A Case Study for Probabilistic Risk Evaluation based on Event Tree Analysis Technique for the Design of Shield TBM

A case study for an application of the probabilistic risk assessment technique for the underwater tunnel excavation using an earth pressure balance (ETB) type TBM (Tunnel Boring Machine) is presented in this paper. An event tree analysis (ETA) has been adopted to quantify the risk at the design stage of the tunnel. Probable results, which may be originated from initiating events, are analyzed, and countermeasures (safety functions) are selected to ensure safety against risks. To identify the initiating events, various data are used such as empirical analyses, tunnel design reports, case studies, numerical analyses, model test results, and hydrologic analysis results. Event trees associated with three significant initial events are constructed: these are a) poor ground condition; b) high water pressure; and c) heavy rainfall. Each event tree consists of five countermeasures: investigation/design; process planning; machine type; construction management; and reinforcement. It results in 32 paths, and the probability of each path is calculated. A quantitative risk assessment is performed so that the occurrence probabilities and criticalities of the paths depending on the initiating events are considered. According to the ETA results, it has been found that the original excavation procedure has a considerable probability of accidents in spite of applying five countermeasures. Additional mitigating actions are suggested to reduce the risk up to allowable level. As a result, pobable risk factors can be minimized in a systematic way. It is found that the ETA is an effective method for the quantitative evaluation of probable risks and for proposing more effective countermeasures under hazardous conditions such as underwater tunnelling.