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Experimental study on failure characteristics of jointed rock mass under staged unloading conditions

Strength and deformability characteristics of rock with pre-existing fissure are governed by cracking behavior. To further research the effects of pre-existing fissures on the mechanical properties and crack coalescence process under staged unloading conditions, a series of uniaxial compression tests were carried out. In this study, cement, fine sand, and water were used to fabricate a kind of brittle rock-like material rectangular model specimen. The mechanical properties of rock-like material specimen used in this research were all in good agreement with the brittle rock materials. The pre-existing fissure was created by mica sheets and the unloading zone was created by pre-filling Polylactic acid (PLA) resin material during the molding processes the model specimen. And the staged unloading test was conducted based on polylactic acid’s hot-melt characteristics. In order to investigate the crack initiation, propagation and coalescence process, inclination angle of pre-existing fissure was set to change from 0° to 90°. The mechanism of crack propagation and coalescence under photographic monitoring was discussed. And these experimental results indicate that the failure modes in the zone above unloading zone mainly consist of four modes: separation failure, shear-slip failure, mixed shear-separation failure and compression-shear failure. The experimental results are also expected to increase the understanding of the strength failure behavior and the cracking mechanism of rock under staged unloading.