Coal seam water injection is an effective technique for dust reduction in coal mines. Imbibition is an important means of water transport in coal seams and is influenced by coal bedding. The spatial distribution of water in the seepage process was obtained, and the spontaneous seepage characteristics and water transport mechanisms of coals containing different layered angles were revealed by NMR. Results show that the bedding mainly serves as the water flow channels in the imbibition. When the angle increases, the imbibition rate of adsorption pores and seepage pores in the early stage decreases. The imbibition is firstly transported to the top quickly along the bedding flow channel, 1.5h for 0° bedding coal sample, 24h for 90°, when the water reached its apex, the curve no longer changed and imbibition proceeded slowly. With the increase of bedding angle, the imbibition equilibrium time increased from 3h to 30h, up to 9 times more, the maximum imbibition rate decreases significantly, reduced from 5.64 g/h to 1.13 g/h, the flow resistance increases, and the remaining capillary force after equilibrium increases from 0.007 Pa to 5.24 Pa. When the capillary force cannot overcome the flow resistance, the imbibition reaches equilibrium.

Uniaxial compression test of outburst coal was conducted to explore the mechanical characteristics of coal with outburst tendency in low gas pressure environment. The mechanical properties of coal were analyzed, the surface crack morphology of coal was obtained and the energy characteristics of coal during loading were determined. Results are as follows. In the pressure range of 0.03–1.0 MPa, the uniaxial compressive strength and elastic modulus of coal show a negative exponential relationship with pressure, and both decrease with the increase in pressure. With the increase in gas pressure, the angle between the failure surface and horizontal surface of coal body increases gradually. With the increase in gas pressure, the fracture morphology of coal failure tends to be complex. When the gas pressure is low, the coal can store a large amount of energy, and its ability to resist damage is greatly improved.

Roof-guided hydraulic fracturing technology presents significant potential for regional gas drainage in outburst-prone coal seams. To investigate the dynamic cross-interface propagation behavior of fractures, we conducted hydraulic fracturing experiments on “roof rock-coal” composite specimens using a self-developed true triaxial fracturing system equipped with visual observation capabilities. Real-time fracture propagation was monitored through an observation window, while digital image correlation (DIC) was employed to analyze the evolution of the strain field during cross-interface fracture extension. Key findings include: (1) Significant heterogeneous deformation was observed during fracture propagation, with distinct zones of tensile and compressive strain localization flanking the fracture. Fractures predominantly developed within the tensile strain localization zones, with propagation paths closely aligned with the morphology of these zones. (2) The influence ranges of the tensile and compressive strain localization zones measured 532–706 times and 459–481 times the fracture width, respectively. During cross-interface propagation, strain fields exhibited marked disparities across the rock-coal interface, with strain localization zones extending into the adjacent medium as fractures advanced. (3) Propagation rates were notably lower in the roof strata, where fracture-tip tensile strain localization zones were short and wide. Conversely, higher propagation rates were observed in lower-strength coal seams, characterized by long and narrow tensile strain localization zones at fracture tips. (4) Fracture path deviation occurred due to shear strain generation at fracture tips induced by coal-rock heterogeneity. Propagation paths consistently followed the direction of maximum shear strain intensity, resulting in a tensile-shear (Mode I-II) hybrid fracture morphology.

To investigate the impact of stress variations on coal permeability during mining, this study combines theoretical analysis with experimental research to reveal the permeability variation patterns under three mining-induced stress conditions. The results indicate that coal permeability exhibits a three-stage variation characteristic under different mining-induced stresses: slow increase, significant increase, and rapid increase. As axial stress increases, permeability initially decreases and then stabilizes. After reaching the strain peak and coal sample failure, through-cracks form on the sample surface, leading to a significant permeability increase. During the quasi-hydrostatic pressure loading stage, permeability decreases with increasing stress, while it gradually recovers during the confining pressure unloading stage, with a notable increase after reaching peak stress. Gas extraction efficiency is particularly ideal after the stress peak, with the most significant effect observed in the pressure relief zone. Based on the Forchheimer equation and the Klinkenberg effect, the calculated results indicate a parabolic variation trend between gas flow rate and pressure gradient, as well as between pore pressure and permeability.