Effect of Premining on Hard Roof Distress Behavior: A Case Study

Considering the complex geological conditions of mining in the fully mechanized 828 working face of the Qingdong Coal Mine, a mining plan was developed for premining the upper No. 7 coal seam to control the hard roof and gas of the No. 8 coal seam. Failure depth of the No. 7 coal seam floor and the caving interval of the hard roof were analyzed using a proposed model based on the Terzaghi's principle and rock beam theory, respectively. The movement rules of the overlying strata under premining (No. 7 coal seam) and non-premining conditions were analyzed using physical material similarity simulations experiment. Furthermore, field monitoring was performed to determine the working resistance of the hydraulic supports. The estimated results from the proposed method showed that the failure depth of the floor of the 726 working face was 13.8m, which is in agreement with that noted during the field observation. The caving interval of the hard roof decreased from 24.9m to 10.6m after premining the No. 7 coal seam. In the simulation, the caving interval of the hard roof of the No. 8 coal seam decreased from 20m to 10m after premining the No. 7 coal seam. The hydraulic supports were effective in the actual production process, and the initial and periodic weighting intervals were 20.4m and 10.4m, respectively. These findings demonstrate that after premining the No. 7 coal seam, the initial and periodic weighting intervals decreased, thereby reducing the pressure intensity of the roof. This method effectively weakens the hard roof of the No. 8 coal seam. The issues pertaining to the hard roof during the mining of the 828 working face could be overcome by premining a protective coal seam of the upper No. 7 coal seam.