In this paper, risk assessment methodology is applied to quantify the impact of uncertainties in the tunnel lining design. Uncertainties and variability concerning geologic conditions, structural parameters and lining loads evaluation are addressed. Firstly, risk mitigation measures used in the design phase are presented to produce reliable design solutions, including a combined procedure used for assessing geological uncertainties, the displacement releasing factor method used to consider the 3-D effect of excavation face and the probabilistic method used to consider the variability of structural parameters. For risk estimation, the failure probability and the corresponding losses for each design solution need to be calculated. An improved Convergence-Confinement method (CCM) is proposed for evaluating its stability. Together with the improved CCM method, the probabilistic approach based on Monte-Carlo is adopted to calculate the failure probability. Overall losses due to lining failure is evaluated with the Analytical Hierarchy Process.
Design of tunnel lining preferably should result in optimal solution which minimizes time and cost. However, due to the inherent risks in the design, choice of the optimal solution is not straight forward. To strive for it, the impact of these risks shall be addressed.
Tunnel lining design is characterized with uncertainties connected with the nature characteristics, geologic conditions, uncertainties in ground-structure interaction and construction performance and level of knowledge. However, in the design common practice normally follows a deterministic approach with the assumption that the behavior of the ground and that of the lining system are well understood and quantifiable. And safety factor is adopted to cover the possible impact of uncertainties. Obviously, this approach is not sufficient to take full consideration of the impact of these uncertainties. Therefore, tunnel design always involves a certain degree of risk, which may lead to under-design and costly failures or over-design and high tunneling costs. Considering these factors, a way to follow is the application of risk assessment in the design.
