Mechanism of Abnormal Surface Subsidence Induced by Fault Instability

Surface movement and deformation with faults differ significantly from that without faults, which the surface movement and deformation at the fault outcrop generally abnormally increase when a fault occurs in the overlying strata and loses stability as a result of mining. To explore the mechanism of abnormal surface subsidence induced by fault instability due to mining in the hanging wall and footwall, mechanical models for the fault slip and soil cantilevers were separately established. Moreover, based on numerical and similarity simulation experiment, the difficulty degrees of fault instability were compared and analysed during mining in hanging wall and footwall. The research results show that: (1) the abnormal surface subsidence at fault outcrop is caused by fracture of soil mass at the fault outcrop due to the cantilever effect. (2) The fault zone that can be stabilised during mining in the hanging wall is broader than that during mining in the footwall, and a fault remains stable when mining in hanging wall and the surface at fault outcrop is more likely to experience stepped subsidence when mining in the footwall. (3) Fault stability worsens with the growth of the fault dip angle when mining in hanging wall, the fault is certainly subjected to slip and instability when fault dip angle is lower than the internal friction angle of the rock mass within the fault zone and fault stability strengthens with increasing fault dip angle during mining in the footwall. (4) When mining in the footwall, the ratio of shear stress to normal stress at the fault outcrop is about five times that when mining in the hanging wall and the fault is more likely to undergo slip and instability during mining in the footwall. (5) As a natural weak plane in the rock mass, a fault exerts a blocking effect on the transfer of mining-induced stress and overburden movements.