Research On The Non-linear Mechanical Mechanism Of Mining Instability Of Gas-rich Coal Rocks

Mining instability of coal rocks containing gas may result in various disasters, such as rock bust, coal outburst, and gas outburst. In the case of gas-rich coal rocks, these disasters occur even more frequently. Therefore, it is very important in engineering practice to study the instability mechanism of gas-rich coal rocks, so that disaster accidents may be reduced and coal mine production safety can be improved. The mining instability of gas-rich coal rocks is a dynamic evolution process. To study the mechanism of deformation, collapse, and instability of coal rocks, non-linear mechanical theory is used. In the dissertation research, laboratory tests, theoretical analysis, and numerical simulation methods are comprehensively used to study the non-linear mechanical mechanism of mining instability of gas-rich coal rocks. The dissertation is divided into 6 chapters, and the major research contents and academic contributions are as follows:(1) Coal samples from two gas-rich coal mines in Shanxi, the Xiayukou Mine and the Cuijiagou Mine, are used to do the EMS tests, uniaxial compression tests, and triaxial compression tests. The test results show that the collapse feature of gas-rich coal rocks accords with Coulomb-Mohr strength criterion and the coal samples from non-outburst coal mines, in comparison with those from outburst mines, have larger compressive strength, larger modulus of elasticity, and smaller Poisson ratio.(2) The transient permeability method is used to do permeability tests for gas-rich coal samples. The test results show that the change of permeability during complete stress-strain process is closely related with the development and change of internal cracks of coal samples. Coal samples from non-outburst coal mines form transfixion cracks easily. While coal samples from outburst mines are relatively loose and soft and they do not form transfixion cracks easily, so their after-peak permeability is far less than those of non-outburst mines. Based on the permeability characteristics of non-Darcy flow of coal samples from gas-rich mines, critical pressure gradient at seepage instability is computed. The result shows that seepage instability of coal samples of non-outburst mines usually occurs after the peak strength; while the seepage instability of coal samples of outburst coal mines occurs both before and after the peak strength, and its critical pressure gradient is relatively large, therefore, the seepage instability process is short and quick. This result accords with actual conditions of seepage instability of coal mines.(3) On the basis of the WY-98B Gas Adsorption Constant Determinator, a gas- desorption exciting and testing system is developed. The impact of temperature, particle size, and exciting force on gas-adsorption and desorption properties of coals is studied. The relation between Langmuir adsorption constant and temperature is fitted from the adsorption isotherm within the adsorption temperature range of 293K and 323K. The relationship between particle size and gas adsorption is obtained, and it is found that when the coal particle size is smaller than 0.045mm, the decreasing of particle size will not increase adsorption significantly. The curve of desorption characteristic of coal samples under the action of low-frequency exciting fore is found. The study shows that exciting vibration has changed the adsorptive micro-pores of coal rocks, caused gas and coal rock to move relatively, so that gas has been removed out of coal rocks. Meanwhile, vibrating process provides desorption with heat energy. Thus, low-frequency exciting force can speed up gas desorption in coal rocks and increase desorption quantity.(4) The multi-scale coupling theory is used to build three-dimensional strength distribution model of tiny element of coal rocks and the weighted adjacent element force-sharing model. Based on the strength distribution properties of gas-rich coal rocks and by using the effective strength principle, the macroscopic failure probability function and the multi-scale sensitivity function of gas-rich coal rocks are established. These functions have damage fraction, nominal stress, temperature, and gas pressure as their control parameters. The study shows that when the damage fraction and the pressure are greater and the temperature is lower, the macroscopic failure probability of the coal rocks is larger and the multi-scale sensitivity of the failure is higher.(5) Based on engineering observation and test results, mechanics models of thin plate are established for layered coal rocks and spalled coal rocks. Cusp catastrophe theory is used to derive the determining criterion of mining instability of coal rocks. And swallowtail catastrophe theory is used to study the impact of dynamic factors, such as explosion, drilling, and dynamic gas pressure on the stability of layered rocks, and to analyze the instability characteristics of rock plate under different potential functions. The study shows that the mining instability of gas-rich coal rocks depends not only on the physic and mechanical properties and the rock size, but also on the in-plane loads, normal loads, and the magnitude and changing path of dynamic normal loads.(6) Catastrophe progression method is used to predict the risk of coal and gas outburst of gas-rich coal rocks. The calculation shows that the prediction yields relatively high accuracy.