عنوان مقاله [English]
نویسنده [English]چکیده [English]
The main goal of this research is to present a process for optimizing the support pattern for Shahriyar dam water diversion tunnel utilizing quantitative risk analysis which takes into account the existing uncertainties in the internal friction angle, cohesion, deformation modulus, joint cohesion and joint internal friction angle. In this study, a methodology to determine an optimal support pattern and for 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. Moreover, 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.
Construction of underground tunnels plays an important role in development of modern cities. The most important factor 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 designing the support system is one of the main challenges in the construction of underground tunnels. Underground excavation made in an environment with high uncertainty is always accompanied with 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 technique, which is used to obtain the probability distribution of safety factors of a tunnel. For this purpose, discrete element method (using 3DEC software) and reliability analysis were respectively applied to estimate the factor of safety (FOS) and expected costs of the failure of the tunnel. In addition, Taguchi method was used with the aim of sensitivity analysis of the uncertainty factors on the support system safety factors from 3DEC software.
Results and Conclusions
According to the results, the optimal support system, which 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, the parameters which affect the rock mass quality (based on rock mass classification (RMR)), and therefore, the tunnel stability are spacing of discontinuities and Young's modulus.