Determination of the Optimum Stabilising Scheme of Foundation Pits for Utility Tunnels

A number of utility tunnels have been installed by open excavation in metropolises around the world. However, there is little knowledge about the stability and failure behaviours of excavated foundation pits for utility tunnels. The engineering properties of the foundation pit materials were obtained by field investigations, laboratory and in-situ tests. A series of numerical analyses considering excavation and stabilisation was conducted to determine the optimum stabilising scheme of foundation pits for the utility tunnels. The factor of safety and shear strain of the foundation pits were obtained through combining the elastic-plastic Mohr-Coulomb criterion and FLAC3D program using the parameter reduction technique, based on which the effect of excavation and stabilisation on the foundation pits was thoroughly discussed. The results show that a steeper foundation pit may produce a more significant vertical shear effect with the greater vertical shear strain at crowns under the action of gravity, and the shape of the computed slip surface gradually transformed from a polygonal line shape to a circular arc shape with the slope gradient of the foundation pit decreasing. The optimum excavation height for the upper slope of the foundation pit varied with the slope, which was smaller when the foundation pit became gentler. Combining both economic and safety requirements, an excavation height of 5 m for the upper slope and a slope gradient of 1:1 and 1:0.3 for the upper and lower slopes, respectively, was considered the optimum excavation scheme of the foundation pits for the utility tunnels in the Tianfu new area, Chengdu, China. For some narrow areas with difficult-to-reduce slopes, stabilising anchors were suggested as an alternative solution to ensure the safety of foundation pits. It is demonstrated that installing anchors could change the failure mode of the foundation pits. Based on the analysis results and easy to operate, the anchors installed from the bottom to top of the foundation pits with spacing of 1 m, length of 4 m and inclination of 15° were suggested as the optimum stabilisation scheme in the Tianfu new area. The findings from this study can provide the constructors and designers with a starting from the analysis and design of foundation pits for utility tunnels worldwide.