Study on the Mechanical Behavior of In-Situ Expansion of Small Clearance and Large-Section Highway Tunnels: Numerical Simulation and Model Test

The existing two-way four-lane highway tunnel cannot meet the growing traffic demand, necessitating the expansion and reconstruction of the existing tunnel to enhance its traffic capacity and safety. Based on a highway tunnel expansion project, this study investigates the in-situ mechanical behavior of small clearance and large section highway tunnel expansion through numerical simulation and model test. In numerical simulations, the patterns of circumferential deformation around the tunnel and stress evolution in the support structure of a two-hole, eight-lane tunnel were compared under four different expansion excavation methods. Considering deformation around the tunnel, proximity effects, stress in the support structure, and ease of construction, the three-bench method offers the greatest advantages for in-situ expansion excavation in existing tunnels. Specifically, the maximum horizontal and vertical displacements for the three-bench method were 4.54 mm and 15.18 mm, respectively, while the maximum tensile stresses in the primary support and secondary lining were 4.48 MPa and 1.27 MPa, respectively. At the same time, based on data from triaxial tests on Grade V surrounding rock and similar materials used in the support structure, model tests of tunnel expansion excavation using the three-bench method were conducted. By comparing the test results for vault settlement with those from numerical simulations, the reliability of the numerical simulation results was verified. Furthermore, model tests have further demonstrated that the impact of the following tunnel on the advance tunnel is primarily concentrated at the arch roof, inverted arch, and arch ribs of the advance tunnel—key locations where stress concentration and release occur during the expansion excavation process. The simulations and tests have revealed the mechanical behavior of large-section highway tunnels during in-situ expansion excavation, which holds significant implications for the construction of similar projects.