Mechanical behavior and permeability of coal and rock under strong mining disturbance in protected coal seam mining

The method of protected coal seam mining has been widely used in low permeability,high gas and short dis- tance coal seams. The research on mechanical behavior and permeability of coal and rock under strong mining disturb- ance in protected coal seam mining provides a theoretical support for further efficient and safe mining of protected seam. Taking the selection of the Ji14 -31010 and Ji15 -31030 mining face at the No. 12 mine of Pingdingshan Shenma Group as a case study,the similar material simulation test and the stress analysis of the protected coal seam were car- ried out. The stress condition of the protected coal seam under the protected coal seam mining layout can be obtained by the similar material simulation test. During the mining process of the upper protective coal seam,the pressure of the coal seam increased initially and then decreased. After the goaf was re-compacted and stabilized,the stress state almost restored to the original rock stress state. The stress state of the coal seam after deformation is obtained by stress analy- sis of the protected coal seam. After mining of the upper protective seam,the protected coal seam has deformed. The top of the protected coal seam is less than that original rock stress,and the bottom of the protected coal seam is larger than the original rock stress. Combining with the similar material simulation test and the stress analysis of the protected coal seam,the stress loading path of the protected coal seam mining in laboratory test has been obtained. According to the stress loading path of the protected coal seam mining,the permeability test of mining coupled stress path has been designed and carried out. Through the experiments,some views can be concluded as follows. During the mining process of the upper protective coal seam,with the same experimental conditions,the larger the disturbance stress of the pro- tected coal seam is,the larger the peak value of failure stress is,the smaller the confining pressure is,and the larger the volume strain is. During the mining process of protected coal seam mining,the expansion point on the stress-strain curves of M group coal samples and N group coal samples are much earlier than those of conventional unloading triaxi- al tests. With the same stress state,the larger the water pressure is,the larger the volume strain of coal is. During the mining process of protected coal seam mining,the initial confining pressure of M group coal sample is 35 MPa,the in- fluence of confining pressure on permeability is greater than that of axial pressure. During the mining process of protec- ted coal seam mining,the initial confining pressure of N group coal sample is 20 MPa,the alternation of confining pres- sure and axial pressure has the main effect on permeability. The permeability curve presents a “ W” type. In both groups of experiments,the permeability of the sample with the largest disturbed stress is generally higher than that of other samples.