Mechanisms and optimization of system bolts in shallow four-track HSR tunnels based on deformation pressure theory

This paper investigates the performance mechanisms of system bolts in China's first four-track high-speed railway (HSR) tunnels, specifically the XBS tunnels, with spans ranging from 25.93 to 27.12 m and depths of 7 to 53 m. First, a theoretical framework, referred to as the deformation pressure theory arch, is introduced to elucidate the role of bolts throughout the sequential construction phases of four-track HSR tunnels. Subsequently, the effectiveness of system bolts is examined through an eight-month field investigation, and the relative functional partitioning for the tunnels is established. Three optimization schemes are then proposed, and their effects are analyzed using numerical simulations. The results indicate that the tunnel with a depth ranging from αh_1c to αh_2c represents a transition from deep-buried, small cross-sectional pilot tunnels to shallow-buried, super-large-span cavities. The function of system bolts can be categorized into stitching and anchoring based on the deformation pressure arch. The mechanisms of rock bolts in shallow four-track HSR tunnels can be summarized as 'anchor first, then stitch; anchor in deep state while stitch in shallow state'. The implementation of the long-short combined scheme reduces the axial force and deformation in Class V rock conditions by 74.15% to 80.04% and 41.91%, respectively. Scheme 3 is recommended for Class V rock, while Scheme 1 is proposed for alternative scenarios.