Hydraulic weakening technology for unsupported top coal at the end of fully mechanized caving face

To address the issue of excessive unsupported top coal length and delayed caving at the end of a fully mechanized caving face, a hydraulic weakening technology for face-end top coal in steeply inclined thick coal seams was proposed based on the coal-breaking and cavity creation method using high-pressure water jet. Taking the 31123-1 steeply inclined fully mechanized caving face of Dongxia Coal Mine as the engineering background, a mechanical model was established for the unsupported top coal condition at the end of a fully mechanized caving face. Based on the principle of linear superposition, the deflection equation under the combined action of bending moment and shear stress was derived. It was found that the deflection at the free end of the unsupported top coal was greatest, while the fixed end generated the maximum tensile stress due to the peak bending moment, identifying this area as the optimal zone for hydraulic weakening. This study revealed that high-pressure water jets weakened the mechanical connection between the top coal and coal pillar through impact-induced coal fracturing, cavity creation, and fracture propagation. A standardized procedure of "drilling positioning→high-pressure jet coal breaking→rotational cavity creation" was proposed. By combining numerical calculation and field application, this study analyzed the evolutionary patterns of stress distribution in the face-end top coal zone, deformation characteristics of surrounding rock of roadways, and collapse effects of unsupported face-end top coal before and after applying the new technology. The results demonstrated that after hydraulic weakening for coals, the peak values of vertical stress inside the coal pillar and advanced support stress of the working face decreased by 29.6% and 17.4%, respectively. The maximum convergence of the roof-to-floor and rib-to-rib in roadways reduced by 35.8% and 37.8%, respectively, and the unsupported top coal length decreased from the original 10 m to 1 m.