# Study on Vibration Effects and Optimal Delay Time for Tunnel Cut-Blasting Beneath Existing Railways
## 5 Conclusions  
Taking Zhongliangshan Tunnel of Chongqing as the engineering background, this study carries out numerical simulation for the controlled blasting of tunnel blasting construction crossing under existing railways. Through the numerical simulation results, the vibration reduction through waveform interference of millisecond blasting is summarized. It also analyzes the corresponding law of the surface stress in the different classes of the surrounding rock conditions. The selection basis of optimal millisecond time for tunnel cutting hole blasting is proposed. The main conclusions are as follows:  
1. According to the superposition mechanism of blasting seismic waves, the vibration reduction through waveform interference can be further divided into two types of vibration reduction: superposition of the main vibration phase and superposition of the main period. 2. The millisecond delay time \(\Delta\)t is less than the main period T of the blasting seismic wave, and, the blasting seismic waves weaken each other. At this point, it realizes the vibration reduction through the main-period superposition with the best vibration reduction effect. 3. With the deterioration in the surrounding rock quality, the dissipation of low-frequency waves in the formation increases. The first arrival time and peak time of stress waves arriving at the surface are delayed. This increases the degree of wave superposition between the blasting seismic waves. 4. Surface wave arrival times progressively increase (from 1.9 ms in Class III to 3.0 ms in Class IV and 4.0 ms in Class V). The maximum surface stress exhibits a progressive decline across different classes of surrounding rock: 4.0 kPa for Class III, decreasing to 3.1 kPa for Class IV, and further reducing to 2.0 kPa for Class V.
5. For cut-blasting in tunnels crossing under existing railways, millisecond delay blasting can be used to reduce vibration effects. Class III adopts a 3 m delay time to achieve the vibration reduction through waveform separation. A delay time of 1 ms can be used for Class V to achieve vibration reduction through waveform interference. By combining on-site blasting monitoring data and determining the relevant parameters of blasting seismic waves, this method can be used to determine the optimal delay time. Limitations: In practical engineering applications, rock mechanics parameters and geological media exhibit complexity, containing features such as faults. Although this study included different classes of surrounding rocks, the mechanism of seismic wave superposition caused by explosions under complex geological conditions is still unclear. Meanwhile, the vibration reduction effect of delayed blasting is also limited by specific parameters of the surrounding rock and the accuracy of electronic detonators. These aspects \begin{table}
\begin{tabular}{c c c} \hline
**Class** & **Range (m)** & **Optimal Delay Time (ms)** \\ \hline \multirow{3}{*}{III} & \(-\)50\(-\)47.5, 0\(-\)10, 45\(-\)50 & 1 \\  & \(-\)40\(-\)0, 10\(-\)40 & 3 \\ \cline{2-3}  & \(-\)50\(-\)35 & 1 \\  & \(-\)7.5\(-\)17.5 & 2 \\ \multirow{3}{*}{IV} & \(-\)35\(-\)7.5 & 3 \\  & 30\(-\)50 & 5 \\  & 17.5\(-\)30 & 7 \\ \cline{2-3}  & \(-\)50\(-\)25 & 1 \\ \multirow{3}{*}{V} & 25\(-\)50 & 4 \\ \hline \end{tabular}
\end{table} 
Table 10: Optimal delay time of the different class rock masses. will be extensively studied in future research. Optimizing delay time in complex geological environments is a key direction for future research. Conceptualization, R.H. and Y.Z. ; methodology, R.H.; software, R.H.; validation, R.H. and W.L. ; formal analysis, R.H. and Y.Z. ; investigation, R.H., W.L. and Y.Z. ; resources, W.L. ; data curation, W.L. and Z.L. ; writing--original draft preparation, R.H.; writing--review and editing, Y.Z. ; visualization, R.H.; supervision, Z.L. ; project administration, W.L. ; All authors have read and agreed to the published version of the manuscript. This research received no external funding. Not applicable.