Research On The Overlying Rock And Coal’s Deformation And Damage Rules And Their Pre-splitting Antireflection Mechanism During The Underlying Protective Layer Mining

Most coal seams in China are high methane content and low permeability with complex mining conditions. That make great differences and limitations for the pressure relief and anti-reflection effects of underlying protective layer mining technology. In some large layer-spacing, small protective layer thickness and key layers existence coal seams, the protection effects of protective layer mining technology may appear insufficient, even failure. Moreover, the outburst prevention and anti-reflection effects of the available technologies work slowly, that is not conducive to the efficient production of coal mines. Therefore, to fully grasp the overlying rock and coal’s deformation and damage rules and explore the auxiliary anti-reflection technology under complex geological conditions, is the key promotion for the large scale use of underlying protective seam mining technology, as well as the guarantee for implementation of coal mine safety efficient green production.In this paper, the outburst coal containing methane and the typical sandstone in coal stratum are chosen as the research object. Their micro-and meso-structure were analyzed firstly. The mechanical characteristics of coal containing methane under a variety of factors influence, were studied use of macro-mechanical tests. The paper also revealed the energy evolution characteristics and damage-permeability characteristics of outburst coal containing methane in complex disturbance stress field, as well as the deformation and damage characteristics of sandstone based on acoustic emission technique. The relationship between the permeability and energy dissipation rate of outburst coal containing methane was established. And the statistical damage evolution model of sandstone is established to reflect its whole process of deformation characteristics. The prefabricated crack propagation law of layered rock were studied by theoretical analysis and numerical simulation. Finally, the stress field and seepage field evolution regularity of disturbance zone, using pre-splitting overlying strata technology, were analyzed in the process of underlying protective layer mining. The above research contents laid the foundation theory for exploring new prevention technology of coal and gas outburst disasters. Through the research, this paper made some progress in the following aspects.(1) The differences in the material composition, micro-mesoscopic morphology characteristics and pore structure characteristics between the outburst coal and non-outburst coal were comparatively analyzed. The damage evolution model of outburst coal under uniaxial compression was established, based on the rock acoustic emission mechanism and the defect distribution assumptions.(2) The effects of different confining pressures, different gas pressures and different loading rates on mechanical properties of briquette coal containing methane were tested. The results showed confining pressure restrained deformation and improve the strength of coal, gas pressure reduced the effective stress of coal, while, a threshold of loading rate was existed to change the mechanical strength of coal. And, the deformation of outburst coal containing methane with confining pressure existed plastic hardening characteristics. Based on this, the quantitative relationship between two hardening coefficients and plastic strain of outburst coal containing methane was established.(3) A series of step unloading tests were carried out to analysis the permeability characteristics and energy evolution characteristics of outburst coal containing methane under complex disturbance stress field. The tests revealed the creep deformation characteristics in the constant pressure unloading steps of coal containing methane. And the energy dissipation rate is used to describe the changing law of the internal porosity of coal samples. Besides, the relationship between the permeability and energy dissipation rate of outburst coal containing methane was established.(4) The statistical damage evolution model of sandstone is established, based on the D-P failure criteria, to reflect its whole process of deformation characteristics. The deformation and damage characteristics of sandstone based on acoustic emission technique was studied. It revealed that the absolute energy emission probability density functions of sandstone under different loading rates were in line with the power law distribution, and the critical exponents of sandstone’s absolute energy power law distribution was obtained using the maximum likelihood estimation method.(5) Theory of elastic mechanics and fracture mechanics were used to analysis the mechanical mechanism of pre-crack initiation and extension. The relationship between the crack initiation angle and the initial size, force, angle and fracture toughness of pre-crack was established. And three-point bending tests of layered rocks containing the pre-crack were carried out using numerical simulation methods. The tests analyzed the effect of many factors on the prefabricated crack propagation laws and deformation and failure rules of layered rock. The factors include the geometric parameters and distribution parameters of the pre-crack, the components and structures of layered rock, and the external factors such as loading rates of layer rock.(6) The stress unloading value, vertical expansion deformation and unloading range of the protected coal seam, using and not using overburden pre-splitting technology, were respectively analyzed with the numerical simulation software Flac3 D. And the influences of pre-splitting zones’ geometry and distribution parameters on the permeability increasing of the protected coal seam were studied. The theoretical guidance for the permeability increasing of the protected coal seam with pre-splitting technology were also provided.