A Study on Construction Control of Extra-Large-Span Asymmetric Variable-Cross-Section Tunnels

In the construction process of a super-long-span short-distance continuous variable cross-section tunnel, multiple geometric mutations along the alignment induce significant discontinuity and asymmetry in the mechanical response of the surrounding rock-support system, which differs significantly from that of conventional single-variable-section tunnels. This aggravates local asymmetric stress concentration and sudden deformation surges, posing severe construction risks. To reveal the influence of different excavation methods on the mechanical response of the surrounding rock-support system during the construction of this type of tunnel, the Tangshan Road Interchange and Connection Line Project was added based on the Qingdao Qingyin Expressway. The large-span continuous variable cross-section sections of A, B, and C in the north line of the Tangshan Road Tunnel were selected as the research objects, and a three-dimensional finite element numerical model was established and rigorously validated against field monitoring data from three representative cross-sections. Three typical construction methods—the Distributed Bench Excavation Method (DBM), Double Sidewall Drift Method (DSM), and Distributed Double Sidewall Drift Method (DDSM)—were systematically compared and studied. The results indicate that the section transition zones (A → B and B → C) are the most sensitive key control areas. Compared with DBM, DDSM significantly reduced the vault initial support stress in Section A by 38.1% (from 6.51 to 4.03 MPa), the left and right spandrel stresses by 17.9% and 26.9%, respectively, and peak bolt axial forces by over 20%. Although DSM achieves maximum lateral convergence reduction (reducing haunch convergence by 32.8% in Section C), DDSM delivers the optimal comprehensive control by effectively restricting vault settlement and balancing support stress distribution. The field monitoring trend was highly consistent with the numerical simulation results, which confirms the accuracy of the established model. The research results can provide a reference for the selection of construction methods and deformation control of large-span continuous variable cross-section tunnels.