A Study on the Quantification of Initial Support Stiffness and the Coordinated Control of Transition Zones in Asymmetric Bifurcated Tunnels with Super-Large Cross-Sections

There are some problems in the transition zone of an asymmetric bifurcated tunnel with a super-large cross-section, such as a sudden change in section size and asymmetric unloading, which can easily cause problems such as uneven stress and deformation of surrounding rock and local stress concentration of support in the transition zone. The existing research lacks a systematic understanding of the quantification of combined support stiffness and the synergistic relationship between surrounding rock and support in this kind of transition zone. In view of the above problems, a super-large cross-section asymmetric bifurcated tunnel project in China is taken as the basis, and the three-dimensional numerical model of the tunnel is constructed. The installation timing of the initial support was determined using the convergence–confinement method (CCM), and the deformation and stress distribution of the surrounding rock under opposite excavation and reverse excavation were compared. Based on continuum mechanics and the superposition principle, a method was established for calculating the equivalent radial stiffness (ERS) of the combined support system. The effects of combined support stiffness on vault settlement and the maximum principal stress in the shotcrete were analyzed, and support adjustments and construction control measures for the transition zone were examined. Similar vault settlement evolution patterns and comparable final settlement magnitudes were observed under the two excavation methods, with excavation-induced disturbances mainly concentrated around the tunnel perimeter. Compared with reverse excavation, opposite excavation was characterized by faster development of vault settlement, a more symmetrical and continuous distribution of high-stress zones in the surrounding rock, and a smaller zone of influence from excavation. Under opposite excavation, vault settlement in the transition zone was reduced by increasing the stiffness of the combined support system. However, further increases in stiffness yielded diminishing benefits in settlement control, while a more pronounced increase in the maximum principal stress in the shotcrete layer was observed. Therefore, both deformation control and the stress response of the support structure should be considered when adjusting support parameters, rather than relying solely on increased support stiffness. Accordingly, three recommendations were proposed for construction safety control in super-large cross-section asymmetric bifurcated tunnels. These findings can inform initial support design and construction control in the transition zones of such tunnels.