Floor Pressure-Relief During Top Slice Mining of Extra-Thick Coal Seams and Its Implications for Gas Drainage Application

This paper investigates the pressure-relief response of the floor in top slice mining, taking No. 2 coal seam in Jinta coal mine as its engineering background. The failure laws of the lower slice along strike and with depth during the top slice mining process are analyzed with comprehensive methods including theoretical calculation, similar simulation, and numerical simulation. It is found that physical experiments on similar materials and numerical simulation with FLAC(3D) software using a Mohr-Coulomb model give comparable results, both indicating that floor destruction in the top slice reaches a depth of about 20m. The empirical formula is found to be non-applicable for predicting the depth of damage in the top slice when mining an extra-thick seam. It is further shown that the lower slice is able to restrict floor failure. In the mining direction, the distribution and change characteristics of stress are highly correlated with the periodic weighting: the range over which stress recovery occurs in the floor after periodic weighting is about 0.6 times that before periodic weighting. The conclusions are of engineering significance for the accurate arrangement of gas drainage boreholes and the avoidance of mine gas disasters. On the basis of all the rules, the practical application of gas drainage of the lower slice was carried out, which is beneficial for coal mining and economic achievements at the same time.