Performance of adjacent metro tunnels during deep excavation: A case study in Hangzhou

This study investigates the construction of a large basement adjacent to two overlapping subway tunnels in a soft-soil area in Hangzhou. A year-long onsite monitoring program is conducted to investigate the effects of excavation and underground-structure construction on the immediately adjacent tunnels. The main monitoring items include the tunnel’s horizontal displacement, the horizontal convergent deformation of the tunnel section, and the roadbed settlement. Measurement results show that excavation triggers the displacement of the subway tunnel toward the pit, the elongation of the tunnel tube sheet, and the uplift of the roadbed, among which horizontal displacement occurs the most readily. The construction of the metro structure increases the displacement of the metro tunnel toward the foundation pit and the bulge of the roadbed but minimally affects the convergent deformation of the tunnel cross-section. The tunnels adjacent to the pit area are affected the most by excavation, whereas the overlapping tunnels increase the overall stiffness of the area and constrains the deformation of the tube sheet. The station facilities connect the uplink and downlink of the tunnel, thus promoting interline deformation and diminishing the effect of pit excavation on tunnel deformation. This study provides a useful reference for similar project cases in the Hangzhou area to ensure the safety of adjacent subway tunnels during basement excavation.