Damage And Permeability Evolution Characteristics And Inducing-disaster Mechanism For Gassy Coal Under Igneous Sill

Coal-rock gas dynamic disasters, including coal and gas outburst, unusual gas emission, rock burst and unusual roof pressure, are essentially the instability dynamic phenomena of gassy coal and rock being damaged and destroyed. The mining practice shows that the coal-rock gas dynamic disasters are especially serious for the mining area over which a hard igneous sill exists.Taking Yangliu Coal Mine as engineering location and igneous sill and gassy coal as research objects, the dissertation researched controlling effects of magmatic event on gas occurrence laws of underlying coal seam, structural instability model of hard roof rock and its shock force and energy effects, damage and permeability evolution mechanism for gassy coal in loading and unloading conditions and mechanism for igneous sill structure instability inducing coal-rock gas dynamic disasters via theoretical analysis, laboratory experiments and numerical simulation. The main conclusions were drawn as followed:1) The controlling effects of magmatic event on gas occurrence laws of underlying coal seam were analyzed based on the experiments for the physical parameters of coal. The thermal evolution effects of magma improved the metamorphism degree of coal, sped up the second hydrocarbon generation of coal and improved gas adsorption capacity and gas diffusion velocity. The gas trapping effects of the igneous sill and annular dyke prevented the the secondary gas escaping. The pushing effect of magma intrusion increased the tectonic stress and destroyed the physical structure of coal. Thus, the outburst risk of coal covered by igneous sills is much greater than that of coal without the effect of igneous intrusion and the soft coal and the enriched gas laid the material foundation for coal-rock gas dynamic disasters.2) Square-form structure instability model of hard roof rock(SFSI model), that it is the most instable with ―O-X‖ form when the goaf is nearly square, was established based on its stress distribution characteristics and instable breaking laws. The shock force and energy effects of hard roof rock structure instability on underlying coal were analyzed from the view of stress and energy: the victor stress summation increased the buried depth of mined coal and induced its irreversible damage and the scalar energy summation was the fundamental cause for inducing coal-rock gas dynamic disasters.3) Based on energy accumulation-spread-dissipation laws of hard roof rock structure instability, the criterion of key inducing-disaster stratum was proposed and thus the roof rock strata were divided as key inducing-disaster stratum, weak inducing-disaster stratum and non-inducing-disaster stratum. The igneous sill #2 is potential to induce coal-rock gas dynamic disasters of coal seam #10 due to its calculated inducing-disaster coefficient of 9.67.4) The stress-damage-permeability experiments for gassy coal were performed with the mechanical paths of fixed confining pressure and increased axial pressure(path #1), fixed axial pressure and decreased confining pressure(path #2) and increased axial pressure and decreased confining pressure(path #3). The loading methods affect coal damage evolation and time-response characteristics of damage and permeability, whose orders are path #3 > path #2 > path #1 according to damage degree and response speed. The damage and permeability characteristics of coal are of significant effects of confining pressure. The different deviatoric stress ratio is the fundamental cause for damage and permeability characteristics diversity of plastic coal. The coal damage and permeability are evolved jointly but the permeability lags behind the damage.5) That the damage variable obeys Welbull distribution assumed and AE accumulation normalized and fitted, the statistical damage model of gassy coal was established based on AE characteristics and deviatoric stress ratio. The porosity variation laws of elastic coal were analyzed with the effects of effective stress compression, coal matrix adsorption expansion and thermal expansion and then the porosity variation laws of plastic coal were analyzed with the effects of damage expansion, thus the permeability evolution model of plastic coal was established.6) The stored energy and inducing-disaster consumed energy of gassy coal under igneous sill were calculated and the energy criterion of inducing coal-rock gas dynamic disasters was proposed. Then the tendency and necessary conditions of coal-rock gas dynamic disasters under igneous sill were analyzed and the inducing-disaster mechanism for igneous sill structure instability was revealed, which was verified in Yangliu Coal Mine.