## 4 Conclusions  
In this study, a set of sympathetic detonation experiments was performed to measure critical distances under blasthole constraints. Meanwhile, to evaluate the feasibility of the new charging structure, full-section tests were conducted in the peripheral holes of the tunnel face. Based on the study results, the main conclusions can be drawn as follows:  
1. [leftmargin=*]
2. The critical distance is an important explosive performance indicator. Based on gap tests under blasthole constraints, the critical distance for No. 2 emulsified explosives is about 1.0-1.1 m. The traditional charging structure was optimized using this critical distance to form a new charging structure that did not require detonating cord and simplified the charging process.
3. Full-section test results show that the new charging structure can achieve smooth blasting without detonating cord in the peripheral holes. This can achieve the same smooth blasting effect as a traditional charging structure. The blasthole traces were clear, the overbreak and underbreak volumes were normal, and tunnel profile control was improved.
4. The new charging structure provided good economic benefits. It could save 200 m of detonating cord and reduce the cost of smooth blasting by 53.6% per excavation round. The new charging structure removed the detonating cord, decreased the intensity of labor, and produced smooth blasting results. Thus, it has broad application prospects.  
The successful application of the critical distance further improved smooth blasting technology. This technology is not very mature and perfect now, which is presently implemented only in the East Tianshan Tunnel. Specific experimental research should be done under other geological engineering conditions. However, the advantages of this technology show that it has significant application value.  
Conceptualization, C.M. and Q.L.; data curation, W.X. and Z.L.; funding acquisition, C.M. and Q.L.; methodology, W.X. and Z.L.; project administration, W.X.; supervision, C.M. and Q.L.; validation, Z.L. and J.X.; writing--original draft, W.X.; writing--review & editing, Z.L., J.X., and G.T. All authors have read and agreed to the published version of the manuscript.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Material**} & \multirow{2}{*}{**Unit Price**} & \multicolumn{2}{c}{**Traditional Charging Structure**} & \multicolumn{2}{c}{**New Charging Structure**} \\  & & **Volume** & **Price/yuan** & **Volume** & **Price/yuan** \\ \hline
**Detonating cord** & 5.25 yuan/m & 200 m & 1050 & & \\ Explosive & 4.5 yuan/roll & 164 rolls & 738 & 184.5 rolls & 830.25 \\ Total & & & 1788 (100\%) & & 830.25 (46.4\%) \\ \hline \hline \end{tabular}
\end{table}
Table 3: Comparison of economic benefits.  
**Funding:** This research was funded by the Major Science and Technology Project of Xinjiang Uygur Autonomous Region, grant number 2018A03003, the Research on Mechanized Safety and Rapid Construction Technology of Extra-long Highway Tunnel, grant number 2018A03003-2, and the Fundamental Research Funds for the Central Universities of Central South University, grant number 2019 zzts676.  
**Acknowledgments:** The authors sincerely thank the support of the above fund project and the help of leaders and operators of the test site. Without these supports, the data and field test required by this research will not be able to completed.  
**Conflicts of Interest:** The authors declare no conflict of interest.