Optimization of a Highway Tunnel Bottom Structure in Three-Layer Upper-Soft and Lower-Hard Composite Ground Using Physical and Numerical Models

Abstract

Tunnel excavation in ground comprising a soft upper stratum and a hard lower stratum posed significant challenges, particularly during invert construction using drilling-and-blasting methods, which could disturb the surrounding rock and the installed support system. To address these issues, this study evaluates the feasibility of an alternative support scheme that eliminates the invert by employing expanded arch feet, thereby utilizing the self-bearing capacity of the underlying hard stratum. Both physical model tests and numerical simulations were conducted to compare the structural performance of a conventional lining with an invert against the proposed expanded arch foot lining. Based on the strain measurements and earth pressure cell readings from the physical model tests, together with the internal force and contact pressure results obtained from the numerical simulations, the axial force, bending moment, and contact pressure between the lining and surrounding rock were compared for the two lining schemes. The safety factors were further calculated using the axial force and bending moment at representative sections. The results showed qualitatively consistent trends between the tests and simulations. Compared with the conventional invert lining, the expanded arch foot lining increased the axial force and bending moment at the arch foot and sidewall by 7.95% and 29.60%, respectively, while slightly reducing those at the crown by 4.44% and 3.09%. This indicates that part of the structural demand is transferred from the crown to the arch foot and sidewall, where the enlarged sections provide greater bearing capacity. The contact pressure distributions of the two lining schemes were different. Owing to its closed structural form, the conventional invert lining produced a more favorable contact pressure condition at the sidewall. In contrast, the expanded arch foot lining showed higher safety factors at the arch foot and sidewall, with increases of 89.80% and 36.00% in the model tests and more than 140% in the numerical simulations. Meanwhile, the internal force distributions of the two lining schemes remained generally comparable. These findings indicated that the lining with expanded arch feet provided sufficient structural capacity at critical sections and could serve as a feasible alternative to the conventional lining with an invert, while avoiding the construction-related disadvantages associated with invert excavation.