Investigation on mining mechanics behavior of deep close distance seam group

Deep close distance seam groups mining accounts for a large proportion in China,but the mining mechanical mechanism of this condition is still not clear,which always results in inefficient mining and the frequent occurrence of accidents. The physical and mechanical properties,deformation and failure characteristics of deep coal are essentially different from those of shallow coal seams. Especially under the conditions of deep close distance seam groups mining, the mutual mining influence of the adjacent working face will lead to more complicated mining stress redistribution process. Aiming at the difficult problems of surrounding rock control in deep close distance seam groups,based on the engineering practice of deep close distance seam group of Ji-14 seam and Ji-15 seam of Pingdingshan Coal Mine No． 12,the in-situ monitoring tests of convergence deformation of roadway and anchor cable stress were carried out at the intake airflow roadway of Ji-15-31030 working face. Furthermore,the floor failure range of the close distance seam group was calculated theoretically,and the formula of deformation velocity of roadway surrounding rock was deduced. And the mining mechanics behavior of deep close distance seam group was revealed preliminarily. The results showed that:① The theoretical calculation of floor failure depth of Ji-14 seam was about 21. 24-30. 88 m. ② Overlying seam mining led to the change of boundary condition of the seam,resulting in the convergence deformation of the roadway presenting a two-stage model of stepwise slow growth stage and exponential rapid growth stage. And the exponential rapid growth stage was the main stage of roadway deformation. Furthermore,the convergence deformation of roof floor of roadway,left and right sides,were about 400 mm. ③ Monitoring results of the anchor cables stress presented a two- stage evolutionary pattern of “nearly linear growth to decrease by step” with the advancing of mining face,while the average change rate and peak stress of roof anchor cables were significantly higher than the corresponding parameters of the wall parts,which indicated that the mining effect of the roadway roof was more obvious than that of the roadway wall. ④ Peak anchor cables stress lagged the maximum convergence deformation about 40 m. Besides,compared to the single seam mining,mining impact aging was significantly extended by approximately 35 m,the changing rate and peak value of mining stress were significantly reduced about 53. 5% and 24. 5% ,respectively. The mining influence range of the Ji-15 seam under closed distance mining conditions was about 105 m. ⑤ The deformation rate of the surrounding rock of the roadway was inversely proportional to the distance from the mining face. Based on the relationship,the pre- diction formula of the deformation velocity of the surrounding rock at different distances to the mining face was further derived,and the field in-situ monitoring data was used to verify the rationality of the formula. The research results could provide guidance for engineering problems such as the deformation prediction of roadway surrounding rock,road- way support and mining technology optimization in similar deep close distance seam groups.