## 6 Conclusions  
To investigate the optimal delay time between holes for pre-splitting blasting and the damage caused to the retained rock mass at the Niukutou open-pit mine in Qinghai, China, this study employed a research method that combines numerical simulation and field experiments. By comparing and analyzing the results of numerical calculations and field tests, the following conclusions were drawn:  
1. The numerical calculation results indicate that, within the same blasting area when only the delay time is changed between holes and when the delay time between holes  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Ways of Initiation** & **Number** & **Damage Depth/m** & **Average Depth of Damage/m** \\ \hline \multirow{2}{*}{Simultaneity} & 1 & 1.51 & \multirow{2}{*}{1.62} \\  & 2 & 1.73 & \\ \hline \multirow{2}{*}{Hole by hole} & 1 & 0.8 & \multirow{2}{*}{0.9} \\  & 2 & 1.0 & \\ \hline \hline \end{tabular}
\end{table}
Table 13: Damage results of pre-splitting blasting acoustic wave test.  
Figure 19: The sound wave velocity curve before and after pre-splitting blasting in one hole.  
is 0 ms, the maximum damage depth in the rock mass is 32.56 cm; however, when the delay time is 18 ms, the damage depth is much lower, at 30.61 cm. This shows that delayed initiation reduces the depth of damage caused to the rock mass after blasting. In the case of simultaneous initiation, the peak maximum effective stress of all selected characteristic elements (A, B, C, and D) is higher than that when the delay time between holes is 12 ms, 18 ms, or 24 ms; when the sequential initiation method is used, the relationship between the delay time between holes and the peak effective stress experienced by the characteristic elements is not a simple inverse relationship. In particular, when the delay time between holes is 18 ms, the peak effective stress of the selected characteristic elements is higher than when the delay time is 12 ms or 24 ms. This indicates that the delay time has a significant impact on the rock fragmentation effect, with a delay time of 18 ms being the most ideal. 3. The field test results show that when the delay time between holes is 0 ms, the damage depth of the retained rock mass is 1.62 m; however, when the delay time is 18 ms, the damage depth decreases to 0.90 m, a reduction of approximately 44%. This result indicates that an appropriate delay time, especially a delay time of 18 ms between holes, can significantly reduce the depth of damage caused to the retained rock mass, thereby optimizing the effect of pre-splitting blasting.  
Writing--original draft, investigation, and formal analysis, J.L. Project administration, funding acquisition, and conceptualization, X.L. Software, C.T. Writing--review and editing, T.H. Data curation, J.X. All authors have read and agreed to the published version of the manuscript.  
This research was funded by National Natural Science Foundation of China (No. 52274083), the Basic Research Project of the Yunnan Provincial Department of Science and Technology (No. 202201 AT070178), and Yunnan Major Scientific and Technological Projects (No. 202202 AG050014), supported by Science and Technology Projects Serving Key Industries in Higher Education Institutions by the Yunnan Provincial Department of Education (FWCY-BSPY2024053).  
Not applicable.  
Some or all data that support the findings of this study are available from the corresponding author upon reasonable request.  
This work has received support from the School of Land and Resources Engineering at Kunming University of Science and Technology and Qinghai Hongxin Mining Co., Ltd. Thank you to the School of Land and Resources Engineering at Kunming University of Science and Technology and Qinghai Hongxin Mining Co., Ltd. for their support. The author is also very grateful to the staff of Qinghai Hongxin Mining Co., Ltd. for their warm hospitality and valuable support during the on-site inspection and testing period.  
The authors declare no conflicts of interest.