Study on influence of fracture development process on compaction area of overlying strata in working face

The key to gas drainage in goaf is to understand the evolution process of overlying strata fissures on working face. In order to study the evolution law of mining-induced compaction area of overburden rock in fully mechanized mining face and efficiently carry out gas extraction in goaf of coal mine, taking No.2207 working face of a mining area in Shendong as the prototype, through physical similarity simulation experiment, the self-developed overburden rock fracture monitoring system is used to perform histogram equalization enhancement and median filtering denoising on the image. After obtaining the binary fracture image, the fractal theory is used to quantitatively describe the fracture rate, and the fracture rate of overburden rock caving under typical periodic weighting is statistically analyzed. The space-time function of fracture development degree affected by the advancing time of working face and the evolution correction model of mining-induced fracture elliptical zone compaction area are established. The results show that the change of the average fracture rate of the overlying rock conforms to the DoseResp function relationship. The development process of the fracture includes three stages : generation, expansion and spread, and compaction and closure. The development rate of the fracture is gentle in the two stages of generation and closure, while the development rate increases sharply in the expansion stage. The fracture field of overlying strata in the goaf forms three areas : old fracture area, compaction area and new fracture area. The difference of fracture development cycle leads to obvious differences in development state, compaction degree and spatial form between the old fracture area and the new fracture area. These differences lead to the occurrence of asymmetric subsidence of overlying strata in goaf. By analyzing the relationship between the stress recovery of the overburden rock and the evolution of the internal cracks in the overburden rock, it is further verified that the DoseResp time function curve can well show the allometric evolution characteristics of the overburden rock cracks in the goaf. Finally, a modified model of the compaction zone of the mining-induced fracture elliptical parabolic zone considering the allometric evolution effect of the overburden fracture is established. Combined with the field engineering practice, the accuracy of the high-level borehole layout of gas extraction is effectively improved, which provides theoretical guidance for the safe mining of the working face.