A coupled simulation method of hydraulic fracturing for relieving mining pressure under hard roof based on DEM method

The technology of pressure relief through hydraulic fracturing method in hard roof has successfully addressed the disaster risk caused by the difficult caving of thick-hard roof strata in coal mines. The formation-scale coupled numerical modeling of hydraulic fracturing and pressure relief mining provides critical insights into hard roof deformation mechanisms and parameter optimization. This study employs discrete element numerical simulation software to analyze hydraulic fracturing parameters in small-scale rock masses. A coupled implementation method of ‘hydraulic fracturing & pressure relief mining’ is proposed, which is segregated solved the integrated solid-fluid module to enhances the computational efficiency of formation-scale high-density mesh models. Afterwards, the corresponding engineering case analyses were also conducted. The results show that: The hydraulic fracturing curve at small-scale can be divided into stages such as the water pressure accumulation period, crack propagation period, crack gentle expansion period, water pressure release period, and the stable period. Increasing the fracturing fluid flow rate is conducive to forming complex hydraulic fractures. When hydraulic fractures interact with bedding weak surfaces, they exhibit phenomena such as halting at the bedding plane due to filtration, propagating through the bedding plane, and resuming propagation after filtration. The precise locations of tensile fractures generated by hydraulic fracturing at the strata scale were extracted and re-integrated into the solid mechanics module, and an efficient calculation method for the model ‘hydraulic fracturing & pressure relief mining’ was established. Analysis of examples of pressure relief mining through hydraulic fracturing of hard roofs shows that hydraulic fractures in hard rock mainly develop vertically. When encountering bedding planes, they mainly manifest as interlayer filtration, and may also occur through layer expansion, torsion, etc. After hydraulic fracturing and pressure relief, the hard roof collapses earlier and more fully, the compaction degree of the goaf is higher, and the hanging roof structure is improved. Finally, the necessity, optimization, and scalability of the upscaling simulation of hydraulic fracturing and pressure relief mining for hard roofs are discussed.