Mechanical responses of surrounding rock mass and tunnel linings in large-span triple-arch tunnel

In this study, based on the construction of a large-span triple-arch tunnel, the authors focus on the surrounding rock mass deformation characteristic, evolution process of rock mass load, and mechanical behaviors of tunnel linings. Numerical modelling is used for rock mass deformation analysis. The leading tunnel excavation would disturb the rock mass of the lagging tunnel and cause its pre-deformation. As the middle tunnel lagged behind the two side tunnels, it showed much larger pre-settlement than the side tunnels. The field monitoring method is used to obtain the rock mass pressure, contact pressure between the tunnel linings, and steel stresses inside the tunnel linings. At the initial phase, the rock mass pressure and contact pressure of the linings rapidly increased; then, the monitored pressures tended to be stable until the rock mass was disturbed by the following excavation steps. Most of the steel stresses within the tunnel linings were compressive stresses and considerably lower than the yield strength, which meant that the tunnel linings were in a safe working state. The middle tunnel excavation is the most significant part of the triple-arch tunneling. It caused a significant increase in the rock mass load of the side tunnels, particularly the lateral load on sidewalls. The secondary lining of side tunnels shared most of the new increased load caused by middle tunnel excavation. Therefore, the lateral load-bearing capacity of the triple-arch tunnel was suggested to be improved. The rock mass load was symmetrically distributed along the central axis, but there was considerable bias load on the top and bottom of the division walls between the middle and side tunnels, which meant that the bending resistance and anti-overturning capacity of the division walls should be strengthened. According to the mechanical responses of the tunnel, some engineering suggestions are proposed for large-span triple-arch tunnelling. The results of this study can provide references for the design and construction of large-span triple-arch tunnels.