Experimental study on the mechanical properties and damage evolution of sandstone specimens with prefabricated cracks of different heights

Taking the thick overlying roof of the 122104 working face in the Caojiatan Coal Mine as the research background, prefabricated crack models of different heights was prepared and CT-based in-situ uniaxial compression tests were conducted to investigate the mechanical characteristics of specimens with prefabricated cracks of different heights, along with their mechanical damage and degradation processes. The experimental results show that compared with intact specimens, the peak strength, acoustic emission energy, total strain energy, elastic strain energy and post-peak strain energy of the prefabricated crack specimens are significantly reduced. The energy dissipation index of the upper, middle, and lower crack specimens increased by 24.7%, 28.5%, and 62.6%, respectively. The strength degradation index increased by 21.9%, 28.1%, and 54.6%, respectively, and the impact energy index decreased by 19.9%, 20.9%, and 49.5%, respectively. Among them, the lower-crack specimen shows the best pressure relief effect, with the lowest initial kinetic energy upon failure. The crack initiation times for the intact specimen and the specimens with upper, middle and lower prefabricated cracks are 89, 44, 42, and 39 s, respectively. The failure mode shifts from tensile-splitting failure in the intact specimen to tensile-shear composite failure in the cracked specimens. As the crack height decreases, the initiation and development of cracks occur earlier, with increased microcracks and rock debris at the fracture surfaces. In the lower-crack specimen, the surface porosity, maximum pore area and probability entropy of the fracture increased by 98.51%, 317.58%, and 5.38%, respectively. Based on the analysis of failure characteristics of specimens with prefabricated cracks at different heights, the damage degradation mechanism is revealed from the perspective of hindered force chain transmission. The findings is indicative for optimizing the selection of pressure-relief stratification in thick roof areas.