Mechanical responses of super-large-span variable cross-section tunnels in weak surrounding rock: A case study and the application of a novel constitutive model

Super-large-span and variable cross-section tunnels have proliferated in urban expressway systems across China. However, the deformation behavior and load distribution mechanisms in super-large and variable cross-section tunnels remain inadequately understood. This study systematically analyzed the mechanical response characteristics of super-large-span and variable cross-section tunnels in weak surrounding rock based on extensive field monitoring data. Mechanical tests were conducted to reveal the strain-softening characteristics of weak surrounding rock, and the nonlinear evolution of strength parameters was summarized. Through secondary development, a novel constitutive model based on the Hoek-Brown strength criterion was proposed and successfully implemented in FLAC3D. Furthermore, numerical simulations were conducted using an improved constitutive model. These simulations investigated the evolution of deformation and internal forces within the support system, accounting for the coupled effects of multiple factors. The research shows that the rock pillar compensates for the insufficient stiffness of temporary middle diaphragms. However, it also alters the mechanical behavior of the steel frame system, which leads to significant stress concentrations at the arch shoulders. Additionally, the "asymmetrical loading effect" commonly observed at variable cross-sections substantially impacts the support system within these span transition zones.