Study on the cross-interface propagation characteristics of hydraulic fractures in composite hard roofs and migration law of overlying strata

While hydraulic fracturing is considered an effective method for mitigating large-area suspended hard and thick roof problems, the mechanisms of fracture propagation orientation and the evolution of overburden migration remain insufficiently understood. To address this gap in literature, this study investigated the cross-layer propagation characteristics of hydraulic fractures under various conditions through numerical simulations. Based on the simulation results, similar experimental studies were conducted to explore the migration patterns of the overlying strata in mining areas under various pre-splitting conditions of composite hard roofs. The experimental findings showed that the direction of the maximum principal stress determines the direction of fracture propagation and that the extent of fracture propagation increases with the increase in the differential principal stress. Hydraulic fractures could not penetrate the layers when the joint bonding strength was low (0.5). When the spacing between the fracturing segments was less than 3 m, the initiation of the second hydraulic fracture had a compressive effect on the first one. Hydraulic fracturing reduced the periodic weighting step distance, the overhanging area, and the degree of stress concentration. Moreover, the pre-splitting effect was significant on both the upper and lower hard roofs. Specifically, under hydraulic fracturing, the periodic weighting step distances under working conditions 1 to 4 were 10.3, 8.2, 9.2, and 7.5 cm, respectively. Under these working conditions, the “square” positions occurred when the working face advanced to the 11th, 13th, 12th, and 14th periodic weighting, and the corresponding stress concentration coefficients were 2.72, 2.57, 2.05, and 1.89, respectively. This study provides significant insights into the selection of the hydraulic fracturing process and its parameters under similar production conditions.