Mitigation of rock burst events by blasting techniques during deep-tunnel excavation
Abstract The buried depths of the auxiliary tunnels of the Jin-ping II Hydropower Station in China are between 1500 m and 2300 m, and some violent rock bursts were encountered during the excavation process. The measures of rock burst prevention adopted at this station are summarized in this paper, and the mechanisms of these measures are studied and discussed. Combined with the monitoring of the in-situ stress state and the burst proneness of surrounding rocks, the spatial distribution of the rock bursts along the deep tunnel axes and the statistics of rock bursts around the blasting working face are studied. It lays the foundation for generating the strategies for rock burst prevention. The results indicate that the development of rock bursts depends primarily on the level of in-situ stress and on the brittleness of rocks. Disturbances from excavation particularly under blasting excavation conditions have an important influence on the intensity and the scale of rock bursts. Therefore, two primary guidelines for controlled blasting are followed to control rock bursts. One is to decrease stress of the surrounding rocks; the other is to control the disturbance from the excavation. First, the stress-relief blasting method is adopted to reduce the stiffness of the surrounding rocks and reduce the concentration of stress in the blast area. Two different stress-relief blasting design schemes are applied and the extent of the damage and stress relieving effects are compared. Second, the effects of the blasting excavation disturbance in high-stressed rocks are examined. The results indicate that reducing the excavation footage can be used to control the blasting excavation disturbance in deep tunnels. The energy release rate (ERR) for different excavation footages is simulated to verify this conclusion. After the proper excavation footage and perimeter blasting parameters are adopted, the method is applicable to the control of blasting excavation and the reduction of rock burst intensity in engineering practices.