Probabilistic Optimization of Support System Based on Quantitative Risk Analysis -A Case study: Shahriyar dam tunnel

In this study, a methodology to determine an optimal support pattern and for the design of a tunnel is introduced based on risk analysis. It can be confirmed quantitatively that the more the tunnel is supported, the higher the reliability index becomes and the more stable the tunnel is predicted to be. Also an optimal support pattern can be determined quantitatively by performing a risk analysis considering the construction cost and the expected cost of losses that can occur due to the collapse of a tunnel.
Summary: The main goal of this research is presenting a process for optimizing the support pattern for Shahriyar Dam water diversion tunnel utilizing quantitative risk analysis which take into account the existing uncertainties in the internal friction angle, cohesion, deformation modulus, joint cohesion and joint internal friction angle.
Construction of underground tunnels plays an important role in development of modern cities. The most important factors affecting safety and economic justification of underground structures is convenient support system. Presentation of a solution to deal with engineering errors and designers experience in the support system designing is the main challenges. Underground excavation done in an environment with high uncertainty and always are high risk. Geotechnical risks in tunneling generally stand for hazardous geotechnical conditions that could unfavorably affect a tunnel project and might – in the worst case – cause human fatalities.
Methodology and Approaches: In this study, a methodology is introduced to determine an optimal support pattern. To this end, a risk analysis was performed to consider the uncertainty of ground properties based on the Monte Carlo Simulation (MCS) technique, which is used to obtain the probability distribution of safety factors of a tunnel. For this purpose, discrete element method (3DEC software) and reliability analysis respectively used to estimate the factor of safety (FOS) and expected costs of the failure of the tunnel and also Taguchi method used with the aim of sensitivity analysis of uncertainty factors on the support system safety factor from 3DEC.
Results and According to the results, the optimal support system is recommended includes injection of rock bolts to 4 meters in length, spaced 1.5 × 1.5 m with 120 mm of shotcrete with index reliability, probability of failure and risk, respectively, 2.112, 1.733% and 70880.32 dollars at 95% confidence level. According to The Taguchi sensitivity analysis, increase and decrease of the rock mass quality based on rock mass classification (RMR), respectively, to increase theinfluence of parameters on tunnel stability will be spacing of discontinuities and Young's modulus.