Investigation of the long-term settlement of a cut-and-cover metro tunnel in a soft deposit

• The long-term settlement of Nanjing Metro Line 10 in China is documented.
• The tunnel settlement is mainly induced by the soft silty clay underneath.
• The components of the long-term tunnel settlement are analyzed.

This study investigates the long-term settlement of Nanjing Metro Line 10 in China. A maximum settlement of 240 mm was measured within 5.75 years after the track system was constructed. Four settlement troughs were observed along the longitudinal direction of the railway line. The settlement of the tunnel developed rapidly in the first two years and then the rate of settlement gradually decreased. The excessive settlement and large differential settlement of the tunnel were primarily caused by the deep and non-uniform distribution of the underlying soft soil. The long-term tunnel settlement mainly consists of two components, including recompression settlement following construction and secondary consolidation settlement of the foundation. Each component of settlement was analyzed and estimated in this study. The computed results correspond well with the measured data, and the recompression deformation and the secondary consolidation deformation represents approximately 78.0–100% and 0–22.0% of the total tunnel settlement, respectively. The measured data and calculated results revealed that the lengths of the cement mixing piles used to improve the tunnel foundation are insufficient to reduce the long-term settlement. Piles entirely penetrating the soft silty clay layer are recommended.