Study on Cross-Section Transition Form and Stability of Super-Large Variable-Span Tunnel

In order to clarify the influence of cross-section change mode of a large-span variable cross-section tunnel on the stability of the surrounding rock of a tunnel, four three-dimensional finite element models were established for four typical cross-section conversion forms: Sudden changes in the cross-section of the symmetric tunnel, Gradual changes in the cross-section of the symmetric tunnel, Sudden changes in the cross-section of the asymmetric tunnel, and Gradual changes in the cross-section of the asymmetric tunnel. The Stress characteristics and deformation laws of the surrounding rock under different cross-sectional changes were systematically analyzed. The simulation results were compared with the field monitoring results, and the construction scheme was optimized based on the results of the numerical analysis and field monitoring. The results demonstrate that the final settlement values for sudden changes in the cross-section of the symmetric tunnel and sudden changes in the cross-section of the asymmetric tunnel were 21.23 mm and 21.98 mm, respectively, representing reductions of 14.6% and 15.7% compared to gradual changes in the cross-section of the symmetric tunnel and gradual changes in the cross-section of the asymmetric tunnel. The final support stress values of sudden changes in the cross-section of the symmetric tunnel and sudden changes in the cross-section of the asymmetric tunnel were 8.59 and 7.88 MPa, respectively, representing reductions of 21.5% and 26.9% compared with gradual changes in the cross-section of the symmetric tunnel and gradual changes in the cross-section of the asymmetric tunnel. Compared with asymmetric construction, symmetrical construction is more likely to lead to a stress concentration effect on both sides of the tunnel. Considering the overall construction feasibility and economy, the asymmetric sudden-change construction scheme provides better comprehensive benefits. The research results can provide a theoretical basis and technical reference for solving engineering problems such as complex stress in a large section transition zone and difficult control of tunnel construction.