Mechanical Behavior and Stability Analysis of Underground Excavation Construction: Case Study of Large-Span and Variable Cross-Section Subway Crossover Tunnel

This paper presents a case study on the deformation characteristics and stress evolution of a large-span variable cross-section crossover tunnel excavated by the mining method. Three construction alternatives are compared: Scheme 1 adopts staggered excavation of twin tunnels with transverse passages, Scheme 2 adopts sequential excavation of twin tunnels with transverse passages, and Scheme 3 adopts gradual expansion excavation without transverse passages. Quantitative comparison shows that: (1) schemes with transverse passages reduce sudden rock deformation by 21.7% compared with Scheme 3; (2) Scheme 1 reduces construction steps by 16 compared with Scheme 2, with only 0.14 mm difference in maximum ground settlement (within engineering tolerance); (3) grouting reinforcement of the middle rock wall reduces surrounding rock deformation by 21.7% and plastic zone width by 18.3%. The recommended temporary support removal length is 5–10 m, which ensures that the incremental vault settlement during removal is controlled within 2 mm, meeting the regional engineering safety standards. Field monitoring results of Scheme 1 show that all deformation indices meet the control requirements. This construction technology can be extended to similar subway crossover projects.