# A Theoretical Study on the Resilience Evaluation Method of Operational Road Tunnel Systems
## 3 Basic Concept of Road Tunnel Resilience
### Road Tunnel System Resilience  
Based on the concept of resilience, road tunnel system resilience can be defined as the ability of a road tunnel system to resist the influence it is subjected to by external disturbances, maintain a certain level of operation, and restore to normal quickly. In this definition, four key points need to be discussed:  
1. What is the function of a road tunnel system? 2. What is a road tunnel system? 3. What are the external disturbances faced by road tunnel systems? 4. What is the research content of road tunnel resilience? Firstly, as a transportation infrastructure, the most basic function of a road tunnel system is to achieve safe, unobstructed, and comfortable transportation. So, the core of the road tunnel system resilience concept will be considered the degree of influence of external disturbances on the traffic capacity of a road tunnel. Secondly, the realization of the basic functions of road tunnels not only depends on the existing basic structure and facilities but also on various operating systems, management mechanisms, and resource allocation to ensure the safe operation of road tunnels. So, the road tunnel system can be divided into two systems: the road tunnel infrastructure system and the road tunnel auxiliary function system. The compositions of the two systems are shown in Figure 2.  
Figure 1: The relationship diagram of resilience concept keyword. Thirdly, \"external disturbance\" in the concept system of resilience refer to the uncertain events that will have a great impact on the system. For the road tunnel system, \"external disturbance\" should be the potential major risk events during road tunnel operation. These risk events are mainly characterized using suddenness, uncertainty, linkage, and coupling, which will greatly affect the structural safety and traffic capacity of the road tunnel. So, the \"external disturbances\" to road tunnels can be divided into natural disaster events and emergency events, as shown in Figure 3.  
Finally, according to the concept of road tunnel resilience, the process of a road tunnel coping with external disturbances can be divided into three stages:  
1. Functional decline stage. 2. Low functional operating stage. Figure 3: Classification diagram of “external disturbance” in the road tunnel. Figure 2: Classification diagram of a road tunnel system. 3. Functional recovery stage. These three stages mainly describe three aspects of the resilience of road tunnels: 1. Degree of function loss of the road tunnel. 2. Level of low functional operating of the road tunnel. 3. Degree of function recovery of the road tunnel. Among them, the infrastructure system is the most important structural component and facility of road tunnels, as well as the main carrier to deal with \"external disturbance\". The auxiliary functional system is the key link to ensure the normal operation of the road tunnel system, improve disaster prevention ability, and improve functional recovery ability. Based on the above analysis of the concept of road tunnel resilience, combined with the concept of the public safety triangle [20], a road tunnel resilience triangle was proposed to specifically describe the relationship between external disturbances, the road tunnel infrastructure system, and the road tunnel auxiliary function system, as shown in Figure 4.  
The road tunnel resilience triangle mainly includes the following basic content:  
1. The infrastructure system is affected by external disturbances and absorbs their influence. 2. According to the state of the road tunnel infrastructure system, the road tunnel auxiliary function system should be adjusted in time to ensure the normal operation of the highway tunnel. The auxiliary function system should be adapted for the infrastructure system. 3. The occurrence of external disturbances will be fed back to the tunnel auxiliary function system to trigger an effective response mechanism to achieve the purpose of disaster prevention.