Objective The foundation pit project of the interchange hall at West Lake Cultural Square Station (Hangzhou Metro Line 19) is located directly above the stacked shield tunnels of the existing Line 1. Since the unloading effect of pit excavation can adversely impact the existing operational tunnels, it is essential to study the excavation scheme and the resulting deformation response of the tunnels.

Method Comprehensive technical measures including circumferential MJS (metro jet system) ground reinforcement, secant pile retaining structure for the foundation pit, and strip-type segmented unloading method are adopted during the excavation. Before construction, numerical simulations are used to analyze the tunnel deformation. While during excavation, real-time monitoring of the underlying tunnels is conducted to obtain the true deformation responses and analyze the corresponding disturbance situations.

Result & Conclusion  The research results indicate that the MJS ground reinforcement around the tunnel has produced a significant "hoop effect," effectively reducing tunnel uplift and convergence deformation during excavation. The use of secant pile retaining structures and strip-type segmented excavation are also effective measures in controlling the uplift of underlying tunnels. In the case project, both the tunnel uplift and convergence deformation caused by foundation pit excavation are relatively small, effectively achieving the goal of protecting the existing operational tunnels. It should be noted that, during the MJS construction, methods such as pressure control and alternate-sequence piling must be implemented to ensure micro-disturbance control of adjacent tunnels and avoid adverse impacts.