Roof cutting by dense drilling is one of the main methods of gob-side entry retaining. Taking the 203 working face of the Ruineng Coal Mine as the engineering background, a mechanical model is established to clarify the roof breaking mechanism. Numerical simulation is conducted to analyze the roof cutting effects of different parameters, and reasonable roof cutting parameters are identified. The results show that: ① The increase in roof cutting height is beneficial to roof cutting, but excessive height will cause stress concentration of the ‘key structure’ on the side of the coal pillar. ② It is difficult to cut off the roof when the roof cutting angle is too small, and the cantilever length of the roof increases when the roof cutting angle is too large. ③ The larger the borehole spacing, the smaller the plastic penetration rate between boreholes. The optimal parameters of roof cutting are determined as follows: roof cutting height 8 m; roof cutting angle 15°; aperture size 48 mm; hole spacing at 200 mm. The deformation of the resulting roadway is controllable, indicating that the key parameter determination method is effective.

Coal gangue slurry filling technology is an effective way of utilizing coal gangue solid waste resources rationally, and its fluidity and sedimentation behavior have an essential influence on filling performance. However, evaluation and optimization methods for the fluidity and sedimentation performance of coal gangue slurry filling materials (CSFMs) are still scarce. In order to solve this problem, based on the fractal grading theory, this paper carried out an experimental study on the influence of the fractal dimension on the flow characteristics of CSFMs, revealed the impact of the fractal dimension on the flow performance of slurry, and constructed a CSFM flow performance evaluation and optimization model based on the fractal dimension. At the same time, the influence of the fractal dimension on solid mass fraction and particle distribution in the CSFM sedimentation process was analyzed using a sedimentation experiment. Combined with fitting analysis and model construction, a CSFM sedimentation performance evaluation method based on fractal dimension D was proposed. The results show that (1) the slump, expansion, and yield stress of CSFMs increased first and then decreased with the increase in the fractal dimension, and the bleeding rate of CSFMs decreased with the rise in the fractal dimension. The analysis of the consistency coefficient of CSFMs shows that the increase in the proportion of fine particles will increase the consistency coefficient. (2) The fitting analysis indicates that the fractal dimension D of CSFMs is negatively correlated with the sedimentation performance P_S. The change in D is most significant in the range of 2.3 to 2.4, where the slurry’s stability is poor. When D exceeds 2.5, the slurry’s stability improves significantly. (3) Based on the evaluation of flow performance and settlement performance, the flow performance and settlement performance of CSFMs with fractal dimensions between 2.50 and 2.59 achieve the best balance, which ensures the reliability of long-distance transportation and construction quality. The research results can provide a reference for the pipeline transportation of whole gangue slurry and have important practical significance for realizing the large-scale disposal of gangue solid waste and green mining of coal mines.

To enhance the ductility of coal mine filling materials using recycled asphalt pavement (RAP) and address the limitations in RAP recycling and utilization, this study processed RAP into crushed materials (CMs) and ball-milled materials (BMs). Supplementary with fly ash (FA) and cement, RAP-fly ash cement paste backfill (RFCPB) was prepared. For 1000 g of RFCPB slurry, the composition was 365 g CM, 73 g cement, 270 g water, and a total of 292 g of FA and BM, with an F/B ratio ranging from 1:7 to 7:1. A systematic test program was carried out, including rheological property tests, unconfined compressive strength (UCS) tests combined with deformation monitoring, microstructure analysis, and leaching toxicity tests. Based on these tests, the influence of F/B ratio on the action mechanism, workability, mechanical properties, ductility and environmental compatibility of RFCPB was comprehensively explored. The results show that the rheological behavior of RFCPB slurry conforms to the Herschel–Bulkley (H-B) model; with the decrease in F/B ratio, the yield stress and apparent viscosity of the slurry increase significantly, while the slump and slump flow decrease correspondingly, which is closely related to the particle gradation optimization by BM. For mechanical properties and ductility, the 28-day UCS of RFCPB first increases and then decreases with the decrease in F/B ratio, all meeting the mine backfilling strength requirements; notably, the increase in BM proportion regulates the failure mode from brittle to ductile, which is the key to improving ductility. Microstructural analysis indicates that Dolomite and Albite in BM participate in hydration reactions to generate N-A-S-H and C-A-S-H gels, which fill internal pores, optimize pore structure, and thus synergistically improve UCS and ductility. Additionally, the leaching concentration of toxic ions in RFCPB complies with the environmental protection standards for solid waste. This study provides a theoretical basis for enhancing backfill ductility and advancing the coordinated disposal of RAP and fly ash solid wastes.