Quantitative risk evaluation based on event tree analysis technique: Application to the design of shield TBM

This paper analyses the risk probability of an underwater tunnel excavation using an earth pressure balance (EPB) type tunnel boring machine (TBM). An event tree analysis (ETA) has been applied to quantify the risk at the preliminary design stage of the tunnel. Probable results, which may be sequenced from specific initiating events, are analyzed, and adequate general countermeasures (safety functions) are selected to ensure safety against risks. To identify the initiating events, various data on underwater tunneling such as empirical analyses; design reports; case studies of practical problems; numerical analyses and model test results; and hydrological analysis results were used. Event trees corresponding to three significant initiating events were constructed. Each event tree consists of five countermeasures that construct 32 paths, and the probability of each path is calculated. A quantitative risk assessment was performed and the occurrence probabilities and criticalities of the paths depending on the initiating events were considered. Based on these ETA results, it was found that the selected underwater tunnel site still has a considerable probability of accidents in spite of common countermeasures. Based on the evaluated risks, improved target probabilities are proposed to reduce the probability of disaster during construction. Additional countermeasures, in other words mitigation actions, corresponding to the new target are considered. As a result, technical risks and economical losses of property can be minimized in a systematic way. It was found that the ETA is an effective method for the evaluation and quantitative analysis of probable risks and for the proposition of countermeasures for hazardous environmental conditions such as the underwater tunnel.

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