## 5 Conclusions  
* The structural analysis of segmental tunnel rings can be categorized into only three classes: Class A is the category of asymmetric problems [13]. Class B includes symmetric problems without sudden failures at the joints during the loading process [35], and Class C is the category of symmetric problems with some sudden failures, such as concrete spalling, at the segmental interfaces.
* An efficient analysis procedure was proposed in this paper. With the help of \(\mathcal{I}_{as}\), the analysis problem was, at first, categorized as an asymmetric or a symmetric problem. For the asymmetric problems, the strategy of general rigid-body displacements was employed. For the symmetric problems, the strategy of symmetric rigid-body displacements was used. The accuracy of the simulations of the symmetric problems was checked by \(\Delta C/t\). If it was found to be unacceptable, the improved strategy of symmetric rigid-body displacements was used.
* The reason for the difference between Class A and Class B is different from the reason for the treatment as an asymmetric problem, characterized by the development of the asymmetric structural behavior in a progressive and subtle fashion [13]. The third class of problems is characterized by the development of the deformations in a sudden fashion.  
Figure 10: Decision tree of structural analysis of segmental tunnel linings.  
When convergence-related serviceability class C is reached, the stress state or the failure states of the interfaces need to be checked carefully. Local failure results in a rapid increase in structural deformations. Serviceability class D will then be reached soon.
* It is suggested that, upon the attainment of convergence-related serviceability class C and the detection of local failure at segmental interfaces, the strengthening of the structure should be considered at serviceability class C rather than only at serviceability class D, i.e., at the SLS.
* If the tunnel is located in an aggressive environment and the cracking at the outer surface is not acceptable, the definition of the convergence-related SLS should be stricter. In this case, \(C_{ref}\) should be set equal to 0.3%\(D_{o}\) rather than to 0.4%\(D_{o}\).
* The accuracy of the extended hybrid method was validated by means of monitoring the data collected during full-scale testing. In the future, it will be interesting to equip real tunnels with advanced monitoring techniques that enable the collection of similar data, and to use these data for the further validation of the applicability of the method presented.  
Conceptualization, B.P. and Z.J.; methodology, [PERSON] and J.Z.; software, Z.J.; validation, [PERSON] and B.P.; formal analysis, [PERSON]; investigation, [PERSON]; resources, Z.J. and X.L.; data curation, Z.J. and X.L.; writing--original draft preparation, Z.J.; writing--review and editing, H.A.M.; supervision, B.P. and H.A.M. All authors have read and agreed to the published version of the manuscript.  
Financial support by China Scholarship Council (CSC) to the first author.  
Data Availability Statement: Data is contained within the article.  
Financial support of the first author by the China Scholarship Council (CSC) is gratefully acknowledged. Open Access Funding by TU Wien.  
The authors declare no conflicts of interest.