Experimental investigation of rockburst in hard rock tunnel with bedding planes of different inclination angles under true triaxial condition

To investigate the influence of bedding dip on rockburst characteristics of layered hard rock in deep-buried tunnels, true triaxial tests are conducted in this study on sandstone specimens with different bedding dip angles. The rockburst evolution process is continuously monitored using an acoustic emission (AE) system. The results show that the average maximum depth and width of rockburst craters, fragment mass, and the peak rockburst stress all exhibit a U-shaped trend with increasing dip angle, with a critical turning point at 45°. Except for specimen with dip angle of 0°, the sidewall failure locations of all other specimens are oriented approximately perpendicular to the bedding planes, indicating that the presence of bedding induces a deviation in the failure orientation. For all specimens, fine fragments after rockburst accounts for more than half of the total debris. The crushing index initially increases and then decreases with increasing dip angle, suggesting that the degree of fragmentation first weakens and then intensifies. The AE ring-down count and absolute energy exhibit similar evolutionary patterns, and the abrupt increase in ring-down count can serve as a precursor for rockburst warning. Two types of mixed-mode cracking are identified under different bedding inclinations, which are tension-dominated cracks (15°, 45°, 60°, and 90°) and shear-dominated cracks (0°, 30°, and 75°). In particular, the proportion of tensile and shear cracks for specimen with dip angle of 60° is closest when compared with that for specimens with other dip angles. The simultaneous abrupt increase in both the number of shear-tensile cracks and the shear-tensile ratio may serve as an additional precursor to failure. In addition, the AE b -value exhibits fluctuations during the initial loading stage, which gradually increases with the increase of applied stress, and drops sharply immediately before the onset of rockburst. This abrupt sustained decrease in b -value can also be considered as an important precursor for rockburst.