Strength and fracture characteristics of coal rock disturbed by protective layer mining under graded cyclic loading

Aimed the coal rock disturbed by the protective layer mining under the action of repeated mining during the recovery was likely to induce the destabilization problem of the roadway surrounding rock, the graded cyclic loading was adopted to simulate the long-term effects of mining stresses in the actual recovery projects, to investigate the mechanical strength and deformation damage characteristics of coal rock disturbed by protective layer mining under graded cyclic loading. Results show that the volume expansion deformation of the coal samples disturbed by the protective layer mining under the process of loading and unloading is obvious, the internal structure damage degree of the coal samples is higher than that of the coal samples undisturbed by the protective layer mining. In the later stage of loading and unloading, the coal samples disturbed by the protective layer mining show plastic failure, while the coal samples disturbed by the protective layer mining show obvious brittle failure. Compared with the coal samples undisturbed by the protective layer mining, the peak strength of the coal samples disturbed by the protective layer mining decreases, the fracture volume of the coal samples per unit volume (50 mm diameter and 100 mm height) increases significantly, and both the peak intensity and fracture volume ratio are distributed more uniformly along the coal seam direction. The physical properties of the coal samples undisturbed by the protective layer mining in the fault zone differ greatly from each other. The coal samples disturbed by protective layer mining is more in line with the law of “there is an inverse corresponding distribution between the complexity of coal rock fractures and the peak strength when coal rock is destroyed” than that of coal samples undisturbed by protective layer mining. © 2022 Zhejiang University. All rights reserved.