Long-term performance of subway tunnels induced by the symmetrical excavation of semicircular deep foundation pits in the Northeast Region hard silty clay

This paper presents the first large-scale deep foundation pit adjacent to an operational subway tunnel in the Northeast Region of China under hard silty clay conditions. To study the impact of the excavation on the existing subway, comprehensive field monitoring was conducted for nearly 3 years. Monitoring items include the tunnel bed displacement, vertical/horizontal convergence of tunnel structures, vertical/lateral deformation of the retaining piles, and surface settlement. In addition, in order to understand the response of soil deformation at different depths caused by excavation in this region, field monitoring tests of soil deformation at different depths were conducted. A thorough analysis of the deformation evolution of the tunnel structure, the retaining structure, and the soil is presented. The characteristics of tunnel-foundation pit coordination deformation under long-term excavation are summarized. Under different excavation activities at different proximity distances, the tunnel either settled or rose. Therefore, a three-dimensional numerical model was developed and a multi-parameter analysis was performed. The results indicate that the direction/magnitude of the tunnel structure deformation is closely related to the construction process and the proximity of the excavation. At the same time, the displacement of shallow soil is larger than that of deep soil due to the cumulative effect of displacement. Finally, the deformation properties of the tunnel and the soil are analyzed under different excavation depths, different proximity and different maintenance structure deformation modes. With the critical surface about 35 m away from the foundation pit boundary, the uplift zone and the settlement zone of the foundation pit split.