# Dynamic Risk Assessment of Ultra-Shallow-Buried and Large-Span Double-Arch Tunnel Construction
## 3 Dynamic Risk Evaluation Index Construction  
The risk assessment process of the construction process of shallow-buried and large-span double-arch tunnel is as follows: (1) The construction risks of urban ultra-shallow-buried and large-span double-arch tunnels are identified using two aspects, risk type identification and risk factor identification, and the risk evaluation index suitable for shallow-buried and large-span double-arch tunnel is then proposed; (2) According to the actual working conditions on site and expert opinion, the primary and secondary risk evaluation indexes are weighed up and determined by using the AHP. The weights of risk evaluation indicators are revised according to changes in the monitoring data and construction status; (3) Determine the quantitative value of the membership degree of the construction dynamic risk evaluation index according to expert opinion, and then use the membership degree function to ensure the fuzzy evaluation matrix is composed of these indexes in each stage; (4) According to the weight and membership degree of the second-level risk evaluation index in each construction stage, fuzzy calculation is carried out to obtain the first-level comprehensive evaluation matrix. According to the weight of first-level risk evaluation index and first-level comprehensive evaluation matrix, the second-level comprehensive evaluation matrix is obtained by fuzzy calculation; (5) A quantitative analysis of fuzzy comprehensive evaluation results is conducted. The establishment of a risk evaluation index system is an important link to risk assessment. The rationality of the index system directly affects the reliability of risk assessment results. The selection of risk evaluation indexes is based on the following criteria: objective; scientific; systematic; comprehensive; easy to quantify; and reflect the research content. The risk evaluation index is determined on the basis of the risk identification results, the risk types, and the risk factors. Risk identification can be divided into risk type identification and risk factor identification. According to the main risk types and risk factors, dynamic construction risk evaluation indexes suitable for urban ultra-shallow large-span double-arch tunnels can be further determined. The construction risk types of urban shallow-buried and concealed double-arch tunnels are divided into tunnel collapse, water and sand penetration, mechanical injury, surrounding environment damage, structural damage, electric shock, fire, explosion, poisoning, and suffocation. According to the key characteristics of the design and construction of urban shallow-buried large-span tunnels [49] and the opinions of tunnel experts, the main risk factors in the construction of urban ultra-shallow buried large-span double-arch tunnels include the engineering geology and hydrogeology, the surrounding construction environment, the overall engineering design, the engineering construction technology, and the engineering construction management. To help risk assessors with indicator weighting, risk classification, and other tasks in the follow-up work, the risk evaluation indexes should be divided into several levels according to a certain logic, which are generally called first-level indexes, second-level indexes, and so on. According to existing research [7; 18; 49] and the opinions of tunnel experts, the first-level index (U\({}_{\text{i}}\)) in the construction risk evaluation system of urban shallow-buried large-span double-arch tunnels was divided into six aspects: engineering geology and hydrogeology; surrounding construction environment; overall engineering design; engineering construction technology; engineering construction management; and monitoring measurement data. Given that monitoring measurement data should be obtained in the construction stage, this indicator is only used during construction, and only the first five indexes need to be used in the preparation stage before construction.