Field Observation of a Deep Excavation Adjacent to Subway Tunnels

Unloading effects of deep excavation will inevitably cause deformation and movement of retaining structures and adjacent ground, thus exert negative impact upon surrounding environment. An in-situ monitoring programme was carried out for a deep excavation adjacent to the existing subway tunnels during the whole process of construction and the observation results were presented and discussed in detail. It is found that substantial displacements of the retaining structures occur during the final stages of the excavation. The maximum displacement of the retaining structures appears between the second and the third level of the horizontal struts, which correspondingly induces high values of axial forces in these two levels of the struts. Unloading due to the excavation leads to the uplift of the vertical props inside the excavation. The magnitude of the uplift near the center of the pit is greater than those near the side and the comers. The excavation leads to the ground settlement and deep-level strata heave near the excavation and consequently the uplift of the subway tunnels. The influence of the deep excavation on the vertical deformation of the subway tunnels is much more significant than that on the horizontal deformation.