The excavation of tunnel portals in shallowly buried bias tunnels in mountainous areas is highly prone to causing instability of the soil‒rock bedding slopes at portals, which threatens project safety. This paper uses the Moziping tunnel outlet section as the engineering background. A research approach that combines theoretical analysis, numerical simulation, and field monitoring is adopted to systematically investigate the impact mechanism of tunnel excavation on the stability of soil‒rock bedding slopes. A mechanical model of a tunnel-soil-rock bedding slope is established on the basis of the transfer coefficient method, and a slope stability evaluation method suitable for composite strata is proposed. Additionally, the slope deformation pattern and failure modes under tunnel portal excavation are revealed. The research results indicate that the stability coefficients of the slope after tunnel excavation obtained from theoretical calculations and numerical simulations are 1.18 and 1.13, respectively, with a difference of only 4%, thereby validating the reliability of the proposed theoretical method. As the tunnel excavation distance increases, the slope deformation exhibits three characteristic stages: the rapid deformation stage (excavation distance ≤ 0.22D), which contributes 78.1% to 81.1% of the total deformation; the slow deformation stage (0.22D-2.63D), accompanied by partial rebound; and the stable stage (> 2.63D), where the deformation tends to stabilize. The interlayer sliding effect at the soil‒rock interface results in significant horizontal displacement, which promotes the formation of a potential slip surface at the slope's forefront. This is manifested as the convergence value of the upper-step surrounding rock being significantly greater than that of the lower step. The plastic strain concentration at the toe of the shallowly buried side slope is a key trigger for slope instability, which is consistent with the development characteristics of shear cracks observed in the field. The research findings provide a theoretical basis for evaluating the stability of soil‒rock layered slopes and safe tunnel construction.
Keywords: Excavation disturbance; Numerical simulation; Slope stability; Soil‒rock bedding slope; Transfer coefficient method; Tunnel portal.
© 2025. The Author(s).