A Case Study of Surface Borehole Wall Dislocation Induced by Top-Coal Longwall Mining

Underground coal extraction causes failure and movement of overlying strata, which could also result in the dislocation of vertical surface boreholes. Investigating the correlation between the dislocation of surface boreholes and the broken of overlying strata is of great significance in deducing the mining-induced dynamic movement of overlying strata, which cannot be seen inside the 'black box'. Field measurement, laboratory experiment and theoretical analysis were employed to study the mechanism of the mining-induced dislocation of surface boreholes, and its relationship with the movement of overlying strata. Due to different deflection angles of adjacent strata, the horizontal movement of strata varies, and this contributes primarily to the dislocation of borehole wall. As the development of strata movement from the mining level towards surface, the position of borehole dislocation also occurred upwards, and the highest borehole dislocation occurred at the same position of overlying Key Stratum (KS). In other words, when a KS breaks, borehole dislocation will occur and develop from the bottom of this KS until the bottom of an upper KS. These key findings can provide a theoretical basis for deducing the mining-induced movement of overlying strata, determining the KS position and assessing the water conductivity of broken strata.