# Fuzzy Synthetic Evaluation of the Long-Term Health of Tunnel Structures
## 2 Determination of the Evaluation Methods and Ideas for the Tunnel Structural Health
### Application of Fuzzy Synthetic Evaluation of Long-Term Structural Health in Cang Ling Tunnel  
#### 3.4.1 Establishment of the Evaluation Model  
When using the fuzzy method to evaluate the health status of a tunnel structure, one of the key points is establishing the evaluation model. Through the in-depth analysis of the various factors affecting the health status of the tunnel structure, coupled with the long-term monitoring project carried out in the Cang Ling Tunnel, the two-level fuzzy synthetic evaluation model of the structural health status for the Cang Ling Tunnel is established. This model is shown in Figure 7.  
From Figure 7, it can be seen that the multi-level fuzzy synthetic evaluation model of the tunnel structural health is divided into three layers. The first layer is the target layer, including a target object. The target object is the health evaluation system of the tunnel structure. The second layer is the control layer, including the factors that may influence the health status of the tunnel structure. The factors are the stress characteristics of the structure \(U_{1}\), the engineering geological characteristics \(U_{2}\), the initial geostress characteristics \(U_{3}\), the tunnel section characteristics \(U_{4}\) and other influencing factors \(U_{5}\). The third layer is the index layer, consisting of 14 indices that may influence the control layer factors (Figure 6). \begin{table}
\begin{tabular}{c p{142.3 pt}} \hline \hline
**Health Levels** & **Evaluation** \\ \hline Level I (Safe) & The structure is safe. The structure is intact or has tiny cracks. In this stage, pedestrians and traffic safety will not be affected. Maintaining daily monitoring is enough \\ \hline Level II (Basically Safe) & The structure is basically safe. The structure is slightly damaged or has tiny cracks. In this stage, pedestrians and traffic safety will not be affected. Maintaining daily monitoring is enough. Whether the structural stress will develop further or not should be determined by the combined judgment of the long-term monitoring project. Strengthening the monitoring frequency and daily maintenance are suggested \\ \hline Level III (Potentially Unsafe) & The structure is potentially unsafe. The structure is seriously damaged or has cracks. In this stage, pedestrians and traffic safety will be affected sooner or later. Further development will lead to the decrease of the structure function or even the failure of the structure. Increasing the monitoring frequency and taking strengthening measures as soon as possible are suggested \\ \hline Level IV (Unsafe) & The structure is unsafe. The structure is seriously damaged and the damage continues to develop. In this stage, pedestrians and traffic are endangered. Immediately increasing the monitoring frequency and taking reinforcement measures are suggested \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Health levels of a tunnel structure. It should be noted that in order to quickly determine the current status of the health and safety of the tunnel structure, the internal force (the axial force and the bending moment) of the secondary lining obtained by long-term monitoring are converted into safety factors U\({}_{12}\) in the evaluation model according to the conventional safety formulas (Equations (1) and (2)) in the Code for Design of Highway Tunnels [30]:  
1. When the eccentricity \(e_{0}\leq 0.2h\), where \"\(h\)\" is the thickness of the secondary lining, the section of the concrete is an axial compression member or a compression member with a small eccentricity. The bearing capacity is controlled by the compressive strength. The safety factor is calculated as follows: \[K\leq\phi\alpha R_{a}bh/N,\] (1) where: \(R_{a}\): The ultimate compressive strength of the concrete, which is applied according to the Code for Design of Highway Tunnels [30]. \(K\): The safety factor. \(N\): The axial force of the section (KN). \(B\): The width of the secondary lining. Take \(b=1\) m. h: The thickness of the secondary lining (m).