Experimental study on rockburst of surrounding rock in an elliptical tunnel with different axial ratios

Abstract

The effect of the ratio of long and short axes on the rockburst of an elliptical tunnel structure was investigated by performing a series of true triaxial tests on red sandstone cuboid samples containing seven types of elliptical holes with different long–short axial ratios, using a self-developed true triaxial compression equipment. The entire process of spalling was monitored and recorded using a miniature camera. According to the test results, the failure stress, geometrical characteristics of the rockburst notch, fragment characteristics, and ejection velocity field characteristics of the rockburst are summarized, and the influence of the axial ratios of the elliptical hole on the failure characteristics and stability of the tunnel are discussed in detail. The test results indicate that the rockburst process of elliptical tunnels with different axial ratios experiences four processes; namely, calm period, small particle ejection period, spalling, and rockburst. The shape of the failure area after the rockburst presents a V-shaped feature. With the increase of the axial ratio, the area, angle, and depth of the V-shaped area show a decreasing trend, the initial failure stress of the cavern and the rockburst failure stress exhibit a trend of first decreasing and then increasing; the tangential stress of the cavern shows an increasing trend; the mass percentage of micro, fine, and medium grain decreases; and the average ejection velocity first decreases and then increases. Under the same stress conditions, rockbursts are more likely to occur on circular sections. Under the same section conditions, the ellipse can bear larger loads when its major axis is parallel to the disturbance stress. These conclusions are helpful to the efficient design of the tunnel structure.