Seepage interaction mechanism of crossing tunnels and existing tunnels: Model test and numerical analysis

Abstract

High-density urban tunnel engineering produces strong spatial seepage and stability interactions of newly built and existing operational tunnels, which is critical to the safety and resilience of transport infrastructure. Previous research mainly focused on the mechanical response of adjacent tunnel construction and the active deformation control of surrounding rock, while little consideration is given to the influence of spatial intersecting tunnels on the seepage field distribution, especially in water-rich regions. This paper investigates the seepage interaction of crossing tunnels and existing tunnels using dedicated seepage tests and corresponding numerical analysis under different hydrodynamic head heights and spatial crossing forms. The schemes are designed based on the Kexuecheng Tunnel in Chongqing City, China, which is a typical small spacing highway tunnel that crosses five existing railway tunnels in complex hydrogeological environment. According to the results, the seepage funnels generated by intersecting tunnels will overlap each other in space, causing significant changes in the seepage field within the impact area. When the newly built tunnel above-crossing the existing tunnels, the water pressure on lining structure of the small interval tunnel can be reduced by 20 ∼ 30 %. While the impact of the existing tunnel on the seepage field of the new tunnel is mainly reflected in its upper part. The construction of new tunnels is equivalent to adding seepage paths to the upper or bottom of an existing tunnel, reducing its water pressure by 10 ∼ 30 %. The spatial overlap effect will significantly reduce the water pressure at the intersection of tunnels, but also exacerbates the asymmetric distribution of water pressure in small spacing tunnel. According to the W-shaped spatial distribution of the seepage field in small spacing tunnels, the predicted seepage influence range may exceed 80 ∼ 100 m. When the newly built tunnel above-crossing or under-crossing the existing railway tunnels, the water inflow of the small interval tunnel and the overlying tunnel can be decreased by 10 ∼ 30 % compared to the situation without crossing. However, the overall groundwater discharge further increases by 53.9 %. The seepage interactions of newly built tunnels and existing tunnels could provide support for both alignment design and construction of newly built tunnel, and service performance improvement of existing tunnels.