Abstract: During tunnel construction, blasting excavation and shotcrete are usually carried out alternately. Under blasting vibration, the bond between shotcrete and tunnel rock may be lost, resulting in local cavities at the interface. In this study, the influence of the cavity on blasting vibration characteristics and safety assessments of the tunnel shotcrete are investigated by using a theoretical model of thin-plate vibration, 3DEC modeling and field monitoring. The results show that when cavities exist at the interface between shotcrete and surrounding rock, the vibration velocity is amplified, the vibration duration becomes longer and the vibration frequency decreases for the shotcrete at the cavity location. Furthermore, the amplitude-frequency spectrum of the vibration has only one prominent peak. In addition, the peak particle velocity (PPV) decays faster with distance based on the regression analysis. For the safety assessments of the shotcrete layer, the extent of the dangerous zone is increased accordingly because the vibration on the shotcrete at the cavity location is amplified. As a result, the support time of shotcrete needs to be delayed and the maximum charge weight per delay needs to be reduced so as to ensure the safety of shotcrete under blasting vibration. This will lead to a decrease in the tunnel excavation efficiency and an increase in the construction cost.

Key words: tunnel, shotcrete, cavity, blasting vibration, peak particle velocity