Aquiclude Stability Evaluation and Significance Analysis of Influencing Factors of Close-Distance Coal Seams: A Case Study of the Yili No. 4 Coal Mine in Xinjiang, China

Aquiclude stability is vital for the realization of water-preserving coal mining. And its evaluation, influencing factors, and their significance analysis are quite topical for the ecosystem conservation. The purpose of this paper was to establish an evaluation index system of weakly cemented aquiclude stability. An evaluation index system was built based on three evaluation factors (subsidence, seepage, and deformation), three subfactors (subsidence gradient, seepage rate, and horizontal deformation), and four evaluation criteria (unstable, weakly stable, medium stable, and stable). The evaluation method was applied to evaluate the index for the case study of Yili No. 4 Coal Mine in Xinjiang, China. Based on the geological conditions of the close-distance coal seams in the mine under study, the main influencing factors and subordinate functions of evaluation index S t a were analyzed. The above three factors’ weights were assessed as 0.1095, 0.3090, and 0.5815, respectively, and the proposed evaluation method’s feasibility was verified by the water level variation in the observation hole. The range and variance analyses were performed to assess the significance of the mining heights of the upper and lower coal seams and the coal seam spacing. The results showed that the aquiclude stability negatively correlated with the mining heights and positively correlated with the coal seam spacing. The decreasing order of influence significance on the aquiclude stability was as follows: upper coal seam mining height, lower coal seam mining height, and coal seam spacing. Water protection mining was an effective measure to control the S t a , and the findings provided a reference value and academic significance for the ecosystem conservation.

Modeling the Spatial and Temporal Evolution of Stress during Multiworking Face Mining in Close Distance Coal Seams

Mining in close distance coal seams (CDCSs) is frequently associated with engineering disasters because of the complicated nature of stress distribution within CDCSs. In order to establish a layout of a roadway to minimize the occurrence of disasters associated with mining CDCS, here the spatial and temporal evolution of stress distribution during the multiworking face mining of a CDCS was explored through numerical simulation based on the engineering and geological conditions of the Nantun Coal Mine. The numerical simulation results indicate that, after the extraction of adjacent multiple working faces, the spatial distribution of stress can be characterized with areas of increased, reduced, and intact stress. The superposed stress of inclined seams that are very close to each other propagates through coal pillars in the bottom floor, and this propagation follows neither the line along the axis of the coal pillar nor the line perpendicular to the direction of the floor. It instead propagates along a line angled with the axis of the coal pillar. The roadway can be arranged in the area with reduced stress, to improve its the stability. Based on the computed spatial and temporal evolution of stress, an optimized layout of roadway was proposed. This layout features a reasonable interval between the mining roadway and a minimal proportion of increased stress areas along the mining roadway and is aligned with geological structures.

Stability Analysis and Derived Control Measures for Rock Surrounding a Roadway in a Lower Coal Seam under Concentrated Stress of a Coal Pillar

Numerical simulations have often been used in close-distance coal seam studies. However, numerical simulations can contain certain subjective and objective limitations, such as high randomness and excessively simplified models. In this study, close-distance coal seams were mechanically modeled based on the half-plane theory. An analytical solution of the floor stress distribution was derived and visualized using Mathematica software. The principal stress difference was regarded as a stability criterion for the rock surrounding the roadway. Then, the evolution laws of the floor principal stress difference under different factors that influence stability were further examined. Finally, stability control measures for the rock surrounding the roadway in the lower coal seam were proposed. The results indicated the following: (1) The principal stress difference of the floor considers the centerline of the upper coal pillar as a symmetry axis and transmits radially downward. The principal stress difference in the rock surrounding the roadway gradually decreases as the distance from the upper coal pillar increases and can be ranked in the following order: left rib > roof > right rib. (2) The minimum principal stress difference zones are located at the center of the left and right “spirals,” which are obliquely below the edge of the upper coal pillar. This is an ideal position for the lower coal seam roadway. (3) The shallowness of the roadway, a small stress concentration coefficient, high level of coal cohesion, large coal internal friction angle, and appropriate lengthening of the working face of the upper coal seam are conducive to the stability of the lower coal seam roadway. (4) Through bolt (cable) support, borehole pressure relief, and pregrouting measures, the roof-to-floor and rib-to-rib convergence of the 13313 return airway is significantly reduced, and the stability of the rock surrounding the roadway is substantially improved. This research provides a theoretical basis and field experience for stabilizing the lower coal seam roadways in close-distance coal seams.

Prevention and Control Technology for Harmful Toxic Gas Intrusion in High-Fire-Hazard-Risk Areas of Close-Distance Coal Seams

Fire hazard-risk area in small coal pits can be found in the southern part of the Shigetai Coal Mine, a close-distance coal seam mining sector in the Shendong mining area, which is susceptible to the risk of harmful toxic gas intrusion, seriously threatening the safety of mining around the working surface. Aiming at this problem, a numerical model representing the mining activity on the close-distance coal seams was established to simulate the movement pattern of overlying strata and the development process of fractures based on the horizontal stress “normalization” technology. Also, the principal air-leak passageways were detected with the SF6 tracer analysis. On this basis, the influencing pattern of harmful toxic gas intruding into the working surface can be comprehensively analyzed, providing a basis for effectively preventing and controlling gas intrusion disasters. The research findings show that, after a lower coal seam has been mined, the caving zone ranges from 73 m to 94 m in height, and the fractured zone tends to develop all the way to the surface. Furthermore, shear fractures are the major passageways for air leakage, and the occurrence of gas intrusion disasters is basically taking place at the same time frame as the occurrence of roof weighting. Meanwhile, the harmful toxic gas intrudes the working surface through the fractures on the security coal pillars and shear fractures on the overlying strata. To prevent intrusion disasters from occurring, the applications of inorganic foaming and curing materials for filling were studied in combination with the actual engineering conditions. The construction grounds in sections where the fire hazard-risk area in small coal pits have not been stripped were drilled, and filling materials were poured into the goaf to create an isolation belt. As can be observed from the applied areas, constructing isolation belts to block the major air-leak passageways can effectively prevent the harmful toxic gases from intruding into the working surface, ensuring the safety of mining on the working surface.