# Experimental Investigations of a Tunnel Lining Segment Strengthened by In Situ Spraying Mortar
## 5 Summary and Conclusions  
After long-term operation, tunnel lining segments encounter various problems. The currently used shield tunnel lining strengthening method has various shortcomings. Therefore, in this paper, we proposed a new shield tunnel lining strengthening method by in situ spraying of cement mortar. Based on the material properties test of H-70 for in situ spraying mortar and a full-scale experimental study on the strengthening of tunnel lining segment structure, and compared with internal tensioned steel ring strengthening method, the main conclusions are as follows:  
1. The mechanical properties of H-70 mortar are not dependent on curing conditions. Under standard curing conditions, the 7- and 14-day compressive strengths of H-70 reached 61% and 90% of the 28-day compressive strength, respectively, indicating that H-70 has high early strength and is convenient for rapid repair.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Type of Segment** & **Elastic Limit** & \begin{tabular}{c} **Ultimate** \\ **Bearing** \\ **Capacity (kN)** \\ \end{tabular} & \
\begin{tabular}{c} **Maximum** \\ **Elastic** \\ **Displacement** \\ **(mm)** \\ \end{tabular} &
\begin{tabular}{c} **Structural** \\ **Stiffness in** \\ **Elastic Stage** \\ **(kN/mm)** \\ \end{tabular} \\ \hline Unstrengthened & 316 & 335 & 35.34 & 10.15 \\ segment & 214 & 368 & 11.55 & 8.53 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Performance comparison between the strengthened and unstrengthened segments.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
\begin{tabular}{c} In situ spraying \\ mortar has better \\ fire resistance \\ \end{tabular} \\ \hline \hline \end{tabular}
\end{table}
Table 5: Comparison of engineering economics of the two strengthening methods.  
2. The strength and surface roughness of the strengthened segment have a certain influence on the interface-splitting tensile strength. When strengthening the shield tunnel lining segment structure, the roughness of the interface between the segment and the mortar can be increased by chiseling, thereby improving the bearing capacity of the reinforced structure. 3. The ultimate bearing capacity of the tunnel lining segment structure strengthened by in situ spraying mortar was significantly increased; the ultimate bearing capacity of the lining segment structure is increased by 10% compared with the unstrengthened lining segment structure. Compared with the internal steel-ring strengthening method, the in situ spraying mortar method has obvious advantages in terms of economy, workability and durability. 4. The full-scale loading experiment clarified the effectiveness of the new strengthening method. The next step will be to carry out continuing research on construction technology, combined with the actual project, and to provide comprehensive technical support for the practical application of the new strengthening method.  
Conceptualization, F.W. and C.Z.; methodology, F.W.; validation, C.Z. and B.M.; formal analysis, F.W.; investigation, F.W., C.G. and Y.Z.; resources, B.M. and B.L.; data curation, F.W., C.Z. and Y.K.; writing--original draft preparation, F.W.; writing--review and editing, F.W., Y.Z., C.G. and C.Z.; visualization, F.W., C.M. and B.L.; supervision, C.Z. funding acquisition, C.Z. All authors have read and agreed to the published version of the manuscript.  
Engineering Research Center of Rock-soil Drilling & Excavation and Protection 20210501.  
Not applicable.  
Data Availability StatementData are contained within the article.  
The authors are very grateful for the technical support provided by Wuhan CUG Trenchless Technology Research Institute Co., Ltd., Wuhan, China.  
The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.