Abstract: Rockburst has become an urgent safety problem in deep mining, and stress gradient plays a significant role in controlling rockburst. To explore the influence of stress gradients on the microscopic failure of surrounding rock, a pneumatic-hydraulic coupled rockburst simulation testing device was used to achieve six-sides loading in three-directions and the gradient loading in the top direction. When the gradient loading was applied at the top, the confining pressure was kept unchanged. Loading-unloading tests were carried out on rock similar material specimens by using rockburst physical models under different stress gradients. The analysis of microscopic morphology of failure surface was conducted using the scanning electron microscope (SEM). The results showed that failure phenomena and characteristics of rockburst were significantly different under varied paths of stress gradient. The greater the stress gradient is, the smaller the pores among the crystals is and the higher the density of the crystals is. The ratio of shear failure to splitting failure is different under different stress gradient paths. The higher the stress gradient is, the greater the ratio of shear failure is. The fractal dimension of detrital crystal contour increases with the increase of stress gradient.

Key words: stress gradient, rockburst, failure characteristics, microscopic analysis, fractal dimension