Long-term performance of metro tunnels induced by adjacent large deep excavation and protective measures in Nanjing silty clay

Abstract

This paper presents the construction of a large-scale basement excavation adjacent to the two running metro tunnels in Nanjing silty clay. To investigate the influences of excavation on existing tunnels, a numerical modelling was carried out to predict the possible deformation of tunnels before construction. Then, referring to the obtained numerical results, a targeted monitoring program was proposed and comprehensive field monitoring was performed to monitor the deep excavation and tunnels response over four years. The monitoring items include the diaphragm wall lateral deflection, the tunnel crown settlement, the springline horizontal displacement, the diameter convergence and the opening width of the segment joint. Based on monitoring results, the evolution of tunnels deformation was analysed, and the safety of tunnel structures was evaluated. According to the evaluation results, micro-disturbance grouting was proposed and adopted to correct the deformed tunnel, and the treatment effectiveness was evaluated. The results indicate that the evolution of tunnel crown settlement, springline horizontal displacement and diameter convergence can be divided into slow growth, rapid growth and stable growth stages, and they correspond to the pre-excavation, main excavation and post-excavation stages of deep excavation respectively. Based on the simple beam principle, the settlement range of tunnel can be taken two times the distance between the monitoring section with the maximum settlement and the section with the settlement of 0. In the process of micro-disturbance grouting treatment, the deformed tunnel goes through a correction stage and a rebound stage respectively, and the additional deformation induced by adjacent excavation is well corrected by micro-disturbance grouting.