Low-Frequency Characteristics of Vibrations Induced by Transient Unloading of In Situ Stress and Its Influence on Safety of a Deeply Buried Tunnel

Abstract

High in situ stress is a typical geological characteristic of tunnel excavation in hydropower projects in Southwest China. First, the spectrum characteristics of the vibration induced by the blasting load and transient unloading of in situ stress in the surrounding rock masses were calculated for the full-face millisecond-delay blasting excavation of a deeply buried circular tunnel. Then, the methods of time–energy density and amplitude spectra were employed to analyze the spectral characteristics of the vibration monitored in the excavating and adjacent tunnels at Pubugou, Shenxigou, and Jinping II hydropower stations. Finally, the low-frequency characteristics of vibration induced by transient unloading of in situ stress, hidden in the monitored vibration, were discovered, and their influence on tunnel safety was analyzed. The results showed that under low in situ stress levels (1–10 MPa), the peak value of the vibration induced by the blasting load was larger than that caused by transient unloading of in situ stress, and the difference in the excavating tunnel was notably larger than that in the adjacent tunnel. With an increase in in situ stress (more than 20 MPa), the peak value of vibration induced by transient unloading of in situ stress exceeded that produced by the blasting load in the adjacent tunnel; the same rule was found in the excavating tunnel under high in situ stress (50–70 MPa). A lower frequency and larger amplitude indicate that the vibration induced by transient unloading of in situ stress is much more dangerous than that produced by blasting vibration.