Time effect of step-by-step excavation unloading in soft soils and its influence on adjacent tunnels

A physical model test with a similarity ratio of 1:20 was conducted to study the time-effect of foundation pit excavation unloading in soft soils and its influence on nearby tunnels positioned below the pit bottom. The experiment data regarding lateral retaining wall displacement, ground surface settlement, basal heave, vertical of the tunnel and horizontal displacement of the tunnel, and convergence deformation of tunnel cross section were obtained. The results show that the maximum lateral wall displacement induced by excavation is 0.61%H (H is final excavation depth), which is greater than the upper limit of the maximum retaining wall deflection of metro station in Shanghai soft soils of 0.50%H. The development of lateral wall displacement and surface settlement during the step-by-step excavation presents a noticeable time effect, with the impact on the pit deformation becoming more pronounced as the depth of the excavation increases. Excavation-caused tunnel deformation is characterized by significant horizontal displacement towards the pit and relative smaller upward vertical displacement when the tunnel is located below the basal surface. Lateral displacement of the tunnel is approximately linear with that of retaining wall at the same depth, and hence the lateral deformation of tunnel caused by subsequent excavation can be predicted. The excavation-caused radial convergence of the tunnel is characterized by transverse elongation and vertical compression, and the transverse convergence is about 1.7 times of the vertical convergence. When the clear distance between the tunnel and excavation surface is within 0.2H− 0.5H, the tunnel deformation response caused by excavation unloading is particularly sensitive and the vertical deformation of the tunnel greatly increases with the increase of basal heave. © 2023 Central South University of Technology. All rights reserved.