Theoretical Analysis of Longitudinal Vertical Displacement of Subway Tunnels Caused by the Construction of Steel Casings for Bridge Piles

The construction of new bridge piles adjacent to existing subway tunnels poses a significant threat to the safety of the tunnels. To accurately assess the impact of the construction of the steel casing of bridge piles on nearby subway tunnels, a simplified analytical method incorporating a mechanical model is developed in this study. Specifically, modified calculation formulas for the additional force are proposed, taking into consideration the characteristics of casing segmental construction and the soil-squeezing effect. The two-stage analysis method is employed to determine the additional stress and longitudinal vertical deformation of the tunnel. Subsequently, a series of calculations are performed to gain further insights into the effects of varying lengths and diameters of bridge piles, as well as the relative distance between the pile and tunnel on the tunnel-pile interactions. As the construction depth increases, the existing tunnel experiences a slight initial rise followed by progressively larger settlement that eventually levels off. The maximum settlement exhibits a linear increase with the increasing length and diameter of the bridge piles, while it exponentially increases with decreasing relative distance between the pile and tunnel. Moreover, impact zones of varying degrees of the bridge pile construction on the nearby tunnel are proposed. Based on the Response Surface Methodology, the relative distance is found to exert the greatest influence on the deformation of the existing tunnel, and an optimal relationship among the parameters is provided. The theoretical results are well validated with field-measured data from a representative engineering project and numerical simulation results. The proposed solution can be proved to offer reasonable theoretical support for risk assessment of tunnels under the influence of adjacent piles construction.