# Safety Assessment of the Cover-and-Cut Method Under Blasting Vibration Induced by Tunnel Excavation
## 4 Vibration Analysis of Subway Tunnel Construction
### Summary and Analysis of Results  
#### 4.3.1 Overview of Field Vibration Monitoring Results  
This study conducted a detailed analysis of field blasting vibrations by setting up monitoring points on the central pillar and diaphragm wall. The NUBOX-8016 vibrometer was used to measure particle velocity, frequency, and duration in both horizontal (x, y) and vertical (z) directions. Monitoring point 6, which experienced higher vibration levels, was selected for detailed analysis. A comparison of field and simulated peak particle velocity (PPV) values indicated that the simulated values closely followed the trend of the field data, reflecting the impact of blasting on the structure (Figure 10).  
In Figure 10, the time-domain signal suggests a lower frequency event, which appears to contradict the previously stated 40-80 Hz frequency range. This discrepancy may arise due to the scaling of the time-domain signal, which corresponds to a short-duration event of approximately 60 ms. Such a short event duration can lead to an apparent broadening of the frequency content in the time-domain representation, as it may introduce lower frequency components that are not representative of the true dominant frequency range. Specifically, the limited duration of the event in the time-domain could widen the signal's spectral components, making it appear as though lower frequencies dominate the event. To resolve this issue, the frequency-domain analysis provides a clearer picture of the true frequency content of the vibrations.  
To further clarify the frequency content, we conducted an analysis of vibration energy distribution across different frequency bands. The frequency-domain analysis, using a Fourier transform, decomposes the time-domain signal into its frequency components, allowing us to assess how the energy is distributed across different frequencies. This  
Figure 13: Peak values of particle vibration velocity at different measurement.  
analysis was performed over key frequency ranges, including 40-80 Hz, to evaluate whether the peak energy aligns with the natural resonance frequencies of the structure.  
The results of the energy distribution analysis (shown in Figure 11) indicate that the peak energy is predominantly concentrated in the 40-80 Hz range, which corresponds to the expected dominant frequencies observed in the vibration signal. However, it is important to note that while there is some energy content in this frequency range, the peak energy does not significantly overlap with the structural resonance range of the tunnel, which typically lies between 50 and 200 Hz. As a result, the energy levels in the 40-80 Hz range are not sufficient to induce resonance in the tunnel structure, confirming that resonance is unlikely to occur.  
By incorporating the vibration energy distribution analysis across these frequency bands, we resolve the previous discrepancy noted in Remark 4 regarding the apparent lower-frequency event in the time-domain signal. The frequency-domain analysis provides a more accurate and detailed characterization of the vibration and demonstrates that the observed vibrations fall within the safe zone with respect to the structural resonance frequencies. Therefore, this analysis ensures that the measured blast-induced vibrations do not pose a resonance risk to the tunnel structure.