The position of hydraulic fracturing to initiate vertical fractures in hard hanging roof for stress relief

A strong and hard hanging roof causes high underground stress in underground mines, leading to rock burst, coal and gas outburst, or large deformations in roadways. To address these problems, hydraulic fracturing can form hydraulic cracks in strong hanging roofs and promote fractures in the hanging-roof. However, it is difficult to ensure that hydraulic cracks propagate vertically through rock strata. Therefore, a novel method has been proposed that uses hydraulic fracturing to generate vertical fractures in hard hanging roofs for-roof cutting and stress relief. The method exploits the additional horizontal tensile stress produced by the bending deformation of the roof so that the fracturing point occurs in a desirable tensile-stress area, and therefore to promote the vertical expansion of hydraulic cracks that cut off the rock strata. In this paper, the position of the maximum additional horizontal tensile stress is taken to be the reasonable fracturing position given by a mechanical model. Solving this model indicates that a shorter hanging roof equates to a thicker hanging roof, a larger elastic modulus, a smaller elastic cushion coefficient, and a fracturing position that is deeper in the coal wall. The various factors that affect the reasonable fracturing position may be ordered in terms of their impact as follows: hanging-roof length > hanging-roof elastic modulus > hanging-roof thickness > elastic cushion coefficient > horizontal stress. The proposed method has been applied to roadway 5103 of the Majiliang Coal Mine of Datong Coal Mine Group Co., Ltd. The results show that hydraulic fracturing forms vertical hydraulic cracks at the point of maximum additional horizontal tensile stress, which can effectively control the deformation in the surrounding rock. The fracturing position calculated by using this model is reasonable and reliable and can be used to guide the on-site construction.