RESUMEN
Thick-coal-seam with complex structures and thick-hard roof in the initial mining phase pose various challenges, including a long weighting interval, strong rock pressure, poor top coal caving performance (TCCP), and significant coal loss. These problems directly affect the safety and efficiency of the mining operations. This study employs the principles of elastic thin plates and ellipsoidal bodies to unravel the formation mechanism of strong rock pressure in thick-hard roof and the influence of parting on the TCCP. In addition, a hydraulic fracturing technique is proposed for safe-efficient recovery during the initial mining phase. The reliability of this technique is verified through numerical simulations and field experiments. The research findings reveal the following. (1) The primary causes of strong rock pressure in the mining face are attributed to a long weighting interval and wide collapse range of the main roof, and the weighting interval is primarily influenced by the thickness and tensile strength of the main roof. (2)The key factor affecting the TCCP is the cantilever beam structure formed by the fracture of the thick-hard parting, as it intersects the ellipsoidal body during coal caving. The simultaneous fracturing of both the top coal and roof can reduce the weighting interval on the working face. This process effectively decreased the strength of the thick-hard parting within the top coal, while simultaneously enhancing its load strength and eliminating the cantilever structure resulting from the parting fracture. It not only reduces the first weighting intensity but also promotes the early and proper coal release, thereby enhancing the TCCP and ensuring safe-efficient mining during the initial mining phase. (3) Aiming at the difference in the strengths of the top coal and roof, a graded hydraulic fracturing technique and system were proposed. Fracturing boreholes with a diameter of 60 mm, spacing of 10 m, and height of 20.85 m, which can economically and effectively ensure the fracturing results. Field applications have demonstrated that fractured coal and rocks in fracturing areas exhibit well-developed fractures. During the initial mining phase, the weighting interval in the working face was reduced by 20 m, resulting in a decrease in the overburden pressure and 26.9% reduction in the lumpiness of the top coal. Additionally, the recovery rate increased by 31.19%.
RESUMEN
Due to the large mining area, the fully-mechanized top-caving mining with thick-hard roof is easy to form cantilever structure on the lateral roof of the working face, which on the one hand causes high stress level of adjacent roadway and serious deformation of roadway, on the other hand causes gas accumulation in corners, which brings severe challenges to safe and efficient mining of the mine. In this study, a mine facing such problems in Jincheng, China was taken as the research object. Based on the mechanical characteristics of coal and rock, the characteristics of overlying strata activity in the mining process of working face are mastered, and the dual effects of controlled transformation of lateral overlying strata structure on stress field and gas field were revealed. On this basis, roadway reinforcement and gas drainage schemes were put forward and applied. The results showed that the strength of the hard rock stratum was high in the triaxial stress environment, and it was not easily destroyed. However, once the strata exceed their strength threshold, they break down. In addition, the strength of coal is relatively low, and it is continuously deformed when the force exceeds its strength. The overlying strata structure after thick-hard roof fully-mechanized top-caving mining evolves in the following manner: "long cantilever length formed by the main roof being broken in the initial stage, voussoir beam formed by the upper hard roof being broken in the middle stage, and double cantilever beam formed by overlying strata compaction." The stress carried by upper hard rock stratum is transferred to coal pillars, which is the main reason for the high stress environment of multi-purpose roadway with large coal pillars. The controlled transformation of lateral overlying strata structure by pre-splitting and roof cutting can realize the "transfer-unloading" of coal pillar stress and the "plugging and driving" of corner gas. Based on the hydraulic fracturing reconstruction of lateral overburden structure, the grouting reinforcement scheme of roadway and dynamic gas drainage scheme were put forward. The results demonstrated that after roof cutting, the maximum deformation of the surrounding rock in the multi-purpose roadway was reduced by approximately 90 %, and the maximum concentration of corner gas was decreased by 15.28 %. This approach successfully achieved a collaborative control effect on roadway surrounding rock stability and gas emission well within the safety limits.
RESUMEN
For understanding the mechanical performance and strain energy evolution mechanism of thick hard roof sandstone samples, a sequence of uniaxial compression trials with acoustic emission (AE) monitoring were carried out. The results indicate: (1) The stress-strain curve of the thick hard roof sandstone specimens exhibits distinct stage characteristics. Based on the evolvement of instantaneous axial stiffness, it is separated into Fracture Closure Phase, Elastic Deformation Phase, Steady Fracture Expansion Phase, Unsteady Fracture Expansion Phase, and Post-Peak Phase. (2) The AE energy and cumulative count curves of the thick hard roof sandstone specimens also exhibit significant stage characteristics and can be mutually corroborated with the stage division of the stress-strain curve. (3) Based on the energy conservation principle, the evolution of strain energy density in the thick hard roof sandstone specimens under uniaxial compression loading was analyzed, and plastic strain energy increment was employed to study the stage characteristics of strain energy dissipation. (4) A damage constitutive model for the thick hard roof sandstone specimens was constructed, considering the characteristics of strain energy dissipation. This model effectively describes the stress-strain relationship among the samples, which undergo strain hardening, strain softening, and sudden destruction.