Fracture mechanical criteria and simulation of floors with Y-shaped branches of major faults in coal mines

Objective Despite wide distribution, the branches of major faults (also referred to as the fault branches) tend to be overlooked. However, they represent one of the primary causes of water inrush.

Methods Under the engineering geological background of the Liuzhuang coal mine in Anhui Province, this study established a fracture mechanical disaster model for fault branches under the impacts of both seepage and stress based on the criteria for compression-shear fracture of rock masses considering the Mohr-Coulomb criterion. Using this model, this study obtained the critical water pressure and the minimum safe distance between the fault branch and the floor's fractured zone in the case of fault branch splitting. It investigated the influence patterns of factors including the fault dip angle, minimum principal stress, length of a major fault, and seepage water pressure on critical water pressure and the minimum safe distance, determining the dominant influencing factors. Furthermore, this study simulated the evolutionary characteristics of the plastic zones and seepage fields of the fault branches under the mining effects and verified the results using in-situ water pressure tests.

Results and Conclusions Key findings are as follows: (1) A higher dip angles of the fault branches corresponded to a higher susceptibility to water inrushes, while a higher minimum principal stress suggested a lower probability of the fissure propagation of the fault branches. (2) In the case where the dip angle of the major fault and the angles between the fault branches and the major fault were both 60°, fault channels were entirely activated and connected when the mining face advanced directly above the fault branch, as indicated by the simulation results. After the mining face advanced 10 m away from the fault branches, fissures at the tip of the fault branches were connected to the fractured zone of the mining face’s floor, resulting in water inrushes from the floor. (3) The in-situ water pressure tests verified the correctness of theoretical calculations, proving that fault branches can cause early interconnection between fissures on the floor and faults, leading to water outflow from the mining face. Overall, the findings of this study provide a reference for setting up waterproof coal pillars in coal mines with Y-shaped fault branches on stope floors.