Coal is the basic resource underpinning energy generation in China, however, constant, large-scale mining of coal results in many problems such as ecological destruction of mining areas. As a result, backfilling of solid waste underground is proposed to control strata and surface subsidence and to protect the environment. At present, these materials, such as granular material, cemented material and high-water-content materials are mainly used for backfilling. This study summarised the types of backfill materials that are used in coal mines in China along with the backfilling process. Moreover, distribution and characteristics of mines backfilled with these backfill materials were obtained and analysed. Considering the socio-environmental aspects that affect backfilling, this research proposed a guideline for the selection of backfill materials and then analysed specific engineering cases of three backfill materials. In addition, the future development of backfill materials was discussed. With extensive extraction of shallow coal resources in China and, therefore, rapid depletion of coal resources in eastern regions of China, coal mining depth is increasing significantly. As a result, it is required to investigate new backfill materials suited for the deep high-stress environment.

China is the world’s largest coal producer country. However, large-scale coal mining has led to severe environmental pollution issues such as surface subsidence and gangue piling up. The gangue discharging amount has ranked the first in the world and coal mine enterprises are facing enormous discharging reduction pressure. This paper summarizes the research progress of the solid backfilling mining technology and then illustrates the realistic demands and significance of implementing underground coal-waste separation. It also focuses on the technical principles, systems and key equipment of the common underground coal-waste separation methods, such as the selective crushing method, the dense medium shallow groove method, the vibro-assisted jigging method and full-size water separation method and ray identification method. In addition, the selection steps of underground coal-waste separation method, the design process of large section separation chamber and the design principle of separation and backfilling system are proposed, finally, the mining-separating-backfilling + X for coal mining is put forward. By combining the technology of mining-separating-backfilling with other technologies, such as gob-side entry retaining with non-pillar mining, gas extraction, solid waste treatment, water protection mining, mining under buildings, railways and water bodies, the integrated mining methods, mining-separating-backfilling + setting pillars, gas drainage, treatment, protection and prevention methods are formed. It also introduced the ‘mining-separating-backfilling + gas extraction’ technology’s whole idea, system arrangement, separation equipment and practical engineering application effects based on the specific engineering case of pingmei no. 12 coal mine. The results indicate that the integration of underground coal-waste separation and solid backfilling technology could achieve gangue discharging reduction, underground washing and surface subsidence control. It is effective at realizing green mining.

There is a huge amount of solid waste generated in coal resource exploitation and utilization, leading to effects on the environment and hindering sustainable development. Aiming at the difficulty of disposal of coal-based solid waste from high-yield, high-efficiency mines, this research proposed a technique of gangue grouting filling in subsequent space (GGFSS) for coal mining and explored the movement of key strata backfilled using GGFSS technique and surface deformation characteristics using theoretical analysis and a field measurement method. The results indicate that the equivalent backfilled ratio (EBR) when using GGFSS is defined as the ratio of the grouting volume to available void volume, which is mainly affected by bed separation void volume, grouting volume, and the void volume of the caving zone. Using an EBR ground control mechanical model, the changes in the displacement and bending moment of key strata with the variation of the EBR were investigated. The critical EBR for the rupture of key strata was determined to be 74.44%. On this basis, the calculation methods of equivalent mining height (EMH) and the subsidence factor of GGFSS were proposed and then a surface deformation prediction model of GGFSS was established based on the principle of the probability integration method. The predictions implied that, at an EBR greater than 75%, the surface deformation could be significantly reduced. Engineering applications verified the effect of GGFSS on the control of ground movement. In-situ measurements of concentration, density, and particle size of the filling body were all close to the those designed values: the GGFSS technique shows longer grouting duration and a larger amount of grouting and its filling body can support impervious key strata above the working face in a mine, significantly decreasing surface subsidence.

The solid backfilling mining method is one of the methods used to solve problems arising from strata and ground subsidence in underground mines. Through 2D physical analog modeling, 3D numerical simulation, and field measurement, the effects of the solid backfilling method were analyzed, providing a better insight into optimizing the configurations of a working face beneath a city for safety, environmental problems, and its use in production. In the physical modeling, MatchID software was employed to capture the movement characteristics of overlying strata and ground subsidence during mining and backfilling. Key parameters such as vertical displacement, subsidence characteristics, and rock mass stress variations were monitored and analyzed. In the numerical simulation, FLAC3D was used to simulate and analyze the effect of the backfill body on strata and ground subsidence above the backfill working face. For the field measurements, the Continuously Operating Reference Station (CORS) system was used to confirm the effective control of ground subsidence. With a filling ratio of 80%, the three methods are consistent and show a maximum subsidence value of 0.46 mm (physical simulation), 50.4 mm (numerical simulation), and 47 mm (experienced), significantly lower than the predicted subsidence, which is 281 mm. Therefore, this study demonstrates the reliability and scientific validity of both the physical analog modeling method and the field measurement method in measuring the efficiency of solid backfilling, providing valuable insights into strata and ground subsidence control in longwall coal mining.

Shallow-buried, hard, and extra-thick coal seams are very common in Xinjiang, China, but there are relatively few studies on the mine pressure law and the development characteristics of water-flowing fracture zones (WFFZs) during the mining of such coal seams. In this paper, the mine pressure of the top coal caving face in a shallow, hard coal seam with a hard roof and full bedrock (SHCSHRFB) is analysed, the laws of the surrounding rock deformation and stress of the open-off cut and roadway in the large-mining-height top coal caving face are studied, the characteristics of roof-breaking and overburden fracture development are analysed using the physical similarity simulation method, supporting suggestions for roadways are put forward, and three development stages of the WFFZ are analysed. Field monitoring shows that the hydraulic support stress in SHCSHRFB is weak, but the coal wall and roadway stability are good, which is significantly different from the results in the typical shallow-buried thin bedrock working faces. The measured height of the WFFZ is close to the physical similarity simulation results, but quite different from those arising from use of the empirical formula.

Coal mining-induced surface subsidence causes many environmental problems and geological hazards, including damage to buildings, depletion of water resources, and land desertification. However, numerous coal resources in China are buried under buildings. To solve these problems, a solid waste backfill mining (SWBM) technique has been proposed to control the surface subsidence under buildings. This paper introduced the fundamental principle of the SWBM technique and then presented a model based on the theory of equivalent mining height (EMH) to predict surface subsidence. In addition, the compaction effects of backfill materials were obtained through an experiment on the compaction properties thereof. Afterward, measuring transects were arranged to conduct field observation of surface subsidence: when the backfill materials are sufficiently compacted, the compression degree is 0.13 with 13% deformation. Moreover, the measured maximum subsidence was 18 mm, which is lower than that predicted for the maximum deformation of a nearby building. In addition, surface buildings were slightly influenced by coal mining and no obvious cracks were found. Therefore, this technique provides a new approach for controlling the surface subsidence induced by coal mining.