Experimental Study On Evolution Of Cracks And Permeability Of Mining Coal

Variation of dynamic and spatial distribution of coal joint fissures due to mining stress was along with the ongoing process of development of mining activities, the coal methane's characteristics of desorption, diffusion and migration and the methods of energy release were controlled by the process, so the occurrence, development and evolution of the gas emission (even coal and gas outburst) in space mining were also controlled by such process. The study on finding the inner variation laws of mining stress-fracture-gas emission, revealing the space-time relationship on the mining mechanics process, gas migration was with basis theoretical significance on the research of joint exploitation of coal and coal-bed methane and gas disaster prevention.Based on the problems of mining pressure field, surrounding rock fracture field, seepage field in the process of mining, the thesis adopted the test system of rock mechanical RMB-150B, tungsten filament scanning electron microscope JSM-6390LV, acoustic emission detector CDAE-1, self-made bidirectional loading testing system of coal-gas coupling permeability and made plenty of coal mechanical tests from micro perspective and macro perspective, and then studied the time evolution of stress field, fracture field and seepage field during the fracturing process of coal materials, especially coal fracturing evolution and permeability evolutionary process under the simulated real boundary conditions of extraction face. At last, the coupling effect mechanism of mining coal and gas was analyzed, and the influences of coupling effect on coal or rock dynamic disasters were discussed.Based on the above systematic study, some main achievements having been made are as follows:Firstly, through the morphology descriptions and mechanical process analysis of uniaxial compression ruptured coal samples, this thesis summed up seven main fracturing features and six stages of fracturing involution, and in particular concepts of fracture slow-growing section, fracture fast-growing section, and fracture expanding and perforating section were brought up. SEM test results on pore and fissures structure and the morphology of the cut indicated that the macro cracks propagation have close relation to the micro-structure and micro-defects. Secondly, for the quality index of coal, uniaxial strength had great influence on its emission properties, but void property and emission property were not related. Based on the features of cracks evolution, concepts of fracture slow-growing pointσ_ci and fast-growing pointσ_cd were given, and the approach, combining fast-growing pointσ_cd with stress monitoring to judge the fracture portent of samples, was put forward. According to the deformation and failure process of coal sample and sandstone under different stress path, acoustic emission features were studied. Based on acoustic emission parameters, a damage model was built.Thirdly, quasi-plane strain bidirectional loading testing system on coal-gas coupling permeability was invented, which was used to simulate the physical activities of unit bodies near extracting coal face. The experiment studied the four evolution stages of coal fracturing under the condition of quasi-plane, and then concepts, yellow precursory point and red precursory point of coal instability, were brought forward. Under the condition of varying quasi-plane, the test related to the variety of stress (strain)- fracture- permeability in coal deformation and failure process showed the time relationship between the three factors.Fourthly, the correlation of axial strain and permeability of coal samples under condition of quasi-plane strain was analyzed systemically, and the mathematical expression of strain and permeability in different strain sections was given. Through studying the the correlation of stress and permeability of coal samples under condition of quasi-plane strain, it found that permeability often increased before the peak, while permeability mutation often appeared after the peak, thus the author drew concepts of stress point of permeability increasing and stress point of permeability mutation.Fifthly, Test results on the change of permeability under the load-unload stress path indicated that unloading had important role on the increasing of samples' permeability, but the flow rates of increment were different under different stress stage. Experimental results on the influence of AE activities because of gas pressure changes under certain stable stress level showed that AE characteristics were different under different stress level, but the density and amplitude of AE parameters were lower than which were produced by variation of stress.