Experimental Research into the Evolution of Permeability in a Broken Coal Mass under Cyclic Loading and Unloading Conditions

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.