Effect of small faults activation on failure and permeability of overburden strata in goaf

Taking the No. 25030 working face of Xuehu Coal Mine of Henan Shenhuo Coal & Power Co.,Ltd. as the engineering background, UDEC was used to simulate the movement and failure process of overlaying strata in the goaf at different dip angles and different horizontal fault distances, and the influence of faults on the failure and permeability of overlaying strata in goaf was analyzed. The results show that:①as the mining face approaches the fault gradually, the fault slips, the displacement near the section presents an inclined downward trend, and the footwall slips along the fault obviously. Under the influence of the supporting force formed by the footwall slip dislocation, the footwall subsidence is obviously larger than the hanging wall, and the height of the “three zones” of the overlying rock on the hanging wall was risen. When the mining face pushes through the fault for 10 m, the overburden displacement begins to lag, and the subsidence of the hanging wall is less than that of the footwall. When the mining face passes through the fault for 40 m, the overburden is gradually weakened by the fault, and its displacement gradually conforms to the movement law of overburden in fault-free mining. The footwall is mainly subsidence, the hanging wall is influenced by fault, and the wedge-shaped head is supported by fault slip. The rock strata collapse is not sufficient, and the subsidence amount is less than the footwall.②The larger the dip angle is, the smaller the influence range of fault on the hanging wall overburden support, the more serious the dislocation caused by fault slip, and the greater the risk of mining, and the closer the position where the fault influence begins to appear; the wider the horizontal fault distance, the greater the support force of the hanging wall by the fault, and the smaller the displacement of the hanging wall overburden after crossing the fault. The wider the horizontal fault distance, the greater the footwall subsidence of the fault, and the greater the risk of mining face crossing the fault. ③The porosity of nearby rocks increases by leaps and bounds. The larger the dip angle is, the less the porosity increases. With the increase of the dip angle, the porosity of the caving zone directly over the fault increases first and then decreases, and reaches the maximum at the dip angle of 45°, while the porosity of the fracture zone increases gradually with the increase of the fault angle. With the increase of the horizontal separation of the fault, the overburden porosity generally increases first and then decreases, while the fracture porosity increases slightly. Generally speaking, the wider the horizontal separation is, the greater the fault dip angle is, and the greater the overburden collapse risk is when the mining face crosses the fault.