Experimental investigation on influence of loading rate on rockburst in deep circular tunnel under true-triaxial stress condition

To investigate the influence of loading rate on rockburst in a circular tunnel under three-dimensional stress conditions, the true-triaxial tests were conducted on 100 mmx100 mmx100 mm cubic sandstone specimens with d50 mm circular perforated holes, and the failure process of hole sidewall was monitored and recorded in real-time by the microcamera. The loading rates were 0.02, 0.10, and 0.50 MPa/s. The test results show that the rockburst process of hole sidewall experienced calm period, pellet ejection period, rock fragment exfoliation period and finally formed the V-shaped notch. The rockburst has a time lag and vertical stress is high when the rockburst occurs. The vertical stress at the initial failure of the hole sidewall increases with loading rate. During the same period after initial failure, the rockburst severity of hole sidewalls increased significantly with increasing loading rate. When the vertical stress is constant and maintains a high stress level, the rockburst of hole sidewall under low loading rate is more serious than that under high loading rate. With increasing loading rate, the quality of rock fragments produced by the rockburst decreases, and the fractal dimension of rock fragments increases.