Experimental study on damage and failure of multi-layer pre-cracked sandstone specimens

Aiming at the problem of strong rock pressure disaster induced by the breaking of hard thick roof, taking the thick roof of 122104 working face of Caojiatan Coal Mine as the engineering background, the weakening effect of multi-layer pre-cracking modification on roof strength is studied. A multi-layer pre-cracked sandstone sample model with different heights was established. The strength and energy characteristics, dynamic crack propagation and micro-damage of the sample were studied by uniaxial compression loading. The test results show that the peak strength of the pre-cracked specimen decreases by 36.9% to 63.4%, and the weakening range of the three-layer (upper, middle, lower) specimen is the largest, and the pressure relief effect is the most significant. The prefabricated cracks promote the conversion of elastic strain energy to dissipated energy. The effective dissipation ratio before the peak increases to about 50%, the residual elastic energy after the peak decreases significantly, and the energy release process changes from concentrated to dispersed. The failure mode evolved from tensile splitting to tensile-shear composite, forming the characteristics of multi-crack synergistic propagation. The porosity, maximum pore area and probability entropy of fracture increase with the increase of fracture horizon, and the microstructure tends to be loose and complex. The synergistic pre-cracking weakening effect of the upper, middle and lower layers is the best, which can significantly reduce the overall strength and failure risk of the thick and hard roof.