Evolution Law of Fractured Rock Permeability under Cyclic Loading and Unloading

Gob is formed by large-scale voids during underground coal mining. The overlying strata of gob collapses from the bottom up. The fractured zone of overlying strata in the gob is subjected to repeated loading and unloading during coal mining. The permeability of the fractured zone constantly changes, directly affecting the gas migration and enrichment law in the gob. The permeability experiment of prefabricated fractured rock samples under cyclic loading is carried out. Results show that permeability of fractured rock samples has a negative exponential function with stress. The permeability difference of the loading and unloading stages represents the closure and opening degrees of pores and fractures in the rock, respectively. The permeability loss between the loading and unloading stages represents the irreversible deformation of pores and fractures. The effects of vertical and horizontal fractures on rock permeability are different. The permeability, permeability sensitivity coefficient, and permeability recovery rate of the vertically fractured rock samples are greater than those of horizontally fractured and intact rock samples. The permeability sensitivity coefficient and the permeability recovery rate of horizontally fractured rock samples are greater than that of intact rock samples, but the permeability has little difference from that of intact rock samples. In addition, the effects of axial and confining stresses on rock permeability are also different. The permeability difference, permeability sensitivity coefficient, and permeability recovery rate of rock samples under cyclic confining stress are greater than that of cyclic axial stress. Finally, the effects of fracture and stress on rock permeability are compared. The permeability change rate caused by prefabricated fracture is greater than that of stress, particularly the vertical fracture. The research results have important theoretical guiding significance for gas extraction in coal mine gob.