Assessment of Cost Variation Risk Caused by Water Inrushing in a Tunnel Excavated in Discontinuous Rock Mass
Hiroyasu OHTSU, Kazuo SAKAI, Yuji IJIRI, Masato SHIMONO and Takayuki MOTOSHIMA
Abstract:It is well-known that encounter of difficult geological condition such as fault and fractured zones in underground construction projects leads to cost overrun and delay of construction term. Particularly, water inrushing from excavated face is a typical example of geotechnical risk factors which cause serious damage. However, cost variation caused by geotechnical risk factors mentioned above has not been investigated sufficiently until now, since it has been born by construction owners with deep pocket. From such a viewpoint, this paper focuses on estimation of cost variation caused by water inrushing from excavated face in a tunnel. In detail, this study adopts DFN (Discrete Fracture Network) model, which is often used for seepage analysis in discontinuous media, to simulate groundwater behavior due to tunnel excavation. Seepage analysis based on Monte Carlo Simulation, which select analytical parameters concerning information on fractures involved in DFN model as random variables, is conducted to evaluate discharge along excavated tunnel. And, in accordance with plan, which prescribes cost of countermeasures against water inrushing, the relationship between cost of countermeasures and corresponding exceedance probability, which is so-called Risk Curve, is obtained. Furthermore, Value at Risk (VaR), which is a popular index to evaluate the occurrence of unwanted events and/or losses based on Risk Curve in financial engineering, is adopted as an index to evaluate cost of countermeasures. The adoption of an index in accordance with financial engineering helps project participants, whose major is not engineering, to make decision on execution of underground construction projects. Furthermore, the methodology proposed in this paper is applied to cost estimation of tunnel countermeasures due to water inrushing in a tunnel to be excavated by TBM method in underground research facility. Finally, results make it clear that methodology proposed in this paper is very effective to investigate cost variation risk caused by encounter of fault and fractured zones during construction. Key Words:Water inrushing, Risk, Discontinuous rock mass, Risk curve, VaR, Construction cost