Projected Effects of a Deep Excavation Pit on the Existing Metro Tunnel and Findings of Geotechnical Monitoring: A Comparative Analysis

With the evolution of modern cosmopolitan cities, subterranean spaces have developed in dense urban environments. Hence, new metro tunnels often intersect with those in operation. The top-priority task of designers is to evaluate the effect of new construction projects. The experience accumulated in this field should contribute to the design of a realistic geotechnical model to simulate long-term displacements in the future. This paper includes a backward analysis of a design scheme developed for a tunnel construction area above an existing tunnel with a 10.3 m diameter, according to the results of geotechnical monitoring performed in PLAXIS 2D. The authors identified the optimum combination of the distance from the tunnel bottom to the lower boundary of the design model, the soil model, and tunnel lining stiffness. The authors derived regression equations describing vertical and horizontal displacements of the tunnel at the stage of excavation to the elevation datum as the excavation pit bottom. These equations can be applied to preliminarily predict the displacements of the tunnel depending on geometrical parameters at the initial design stage. Geometrical parameters include the distance from the tunnel to the excavation pit, the depth of the tunnel from the surface to the crown, the depth of the designed excavation pit, and the distance from the bottom of the excavation pit to the bottom of the tunnel. In addition, the effect of the Muir–Wood coefficient on the vertical displacements of the tunnel was investigated. This work found a reduction in the stiffness of the bearing structure of the tunnel and an increase of 4.8% in deformations on average when this coefficient was considered.

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