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تعداد صفحات این فایل: ۳۰ صفحه

بخشی از مقاله انگلیسیعنوان انگلیسی:Mechanical behavior and permeability evolution of gas infiltrated coals during protective layer mining~~en~~

Abstract

This study investigated the stress–strain–permeability relationship of a Chongqing coal under the stress path during the mining process. The abutment stress was first measured at the longwall (LW) face 3211 of Songzao Mine in Chongqing, China. The field monitoring results revealed that the concentration coefficient of the abutment stress was approximately 1.5–۲۰ during protective layer mining. Then, triaxial compression tests for the gas-infiltrated coals were conducted under the above stress path and different gas pressures. These tests, with the simultaneous actions of unloading confining stress and loading axial stress, are called SUL tests. The triaxial compression tests revealed that the peak deviatoric stress and the corresponding strain of coal under SUL tests were lower than those under conventional triaxial compression (CTC) tests. Poisson’s ratio was higher, but the elastic modulus was lower in SUL tests. The permeability evolution of coal under the SUL tests underwent four distinct stages: the increasing stage in the process of SUL, decreasing stage, slowly increasing stage beyond the yield point, and sharply increasing stage after the peak stress. With the increased gas pressure, the peak deviatoric stress and corresponding axial strain decreased, Poisson’s ratio increased, and elastic modulus decreased. Further, the permeability of coal increased with increasing gas pressure in the complete deformation process.

۱ Introduction

Coal mining induces different stress zones in front of the working face of coal—a relief stress zone, abutment stress zone, and recovered stress zone—from the initial in-situ stress state.1 During this coal mining process, the stress path experiences the loading of axial stress and the simultaneous unloading of confining stress. However, current investigations on the coupling mechanism between the mechanical behavior and permeability of coal are almost all based on conventional triaxial compression (CTC) tests. This CTC path may not represent the coal mining process. It is necessary to validate the applicability of the current investigations to the coal mining process.

The stress evolution of coal seams has been widely investigated. A series of three-dimensional numerical models were developed to examine the effect of the mining depth, in-situ stress and stope geometry as well as the orientation on the overbreak of a stope wall.2 For example, Wang et al. took the cutting face from disaster sites as prototypes to study the effect of the stress distribution on dynamic disasters of coal mines.3 Guo et al. presented a comprehensive study on the longwall in a deep underground coal mine.4 They investigated the mining-induced strata movement, stress changes, fracture openings, and gas flows. Their studies included the field monitoring of overburden displacement, changes of stress and water pressure at the LW face. They concluded that the vertical stress increased and the horizontal stress decreased during mining. All of the above investigations revealed that deeper mining faces a higher risk of mining disasters.

Coal seam gas couples with coal deformation to affect mining safety. In China, coal seams are rich in coal seam gas. There is approximately 10 billion m3 of the recoverable coalbed methane (CBM) in China. The Erlian basin in Inner Mongolia contains 2 billion m3 of recoverable CBM. The Ordos basin and Qinshui basin contain more than 1 billion m3 of recoverable CBM. The gas content gradually increases with coal burial depth. The accumulation of coal seam gas during mining may trigger dynamic disasters, such as gas emission, and even the occurrence of coal and gas outburst in front of the working face. The gas accumulation depends on many parameters, of which the permeability evolution of coal is the most important. Therefore, the investigation of the permeability distribution within coal and the surrounding rocks is the core work of the simultaneous extraction of coal and gas.

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