Abstract
The permeability characteristics of a broken coal mass under repeated loading and unloading conditions exert significance on spontaneous combustion of coal in goaf during the mining of coal seam groups. Considering this, by using the seepage test system for broken coal-rock mass, seepage tests under cyclic loading and unloading conditions, were carried out on broken coal masses. The test results show that the fitting curves between permeability and effective stress, strain and porosity are a logarithmic function, cubic function and power function, respectively. Besides, the permeability of a broken coal sample under cyclic loading and unloading conditions is determined by its porosity, which conforms to the cubic law. With increased cyclic loading and unloading times, the permeability loss, stress sensitivity and the crushing amount of the broken coal sample were gradually reduced, but the particle size gradation of the broken coal sample gradually became better. During one loading and unloading cycle, the stress sensitivity of the permeability of coal samples in the loading stage was far higher than that in the unloading stage. In the loading stage, the re-arrangement, breakage and compressive deformation of coal particles can lead to a reduction in porosity, consequently resulting in a decreased permeability. In the unloading stage, only the permeability reduction of coal samples due to particle deformation can be recovered.
Highlights
Coal spontaneous combustion (CSC) threatens the safety and production of coal mines and it often causes great economic loss, casualties and large-scale environmental pollution [1,2,3]
Goaf is subjected to CSC, which occurs due to the autothermal reaction between broken coal masses and air [4,5,6]
It was supposed that the nitrogen flow in broken coal samples conformed to Darcy’s law and the permeability of coal samples was calculated according to various parameters including the flow of coal samples and gas pressures on two ends of coal samples, which can be expressed as follows: 2P0 Q0 μL
Summary
Coal spontaneous combustion (CSC) threatens the safety and production of coal mines and it often causes great economic loss, casualties and large-scale environmental pollution [1,2,3]. Goaf is subjected to CSC, which occurs due to the autothermal reaction between broken coal masses and air [4,5,6]. Air leaking from the working face is the main factor triggering fire in the goaf, which depends on the permeability of the broken coal-rock mass in goaf [8,9]. Investigating the permeability characteristics of broken coal-rock mass during compaction is important when trying to prevent CSC in goaf. When multiple coal seams are present in mines, the stress and permeability of goaf are disturbed again during the mining of subsequent coal seams [10,11,12]. Repeated loading and unloading cause further breakage of broken coal-rock masses in goaf, resulting in changes in porosity and permeability, the evolution of the permeability of a broken coal-rock mass in goaf during the mining of coal seam groups differs from that during the mining of a single coal seam
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