Mechanical Failure And Permeability Evolution Mechanism Of Coal Based On Equivalent Matrix Scale And Its Application

Due to the effect of geological structure in our country, the coal seam often contains several normal coal and deformed coal sub-layers in the same place. In order to better gas outburst disasters control, it is necessary to further understand the role of deformed coal in gas outburst. However, the permeability of deformed coal is greater than that of natural coal in laboratory, and the mechanical failure mechanism of the deformed coal and its combination is not undefined. The mechanical failure mechanism and permeability evolution behavior of coal based on equivalent matrix scale are researched. Main conclutions are as follows:1) The pores characteristics of natural coal, deformed coal, briquette coal and coal powder are obtained. The pores with a diameter less than 100 nm of deformed coal are slightly greater than those of normal coal, and the fractures with a diameter moer than 100 nm of deformed and briquette coals are greater than those of normal coal. The initial structure permeability of normal coal is far less than those of deformed and briquette coals. The equivalent matrix scale and equivalent fracture width of normal coal are 49 times and 2.1 times than those of deformed coal and 4-62 times and 0.2-1.6 times than those of deformed coal, reseparately.2) The constitutive relation of normal coal can be simplified as the strain-softening model, and the constitutive relation of deformed coal can be simplified as the ideal elastic-plastic model. Due to the larger equivalent matrix scale and less developed pores in normal coal, the compressive strength, cohesion and inner friction angle, elasticity modulus of normal coal are 15 times, 8.6 times and 14 times than those of deformed coal, reseparately. The free gas and the absorbed gas both can weaken the strength of coal. In normal coal, the weakening degree of free gas is 1.3-3.4 times than that of absorbed gas. In deformed coal, the weakening degree of free gas is 8.4-19.8 times than that of absorbed gas, which is related to the larger equivalent matrix scale and less developed pores in deformed coal.3) The permeability evolution expermets of coals under different equivalent matrix scales are done. At the same condition, the permeability of normal coal with larger equivalent matrix scale is far smaller than that of deformed and briquette coals. During complete stress-strain process, the permeability increases 200-300 times after the peak, and the permeability increases 1.4-2.4 times after the peak. An analytical coal permeability model is developed and four of their forms are validated by matching the corresponding field data or experimental data. The equivalent plastic strain and the blast coefficient of permeability are introduced, and the permeability model during plastic failure phase is built and validated.4) The internal mechanism of permeability evolution is obtained. Based on the permeability tests and porosity experiments, we think that the gas seepage channels in deformed and briquette specimens are mostly formed in the reconstruction process. The original fracture system in coal is destroyed during sample selection process. Unfortunately, it makes there are many differences in fractures between the deformed, briquette specimens and the normal specimens. Due to the huge difference on the equivalent matrix scale, the permeability is controlled by the equivalent matrix scale, and the initial structure permeability of deformed and briquette specimens is 5-365 times than that of normal specimen.5) Duo to the difference of horizontal deformation for gas-saturated coal combination, there is extra horizontal stress on the interface, which changes the horizontal effective stress. Using the user-defined uncoordinated deformation and Mohr-Culomb criterion, the mechanical failure forms of gas-saturated coal combination can be divided into seven kinds. The vertical stress for coal failure without the confining stress is far less than that with the original confining stress. With unchanged confining stress, the damage occurs first in the interface of normal coal, and the minimum vertical stress is 27.64 MPa. With confining stress of 0 MPa, the damage occurs first in the deformed coal, and the minimum vertical stress is 1.66 MPa. The damage caused by the minging stress tends to the latter.6) Based on permeability model, the mechanical constitutive equation and dual-pore coal, the fluid-solid coupling model is built.The existence of deformed coal can increase the plastic area and reduce the efficiency of gas drainage. Due to the coal seam often containing several normal coal and deformed sub-layers, the in-seam directional longholes are used for regional gas drainage, and the branch-holes are used for auxiliary gas extraction from the deformed coal sub-layer. The regional gas drainage models for the longwall panel and the main entries are built.