Evolution of Construction Mechanics Behavior in Widening Section of Urban Highway Tunnel with Super Cross-Section

Urban tunnel widening is increasingly used to accommodate growing traffic demand. However, excavation-induced unloading in complex urban geological environments may degrade surrounding-rock properties and compromise structural safety. Previous studies have mainly focused on tunnels with cross-sectional areas below 300 m2, while field-based investigations of tunnels with excavation areas exceeding 400 m2 remain limited. In addition, structural safety assessments often rely on individual indicators. This study investigates the Huofengshan Tunnel in Chongqing, China, an urban tunnel characterized by a densely built-up setting, complex geometry, a low height-to-span ratio, and a maximum excavation area of approximately 440 m2. Field monitoring of deformation and internal forces was conducted at four sections with clear widths of 13.25, 17.45, 20.00, and 25.00 m. Three-dimensional FLAC3D numerical simulations were conducted using a dynamic mechanical-parameter weakening model that accounts for excavation-induced unloading, and the simulation results were validated against field monitoring data. A structural safety assessment framework integrating tunnel deformation, ground-surface settlement, primary-support stress, and secondary-lining stress was established. The analytic hierarchy process–entropy weight method (AHP-EWM) was used to determine indicator weights and establish the tunnel safety redundancy index (TSRI). The results show that tunnel widening induces substantial internal-force redistribution in the support structure, shifting the load-bearing mechanism from arch-waist dominance to joint bearing by the arch shoulder and arch waist. As the clear width increases from 13.25 to 25.00 m, the TSRI increases from 0.231 to 0.385, indicating a reduction in structural safety reserve. This study provides a field-validated analytical framework for characterizing the evolution of mechanical behavior and quantitatively assessing structural safety during tunnel widening, offering a theoretical and methodological reference for the design, construction, and safety assessment of comparable super-large-section urban highway tunnels.