Abstract: Aiming at the problems of poor caving，difficult caving and frequent rock burst of top coal in steeply inclined thick coal seam in north mining area of Wudong Coal Mine，based on the analysis of geological occurrence conditions，the mechanical degradation characteristics，stress field evolution law and pre-splitting effect of top coal under water-force coupling are systematically studied. Through uniaxial compression test，the uniaxial compressive strength，elastic modulus and poisson 's ratio of coal under nat⁃ ural water-bearing state are 13.59 MPa，1.324 GPa and 0.328 respectively，and the corresponding values under saturated state are 11.22 MPa，1.122 GPa and 0.383 respectively. The water action reduces the com⁃ pressive strength and elastic modulus of coal by 17.4% and 15.3%，respectively，and the Poisson 's ratio increases by 16.8%. The stress-strain curve shows that the natural water-bearing coal sample is dominated by tensile-shear composite failure，and the saturated coal sample is X-shaped conjugate shear splitting failure，and the fracture connectivity and fragmentation degree are significantly improved. Based on FLAC3D ，a three-dimensional numerical model of 600 m×350 m×96 m is established. The results showed that the maximum compressive stress of 45# coal seam decreased from 8.47 MPa to 7.22 MPa after water in⁃ jection，with a decrease of about 14.8%. The local stress gradient slowed down，and the high stress migrat⁃ ed to the adjacent complete coal body and deep area. Based on the effective stress principle，Mohr-Cou⁃ lomb strength criterion，water-induced weakening characteristics of coal samples and the safety boundary of on-site construction，it is suggested that the water injection pressure should be controlled at 10~15 MPa to promote the opening and shear slip of primary fractures and provide low-resistance channels for subse⁃ quent blasting fracture expansion. The research results can provide theoretical basis for top coal pre-split⁃ ting and safe and efficient fully mechanized caving mining in steeply inclined thick coal seam.

Key words: steeply inclined thick coal seam, water-force coupling, pre-splitting of top coal, uniaxial compression test, numerical simulation, water injection pressure