Abstract
Study on permeability evolution of an aquifer coal seam in Western China is of great significance for preventing water inrush disaster and realizing water-conserving coal mining. The permeability evolution of an aquifer coal seam is related to a loading path closely under plastic flow. In this work, permeability variations of the Xiaojihan water-bearing coal seam and Longde nonwater coal seam are researched using a transient method under plastic flow. The experiment results indicated the following: (1) Under the same axial strain, the permeability, relative residual strain, and confining pressure influence coefficient of Xiaojihan coal specimens all decrease in plastic flow with the increase of loading-unloading times and confining pressure, while the permeability recovery coefficient increases during this process. (2) The permeability of Xiaojihan water-bearing coal specimens decreases with the growth of axial strain in plastic flow, resulting in the increase of relative residual strain and reinforcement of plasticity. Besides, the confining pressure influence coefficient decreases and the permeability recovery coefficient decreases slightly with the axial strain. (3) Finally, the permeability of Xiaojihan coal specimens is greater than that of Longde coal specimens, while the confining pressure influence coefficient and permeability recovery coefficient of Longde coal specimens are greater than those of Xiaojihan coal specimens. The closure rate of internal cracks of the water-bearing coal specimen is lower than that of the nonwater coal specimen, which is beneficial for water storage and transport.
Highlights
It is necessary to set up safe coal pillars with different supporting and isolating functions during coal seam excavation
The permeability, relative residual strain, and confining pressure influence coefficient of Xiaojihan coal specimens all decrease with the increase of loading-unloading times and confining pressure
The plastic deformation is mainly produced during the first loading and unloading process, while the water-bearing coal specimens are mainly subjected to neutral loading in the last three loading and unloading processes, which is the elastic deformation after plastic deformation
Summary
It is necessary to set up safe coal pillars with different supporting and isolating functions during coal seam excavation. Plastic flow occurs in some areas of a coal pillar after shear yield occurring in these areas, and the deformation of coal pillars causes the change of permeability correspondingly. Multiple redistribution of the stress field leads to the coal pillar being continuously in the abutment pressure area or pressure relief area; that is to say, the coal pillar is under a cyclic loading and unloading process. Coal outburst and coal caving support lifting and crushing of top coal caused by unloading confining pressure generated when Shimen is connected to coal. It is a process of repeated loading and unloading of coal, and unloading is more likely to cause instability of coal rock
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