IntroductionThe design and performance of tunnels are usually affected by some uncertainties that can be costly and time-consuming for tunnel construction projects. Traditional empirical and deterministic design approaches do not include uncertainty in tunnel support design [1][2][3], but tend to be based on trial-anderror processes that consider safety and cost [4][5][6]. Reliabilitybased optimization (RBO) makes provision for the uncertainty of structures by adding probabilistic constraints. This is quite straight forward if the results of the reliability analysis are accurate and precise so that no question arises as to whether a given design satisfies safety requirements. The purpose of RBO is to find a balanced design that is not only economical but also reliable in the presence of uncertainty [7].Over the past few decades, numerous reliability optimization techniques have been proposed [6,8,9]. Younes and Alaa overviewed the various RBDO approaches using mathematical and finite element models with different levels of difficulties [10]. Marcos and Gerhart (2010) produced a detailed literature review on reliability-based optimization [11]. Although RBO has some evident advantages overdeterministic optimization design, it is often computationally inefficient. Response surface methodology has been applied in RBO in attempts to improve its efficiency [12,13]. Zhang et al. applied the mean first-order reliability method (MFORM) to the optimization of geotechnical systems [6]. Those methods improved the computational efficiency but decreased the accuracy of the reliability analysis, which affects the results of RBO. The selection of an optimization method is critical to RBO applications, especially for complex nonlinear optimization problems. Gen and Yun reviewed the application of soft computing methods in reliability optimization [14]. Genetic algorithms and particle swarm optimization have also been applied to RBO [7,15]. Lee et al. proposed a methodology to convert an RBDO problem requiring very high reliability to an RBDO problem requiring relatively low reliability by appropriately increasing the input standard deviations for efficient computation in sampling-based RBDO [16].