Abstract: Projects near metro shield tunnels in loess areas encounter various challenges, including high construction difficulty, complex support system design, and difficulty in controlling tunnel deformation. A case study is conducted to examine a foundation pit situated directly above a metro tunnel in Xi′an, China, and evaluate the effectiveness of different methods for controlling tunnel deformation, including the traditional segmented and block excavation method, the "isolation pile + soil reinforcement" method, and the advance pipe curtain method. The analysis employs a finite element method that considers the small strain hardening property of the soil. Based on the concepts of pile isolation and pipe curtain restraint, a novel "pipe curtain + isolation pile" method is proposed for controlling metro tunnel deformation. The deformation of the metro tunnel during both construction and post-construction periods is monitored and analyzed. The results reveal the following: (1) The proposed method effectively manages pipe curtain displacement by linking the pipe curtain with the isolation pile, significantly reducing tunnel uplift by distributing the rebound load of the soil between the weight of the pipe curtain and the pilesoil friction. (2) Numerical simulations demonstrate that the maximum uplift of the metro tunnel using the proposed method is 68%, 57%, and 72% lower than the traditional segmented and block excavation, "isolation pile + soil reinforcement", and advance pipe curtain methods, respectively, thus highlighting the notable uplift control effect of the pipe curtain. (3) Monitoring results reveal that the connection node design of the pipe curtain and isolation pile, based on the proposed method, is effective in constraining displacement at both ends and the mid-span of the pipe curtain. (4) The measured maximum uplift of the tunnel crown and the maximum settlement of the tunnel are 0.8 and 1.4 mm, respectively, both complying with relevant deformation control standards.

Key words: foundation-pit engineering, tunnel deformation, deformation control, pipe curtain, isolation pile