Deformation and fracture at floor area and the correlation with main roof breakage in deep longwall mining

Water inrush at floor area is a natural hazard during coal mining. Especially in the northern coalfield of China, more than 55% of coal mines are threatened by water inrush at floor area. The hazard of water inrush is becoming more serious with increasing mining depth. In this paper, the deformation and failure behavior at floor area in deep longwall mining site were analyzed. The correlation between the floor failure and main roof breakage was studied using kinematics theory. Meanwhile, the impact loads of cantilever beam breakage on the floor area at both longwall face and gob area were calculated. Combining with “Pressure arch hypothesis”, the compression and unloading mechanics processes at floor area were analyzed, and the unloading deformation model of floor structure after the breakage of main roof was established as well. In addition, the correlation between unloading deformation at floor area and the main roof breakage, mining depth and unloading stresses were also obtained. Finally, these studies have been verified by using micro-seismic monitoring data in deep longwall mining site. The results show that the impact loads are proportional to the span and loads of cantilever beam. After the breakage of cantilever beam, the impact loads were transferred to the floor area at longwall face side and gob side, and the rock masses at both sides were failed in compression. Consequently, the position of back arch foot of the pressure arch was rapidly transformed into the contact gangue zone at gob area from the last breakage position of main roof. While as the unloading stress of rock masses inside of the floor pressure arch is increasing, the depth of unloading fracture and heave below longwall face are greater than those before cantilever beam breakage. In addition, the unloading deformation at surface floor increases nonlinearly with the increase in mining depth and unloading stresses.