Since the weight of overburden is sustained by both the backfill body and the unmined solid coal in coal mining with compacted backfilling (CMCB) panels, the stress and deformation characteristics of the surrounding rocks in coal mining are radically changed. The overburden movement control mechanism by coordinating with backfill body and shield in CMCB was studied systematically in this paper. Based on the analysis of deformational and structural characteristics of surrounding rock in CMCB panels, the methods of theoretical analysis, numerical simulation and engineering test are employed. The results show that the fracture of the main roof is mainly controlled by the filling ratio φ and is non-correlated to the shield supporting pressure p. However, p has a significant control effect on the deflection of roof within the shield canopy length, and adversely affects the filling ratio. With the increase of the filling ratio of the gob, the maximum sagging of the immediate and the main roofs, the peak front and the influence range of the abutment pressures are gradually reduced. Correspondingly, the stable period of internal pressure of backfill body in the gob is shortened. Engineering practice shows that the sagging of the gob roof, the distribution of the abutment pressure, the distribution of the internal pressure in the backfill body, and the ground surface sagging results obtained by the in-situ measurement are approximately corresponding to the theoretical analysis and numerical simulation results.

In solid backfill coal mining (SBCM), loose gangue backfill material (LGBM) is used to backfill the goaf after coal resources are exploited from the underground mines. Under certain geological conditions, LGBM with a certain height may be soaked in the water, and then becomes saturated, significantly altering its mechanical properties. The confined compression experiments were used in this paper to analyze the deformation and the crushing characteristics of LGBM with varying water soaked heights in coal mines. The results showed that a large number of small holes that were distributed in the gangue blocks were the main reason why the material absorbed water and was softened. The crushing ratio and the maximum axial strain of LGBM samples gradually increased with the water soaked heights of the samples. In addition, there was a strong linear correlation between the crushing ratio and the maximum axial strain. When LGBM was used as a solid backfill material in SBCM, its deformation resistance would significantly decrease after it was soaked in the water. Higher water soaked height of LGBM led to lower deformation resistance and greater influence on the quality of backfilling. This research has great significance in getting a deep and better understanding of the mechanical properties of LGBM, as well as guiding engineering practice.

Coal gangue slurry filling is an important technical means for harmless and large-scale disposal of gangue under low-interference conditions, and is one of the most important ways to achieve green mining, which is in line with the national concept of green development. This paper systematically expounds the technical background and scientific connotations of the birth of slurry filling, clarifies the key technology and process principles of slurry filling, and constructs the lag distance and optimization method of slurry filling based on the bulking coefficient. In order to explore the distribution law of the bulking coefficient of the overburden broken zone in the mining process, UDEC numerical simulation and similar simulations were used to analyze the movement law of a coal seam roof and the distribution characteristics of the bulking coefficient. The results show that with the evolution of the spatial structure of the overlying strata of the goaf, the subsidence of the coal seam roof decreases from the bottom to the top, and finally becomes stable. In the advancing direction of the working face, the bulking coefficient decreases continuously, and shows certain zoning characteristics. With the mining, it moves forward periodically with dynamic changes. In the strike direction, it can be divided into three areas: the natural accumulation area, the load-affected area and the gradual compaction area. Finally, the lag distance of slurry filling is determined to be 60 m, and the effect of adjacent grouting filling is good in the field test.

In recent years, as one of the green coal mining technologies, coal gangue filling and coal mining technology has been widely used. The coal gangue filling body in goaf may be in an acidic or alkaline mine water environment for a long time, and the rich heavy metal elements in coal gangue may pollute the groundwater environment. In order to predict the migration distance of heavy metal elements in the goaf floor more accurately, this paper first designed the coal gangue static immersion experiment. By using ICP, the concentration of heavy metal elements in the soaking solution was tested, and then the permeability stress sensitivity of mudstone was tested. Finally, based on the above experimental results, the numerical simulation model for migration of heavy metals in coal gangue under the condition of seepage-stress coupling was established by using COMSOL multiphysics, and the concentration distribution and seepage law of heavy metals in the mudstone floor of goaf were analyzed with Mn element as an example. The research results showed that the heavy metals with pollution risk in coal gangue soaking liquid were beryllium (Be) and manganese (Mn). The diffusion distance of heavy metal elements in all stress states was 9.537 m when using the traditional model. When calculating after combining with the modified model of permeability stress sensitivity analysis, when the stress states were 0 MPa, 3 MPa, 6 MPa, 9 MPa, 12 MPa and 15 MPa, the diffusion distance of heavy metal elements was 9.326 m, 6.748 m, 5.9 m, 5.657 m, 5.558 m and 5.55 m, respectively, and the reduction rate was 2.21%, 29.24%, 38.14%, 40.68%, 41.72% and 41.80%, respectively. The migration and concentration distribution of heavy metal elements were predicted more accurately, which provided a basis for predicting and evaluating the impact risk of coal gangue on groundwater in filling coal mining goaf.

The mining areas in Western China are characterized by water shortage and ecological fragility, and the coal resources in these areas are extracted in a large-scale and highly intensive manner, which is highly likely to induce ecological problems, such as soil erosion and grassland degradation. In addition, there is a secondary protection zone of Hongshixia water source near Guojiatan minefield. If the water resources loss of Salawusu Formation is too large, it will affect the normal water supply of Yulin city. To reasonably coordinate coal exploitation and water protection, the development characteristics of water-conducted fissures with different backfill ratios are obtained in combination with theoretical analysis, field test, and numerical modelling. The panel layout of the layered-backfill-based water protection working face and the parallel operation of mining and backfilling are designed, and a feedback regulation system integrating hydrological monitoring and backfill ratios is established, which assists in evaluating the effectiveness of water protection. The results indicated that when the slicing mining method was adopted with the roof naturally caving, the maximum development height of the water conducted fracture zone was 220.0m (developed to the Yanan Formation), and the water conducted fracture zone could only develop to the Yanan Formation without crossing the Zhiluo Formation when the backfill ratio reached 80. The water emission rates in the west of the field testing area reduced from 334.95m³/h to 60.60m³/h, dropping by 86.86. Compared to the scene prior to the layered backfill mining, the shallow water outflow was reduced by 72.74m³/h, and it helped annually reduce the shallow water loss of 637200m³, which was equivalent to 1.74 of the actual water supply to Yunli city every year. The results of this study can provide guidance on effectively avoiding the loss of shallow water resources that are regarded as indispensable sources of domestic water.