Abstract: A rock burst experiment of granite with prefabricated single crack under the true-triaxial stress condition with a free face was carried out in this paper, and the rock burst process was monitored by high-speed camera system and acoustic emission (AE) system. The failure mode, strength, deformation and AE evolution characteristics of the rocks with different orientations of cracks were investigated. The relationships between crack orientation and rock burst process and ejection kinetic energy was analyzed. The rock burst mechanisms of the rocks with prefabricated single crack and the intact rocks were compared. The analysis of mechanical properties shows that, with the decrease of crack dip angle, the failure mode of rock samples generally changes from "internal shear and external splitting" to "Z-type oblique shear", and the weakening effect of cracks on rock strength is increasing. When the crack dip angle is less than 30°, the peak stress of rock is generally only about half that of the intact rock samples. The larger the length of the small-dip-angle crack is, the more obvious the splitting phenomenon of the rock slab will be, the slightly larger the rock burst pit will be, the larger the strength reduction amplitude will be, and the smaller the peak axial strain will be. The crack close to the free surface of rock samples will aggravate the splitting effect of the rock slab, large deformation and numerous cracks will occur in the plastic stage generally. When the crack is exposed and cuts off the free surface, it will be difficult to form rock burst pits. The analysis of rock burst process and ejection kinetic energy shows that, with the decrease of crack dip angle, the ejection kinetic energy of rock samples decreases significantly first and then increases slightly, the dip angle of 30° is the turning point of transformation. The closer the internal cracks of the rock sample are to the free surface, the smaller the ejection kinetic energy of the rock sample will be. The resin-filled crack greatly increases the ejection kinetic energy of rock sample, but not when cement fills cracks. The analysis of AE characteristics shows that, the longer length and the smaller dip angle of the crack are, the larger the time proportion of "quiet period" to rock burst process is, and the smaller the absolute energy proportion of rock burst in the post-peak stage is. If the crack is close to rock free surface, two "quiet periods" generally appear, and one more sudden rise will also appear in the AE absolute energy evolution curve. With the increase of crack dip angle, the duration of the initial fluctuation stage of b value of rock samples with crack becomes longer, the decrease rate of decline stage becomes larger, and the minimum b value point is closer to the rock burst time. In general, the mechanism of strain-type rock burst in rocks with cracks differs from that in intact rocks. The mechanism of strain-type rock burst in rocks with cracks can be classified into two types: shear-fracture type and tension-slabbing type. The research results provide a scientific basis for the mechanism analysis of rock burst disaster of fractured rocks and the early-warning by using AE technology.

Key words: rock burst, rock, crack, acoustic emission, ejection kinetic energy