With the advancement of mechanization and the deepening of mining operations, safety hazards at excavation faces, especially those related to elevated dust and methane concentrations, have become increasingly prominent and pose serious threats to miners' health and operational safety. To address these issues, a novel ventilation system was developed by improving traditional airflow strategies. This system integrates a specialized air distribution device and a dust removal fan to enhance the control of dust and methane levels. This study combines numerical simulation and orthogonal testing, using data and operational parameters from an actual fully mechanized coal mine excavation face. The optimization focuses on three key parameters of the air distribution device: air volume (50%, 60%, 70%, 80%, 90%), installation distance from the pressure outlet (10, 15, 20, 30, 40 m), and air outlet angle (−30°, −15°, 0°, 15°, 30°). The results show that the optimal configuration consists of a 70% air volume, a 10 m installation distance from the pressure outlet, and a 0° outlet angle. Under these conditions, the average dust concentration at the driver's breathing zone within approximately 5 m of the excavation face remains below 10 mg/m3, while methane concentrations stay under 0.5%, both meeting safety standards. This optimized ventilation arrangement improves the air velocity distribution and effectively reduces dust and methane concentrations at the excavation face, providing a practical reference for cleaner and safer underground operations.

Underground engineering plays an important role in modern urban construction, transportation infrastructure, and resource extraction. The influence of the geometric characteristics of cracks in rock masses on their mechanical behavior and failure modes is a key scientific issue for ensuring the safety of underground structures. This study systematically investigates the effects of prefabricated crack angle and length on the mechanical response, crack propagation, and stress distribution of rock-like samples through uniaxial compression tests and particle flow code in two dimension (PFC2D) numerical simulations. The results show that crack angle significantly affects the sample strength, exhibiting a nonlinear trend of initially weakening and then strengthening. Increasing crack length exacerbates stress concentration, leading to a gradual reduction in peak strength and ultimately to saturation. Crack angle and length jointly influence the crack propagation path and failure mode. As the crack angle increases, the failure region shifts from the sample edge to the center. Longer cracks enhance the dominance of crack-induced failure and reduce the influence of preexisting holes. When the crack angle approaches 45°, acoustic emission (AE) energy release becomes more concentrated and abrupt. Moreover, longer cracks lead to earlier AE onset and faster energy accumulation. At the high-stress stage, the influence of crack geometry on stress distribution becomes more pronounced. The highest stress concentration occurs at a crack angle of 22.5°, and increasing crack length results in more complex damage evolution. Numerical simulations further validate these findings, revealing that both crack angle and length play dominant roles in stress redistribution.

Sandstone and cemented coal gangue-fly ash backfill (CGFB) form composite load-bearing structures in goafs, where repeated water ingress and drainage may weaken stability. This study investigated sandstone–CGFB composite specimens subjected to 0, 3, 6, and 10 water immersion–drying cycles using uniaxial compression, energy analysis, acoustic emission (AE) monitoring, scanning electron microscopy (SEM), and digital image analysis. The results show that increasing the cycle number prolonged initial compaction, reduced the pre-peak slope and peak stress, and promoted gradual post-peak deformation. From 0 to 10 cycles, the mean uniaxial compressive strength decreased from 4.78 to 1.70 MPa, while the 50% secant modulus E₅₀ decreased from 579.63 to 125.08 MPa, representing reductions of 64.44% and 78.42%, respectively. Meanwhile, the peak strain increased from 0.0098 to 0.0146. The peak total input energy decreased from 23.93 to 13.01 kJ·m⁻³, and dissipated energy accounted for 97.9% of the total input energy at test termination after 10 cycles. AE counts and energy shifted from concentrated release near and after the peak to earlier, more dispersed activation. Tensile-type AE events remained dominant, reaching 95.1% at 6 cycles, whereas the shear-type proportion increased to 21.2% at 10 cycles. The dynamic b-value showed stage-dependent rather than monotonic variation, reflecting a transition from fracture localization near the peak to early defect activation and subsequent small-scale damage. In the selected post-failure CGFB-side SEM fields, the apparent fracture-surface defect fraction increased from 21.23% to 35.89%. Together with the mechanical and AE results, the fracture-surface observations are consistent with a progressive weakening of the cemented structure and a shift toward earlier, more dispersed damage during loading. These laboratory findings provide indicators for assessing the water-related deterioration of sandstone–CGFB composite structures in backfilled goafs.