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The artificial fracturing technique under coupled hydro-mechanical effects is widely used in many rock engineering. Therefore, the study on the fracturing behaviors and mechanical properties of hydro-mechanical coupled cracks is very crucial. In this study, a series of fracturing tests were conducted on the cylinder gypsum specimens with single pre-existing cracks using triaxial compression loading system. Water pressure was applied inside the pre-existing cracks and led to the specimen failure with external compression loading. A new type of cracks, namely horizontal coupled cracks (HCC), were found in some specimens. Macroscopic observations reveal that HCC, which were mainly caused by the hydraulic pressure, were different from any tensile wing cracks, shear secondary cracks, or shear anti-wing cracks. Subsequently, a microscopic study was performed using scanning electron microscope (SEM), the outcomes suggest that: (1) Shear fracturing zones (SFZ) and tensile fracturing zones (TFZ) under coupled hydro-mechanical effects displayed distinct characteristics on orientations, length, and independence of gypsum grains; and (2) the HCC were tensile cracks when they just initiated from outer tips of pre-existing cracks. While tensile stress made major contribution to the specimen failure during the whole fracturing processes, the HCC became tensile and shear mixed cracks when the specimen was about to fail.

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To deeply understand the cracking mechanical behavior of brittle rock materials, numerical simulations of a rock specimen containing a single preexisting crack were carried out by the expanded distinct element method (EDEM).Based on the analysis of crack tips and a comparison between stress-and strain-based methods, the strain strength criterion was adopted in the numerical models to simulate the crack initiation and propagation processes under uniaxial and biaxial compression. The simulation results indicated that the crack inclination angle and confining pressure had a great influence on the tensile and shear properties, peak strength, and failure behaviors, which also showed a good agreement with the experimental results. If the specimen was under uniaxial compression, it was found that the initiation stress and peak strength first decreased and then increased with an increasing inclination angle α. Regardless of the size of α, tensile cracks initiated prior to shear cracks. If α was small (such as α ≤ 30 ), the tensile cracks dominated the specimen failure, the wing cracks propagated towards the direction of uniaxial compression, and the propagation of shear cracks was inhibited by the high concentration of tensile stress. In contrast, if α was large (such as α ≥ 45 ), mixed cracks dominated the specimen failure, and the external loading favored the further propagation of shear cracks. Analyzing the numerical results of the specimen with a 45 inclination angle under biaxial compression, it was revealed that lateral confinement had a significant influence on the initiation sequence and the mechanical properties of new cracks. K E Y W O R D Sconfining pressure, crack inclination angle, EDEM, mechanical properties