Mining fracture is the key factor in realizing water-conservation mining, stability evaluation of coal mining subsidence areas, and accurate restoration of the surface eco-environment. To clarify, the evolution and distribution of mining fractures, based on the key strata theory and the mining degree of overburden failure, the contact state of rock stratum after overburden critical failure was analyzed, the formation mechanism of “saddle shaped” water conducting fracture zone was revealed, and the permanent mining fractures were determined to be primarily concentrated in the overburden structure at the boundary of the working face. According to the shear test of sandstone in the fracture zone, the damage degree of the rock sample was positively correlated with shear stress, and the overburden structure exhibited no instability. Based on the distribution of mining fractures, the fracture zone was divided into the original fracture, tensile fracture, structural void, and void compaction areas in the horizontal direction. The structural void area had the largest residual deformation in the goaf. A regional division method based on overburden critical failure was proposed and applied in engineering by using surface movement monitoring. The results showed that the surface subsidence near the boundary of working face was greater than that in the middle, and the residual surface deformation in the goaf was closely related to the mining time, which has long-term characteristics. The rationality of the regional division method of mining fractures was also verified.

Backfill mining is an effective measure to control surface subsidence and restrain floor water inrush. It is an essential part of green mining technology. To solve the problem of confined water in mines under buildings, this study was carried out by combining theoretical analysis, laboratory simulation, and numerical simulation, taking the Liangbei coal mine as the research area. The coal seam floor failure characteristics of traditional longwall caving and paste filling mining methods were compared and analyzed. Based on the relevant mining theory, the key parameters, such as mining thickness and filling rate under the critical state of water inrush, were obtained. Then, the feasibility of backfill mining was expounded, and the surface subsidence of paste backfill mining was predicted and measured on site. The results demonstrated that the longwall caving mining method not only had the risk of water inrush, but also the possibility of step cracks at the surface, with the potential to result in serious damage to buildings. However, the backfill mining method reduced the floor damage depth of the coal seam from 12 m to 7 m, which reduced the water inrush coefficient by 12%, the maximum vertical concentrated stress by 42.1%, and the displacement subsidence value by 78.8%. These parameters correlated negatively with backfill strength. Meanwhile, the maximum subsidence, maximum inclined deformation, and horizontal displacement deformation were estimated as 730 mm, 2.5 mm/m, and 1.1 mm/m, respectively, consistent with the measured values (608 mm, 2.1 mm/m, and 0.9 mm/m, respectively). More relevantly, there were no obvious cracks found in the surface buildings, ensuring the safety of mining above confined water on the working face, and realizing the effective protection of surface buildings.

Mining-induced surface subsidence causes a series of environmental hazards and social problems, including farmland destruction, waterlogging and building damage in the subsidence area. To reduce mining damages, an innovative method of controlling the surface movement scope via artificial weak planes generated by hydraulic fracturing technology was proposed in this paper. Numerical models were built to analyze the influence of weak planes with different heights and dips on the overlying strata movement. The numerical simulation results showed that the weak planes structure cut off the development of the overlying strata displacement to the surface and affected the surface movement scope. When the weak planes’ dips were bigger than the angle of critical deformation, with the increase of the weak planes’ heights (0–120 m) the advance angle of influence changed from 53.61° to 59.15°, and the advance distance of influence changed from 173.31 m to 140.27 m which decreased by 30.04 m. In applications at Sihe coal mine in China, directional hydraulic fracturing technology was used in panel 5304 to form artificial weak planes in overlying strata. The measured surface subsidence and deformation value met the numerical simulation results and the mining-induced surface movement scope reduced. Moreover, no damage occurred to the surface buildings which were predicted to be in the affected area after extraction. This technology provided a new method to protect the surface structures from damages and had great benefits for the sustainable development of coal mines.

The deformation and re-crushing characteristics of different lithological caving crushed gangues in mine goaf directly affect the overburden strata movement, which significantly affects the surface subsidence of mining goaf. The effect of particle size on the re-crushing characteristics of different lithological caving crushed gangues in mine goaf is investigated in this study based on an innovative compression–AE (acoustic emission) measuring method. The results showed the following: (1) The compression deformation was divided into three stages: rapid, slow, and stable compaction. With the increase in axial pressure, the large particle skeletons were destroyed, medium particles were displaced and slid, and small particles filled the pores. (2) For singular lithologies, stress was positively correlated with pressure, and porosity was negatively correlated with stress. The composite sample was between the singular gangue samples. (3) The fractal dimension of crushed gangue samples was exponentially related to the proportion of gangue in singular and combined lithologies. (4) The cumulative AE count and energy of the combined lithological gangue samples were between those of the singular samples. The research results provide a theoretical foundation for further research into the characteristics of the overlying strata, surface movement, and safety management of the goaf.

The mining of the part of the inclined ore body below a goaf is crucial for improving resource extraction and safe production. In this study, the cementation properties of crushed stone during the mining of the inclined ore body were investigated by means of laboratory experiments, theoretical analysis, and numerical simulation. Additionally, orthogonal experiments were performed to assess how factors like water–cement ratio, crushed-stone particle size, and cement–sand ratio affect the strength of the grouting concretion body (GCB). Furthermore, the fluidity of the slurry under different ratios was also measured. Considering both the fluidity of the slurry and the strength of the GCB, the optimal ratios of the slurry were determined to be a water–cement ratio of 2.5:1 and a cement–sand ratio of 1:4. This ratio was then used for crushed-stone cementing under the poorest crushed-stone particle size conditions, based on which mechanical parameters were obtained from experiments. Theoretical analysis equated the problem of the grouting range to the width of the plastic zone of surrounding rock, and a conclusion was reached that the width of the GCB should be at least 29 m. The numerical simulation results reveal that among 30 mining rooms formed below the GCB, 24 mining rooms are in a stable state and 6 mining rooms are partially damaged on a small scale. As a whole, the GCB formed by grout filling into the goaf manages to effectively support the stope below, and it is verified that the theoretical calculation method of the width of the GCB is feasible.