Reliability-Based Design of Minimum Safe Net Distance Between Small-Space Tunnels

Due to the low occupation of land and the suitability in environmental protection, small-space tunnels are widely used in underpass projects in urban expressways. The determination of the minimum safe net distance is one of the major challenges in small-space tunnels. Based on a tunnel project in Jiangsu Province, the expanded reliability-based design (RBDe) method is used to investigate the influences of tunnel burial depth, aspect ratio, and middle rock strengthen methods on the design of the minimum safe net distance between the tunnels, and comparisons between results from RBD and traditional safety factor-based design are made. Results show that for shallow tunnels, the minimum safe net distance increases with the aspect ratio, while for deep tunnels the influence of aspect ratio is insignificant. The combination of grouting and pulling anchors can effectiveness strengthen the middle rock. Besides, the RBDe method can easily cooperate with different reinforcement methods and different functional expression forms of the same reinforcement method. The RBDe method for minimum safe net distance between small-space tunnels is convenient and effective, and can provide theoretical guidance for similar projects in the future.