Seepage and damage evolution characteristics of different gas-bearing coal under cyclic loading-unloading conditions

Cyclic loading exists widely in the mining activity and has a strong influence on the strength,damage and penetration property. For example,in the poly-protection engineering of multi-seam mining,the coal in the protected seam are subjected to cyclic loading and unloading,which significantly change the mechanical and seepage character-istics of the coal. The gas has an obvious influence on the mechanical properties and the energy dissipation characteris-tics of the coal. There are significant difference of the mechanical and seepage properties of the coal under different loading and unloading paths and general methods. Therefore,it is necessary to study the seepage and damage evolution characteristics of different gas-bearing coals under cyclic loading and unloading paths. According to the result of a sim-ilarity simulation experiment on the real-time monitoring of stress state of a pro-tected seam on the mining conditions of a coal seam group,three simplified cyclic loading-unloading stress paths were designed. That is stepped-,stepped-in-creasing-,and crossed-cyclic loading and unloading. The experiment was carried out by using a triaxial seepage experi-ment device for heat-fluid-solid coupling in gas-containing coal made in Chongqing University,China. The coal sam-ples used for the experiment were taken from Pingdingshan No. 10 coal mine,Henan Province,China and Yuanzhuang Coal Mine,Anhui Province,China. The results showed that under the three cycle loading and unloading stress paths, the curve of the permeability and stress-strain of the two coal samples have a significant consistency,and the permea-bility of the coal has a decrease trend with the increase of stress and the cycle number. The influence of unloading and loading stresses on the permeability of coal is different. The permeability was subjected to the dual influence of stress and damage accumulation. At the same stress level,after being subjected to stress unloading and loading,the permea-bility of coal samples gradually decreased and its relative recovery rate declined at first and then increased with in-creasing axial stress. The per-meability of coal samples did not increase before the stress exceeded the yield stage of coal samples. The main conclusion is:① Under the three cyclic loading-unloading paths,the mining-enhanced permea-bility of coal samples in loading stage all showed a three-phase increase characteristic with the growth of stress and number of cycles. The mining-enhanced permeability caused by the change per unit volume increased. ② With in-creasing peak stress and damage accumulation in the coal samples,the sensitivity of the permeability to stress gradually decreased. As loads were applied to the coal samples,the influence of stress unloading on their permeability was first strengthened,then reduced. ③ By calculating the LURRs at various stages of cyclic loading,the evolution of the dam-age variable of the coal samples was obtained. Through regression analysis,it can be seen that the rela-tionship be-tween the damage variable and number of cycles can be characterized by applying the Boltzmann function. The func-tion can be used as an empirical formula for damage prediction. ④ The effect of cyclic loading and unloading on the permeability and damage of coal samples is not obvious due to the different coal types. The research results provide a theoretical and practical support for further revealing the permeability improvement mechanism of coal seam and gas drainage engineering design under multiple protections. It can provide a theoretical basis for the anti-reflection of coal seam by cyclic loading (pulsating hydraulic fracturing,electric pulse,etc).