the confining pressure is lower,the fractal characteristics before cyclic loading are stronger than the after;vice versa.（3）The uniaxial recompression tests are carried out on the damage granite,of which the test process is surveyed by the digital speckle technology and acoustic emission equipment.The energy evolution characteristics and failure modes of granite with different damage degrees are revealed by uniaxial compression tests.When the number of cycles is fixed,the pre peak energy of damaged granite increases first and then decreases with the increase of pre loss confining pressure.The failure mode granite after damage gradually changes from shear failure to tensile and shear mixed failure.（4）Considering the damage effect of cyclic loading,a visco-plastic fatigue element is serialized into the fatigue model.The improved Poyting-Thomson（P-T）fatigue damage model of rock is established.The reliability of the improved P-T fatigue model is verified by fitting the test data.The nonlinear theoretical analysis of the whole stage of rock fatigue deformation is realized under cyclic load.（5）Based on the improved fatigue damage model of rock considering the damage effect of cyclic loading,PFC^2D is used to simulate the triaxial cyclic loading pre-damaged tests and uniaxial compression tests.The meso stress field and the dynamic growth law of microcracks are obtained.In the process of triaxial cyclic pre-damage simulation,the number of microcracks decreases with the increase of confining pressure when the number of cycles is fixed.The microcracks generated during uniaxial compression of specimens with different damage degrees basically begin to expand along the location of local damage,which is caused by the damage effect of cyclic loading.The crack extension of the damaged granite under uniaxial recompression loading is different from that of the natural granite.（6）Combined with the actual mine conditions,the macro-mesoscopic coupling model of deep ground ore chamber and ore column is established by FLAC^3D-PFC^3D to simulate the mining process of pillar-and-room method.The meso-damage evolution rule of ore column caused by stress disturbance is obtained.The plastic zone gradually evolves from both sides to the surrounding of mining field,as the mining proceeds.The macroscopic shear failure is greater than the tensile failure.At the same time,the meso-and micro-cracks inside the ore column also gradually increase,and the development of microcracks is mainly located on the ore column near the goaf.It is determined that the main influencing factor of the propagation mode of microcracks in the ore column is the change of surrounding rock stress environment.This paper contains 134 figures,15 tables,180 references...