The roof falling accident is a serious threat to the lives of miners in deep coal mining, especially when the coal mine is more than 1000 meters deep. In regard to the 5306 coalface in the Tangkou coal mine, Shandong, China, the depth of coal seam is 992.8 m and the stress concentration coefficient of the roadway surrounding rock is 3.33. This leads to a serious deformation of the roadway roof, thereby producing a high risk of the roof falling disaster. In this pursuit, based on the mechanical analysis of roadway roof subjected to a high abutment pressure, the mathematical expressions of the setting load and movable column length of supports were introduced. Furthermore, the stability control mechanism of the roadway roof was analyzed and the optimized support parameters of supports are provided. The results showed that the longtime effective support of the roadway roof required the strength and deformation coupling of supports and anchored surrounding rock. The support length of the belt roadway should be at least 57.7 m, with 0-30 m away from the coalface supported by hydraulic supports and 32-57.7 m supported by single props. In addition, the maximum setting load and movable column length of hydraulic supports were 21.67 MPa and 280.3 mm and 12.44 MPa and 177.1 mm for single props, respectively. By applying the optimized support parameters of supports to the belt roadway of the 5306 coalface, the effective control of the roadway roof and the disaster control of roof falling were realized.
The permeability evolution law of saturated rock under cyclic loading–unloading after shear yield is an important basis for revealing the water resistance performance and water inrush risk of overlying rock under multiple mining conditions. In this paper, the influence of the confining pressure, the cyclic loading–unloading times (CLT), and the volumetric strain on the post-peak permeability of saturated sandstone was studied by carrying out a post-peak permeability experiment. Based on SEM images and an improved simulated annealing algorithm, the 3D internal structure characteristics of sandstone samples before and after the experiment were reconstructed. The influences of the confining pressure on pore diameter, effective porosity, connectivity, seepage path length, and tortuosity of the sandstone before and after the experiment are discussed. Research results indicated that (1) In the post-peak cyclic loading–unloading stage, the volumetric strain is negatively correlated with permeability. At the unloading and initial loading stage, the volumetric strain showed a gradually decreasing trend as the specimen was slowly compressed. However, at the middle and final loading stages, the volumetric strain curve shifted to the left and showed a decreasing trend, resulting in an obvious increase in permeability. (2) The influence of CLT on k is closely related to the confining pressure level. When the confining pressure changed from 4 MPa to 12 MPa, the volumetric strain–average stress hysteretic curve shifted to the left in turn and the corresponding permeability gradually increased. When the confining pressure increased to 16 MPa and 20 MPa, the volumetric strain–average stress hysteretic curve shifted to the right in turn and the corresponding permeability showed a decreasing trend. No matter what the value of CLT, the magnitude of sandstone permeability gradually decreased and the decreasing trend became flat as the confining pressure increased, especially for σ3 = 16 MPa and 20 MPa. (3) No matter what value of the confining pressure, the hysteresis area of the first cycle was larger than that of last three cycles, indicating that the plastic deformation generated in the first cycle was larger than that generated in the last three cycles and the recovery rate of the permeability increased with an increase of CLT. (4) As the confining pressure gradually increased, the pore diameter, effective porosity, and connectivity all approximately showed a linear decrease due to more easily compacted pores and cracks under high confining pressure, lower connectivity, and permeability, while the length and tortuosity of the seepage path increased nonlinearly, roughly due to a more significant shear failure phenomenon where the seepage path became more tortuous, that is, the greater the tortuosity, the longer the seepage path. The research results can provide an important theoretical basis for water resistance performance and water inrush risk assessment of overlying aquifer under the influence of mining stress.
The effective discrimination of aquiclude mining stability is one of the important indexes for the feasibility judgement of water-conserved mining. Based on the mining-induced deformation characteristics of weakly cemented aquiclude and the water level change of weakly cemented aquifer in northwest China, a mechanical model of mining stability of weakly cemented aquiclude is established, and the mining instability criterion of weakly cemented aquiclude and its influencing factors are analyzed. The results show that the weakly cemented aquiclude has strong plastic deformation ability and mainly undergoes bending deformation during coal mining. Considering the mining-induced bending deformation of weakly cemented aquiclude and the groundwater pressure variation of the weakly cemented aquifer, the expressions of the deflection, stress components, and strain components of weakly cemented aquiclude are derived. Furthermore, the stress instability and strain instability criteria of the weakly cemented aquiclude are proposed. The influences of aquiclude thickness, elastic modulus, Poisson’s ratio, groundwater level, coalface length, and longwall panel length on the mining stability of weakly cemented aquiclude are analyzed. The research results are applied to the feasibility judgment of water-conserved mining in Xinjiang Ehuobulake Coal Mine, and the validity of the mining stability criterion of weakly cemented aquiclude is verified.
To unravel the permeability variation mechanism of weakly cemented rocks (WCR), the paper conducted triaxial permeability tests on weakly cemented sandstones (WCS) collected from the Jurassic formation in northwest China. The paper identified the correlation of WCS permeability versus porosity, cementation structure, and mineral composition, further developing a model to characterize the WCS stress–damage–permeability relationship. The research indicated that the WCS permeability was initially high due to the naturally high porosity, large pore diameter, and loose particle cementation, thus favoring a significant decline as pore convergence in the compaction stage. In the residual stage, kaolinite and montmorillonite minerals disintegrated into water and narrowed fractures, causing a slight permeability increase from the initial to the maximum and residual stages. The WCS matrix fracturing was phenomenologically accompanied by clay mineral disintegration. By assuming that the matrix can be compressed, jointed, and fractured, the paper defined a damage variable D and accordingly developed a stress–damage–permeability relationship model that incorporated matrix compression, jointing, and fracturing. The model can describe the WCS permeability regime regarding the high initial permeability and slight difference of the maximum and residual permeabilities versus the initial.
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