Investigation into the Performance of a Covered Top-Down Pit-in-Pit Deep Excavation in Shenzhen Soil-Rock Mixed Strata

Abstract

An oversized pit-in-pit (PIP) excavation, which comprised an octagonal pit outside and two cross-shaped pits inside, was excavated in soil-rock mixed strata, Shenzhen, China. There were two shield tunnels connected to the north and south of the PIP excavation after the main structures completed. Extensive instruments were adopted throughout the construction process to guarantee the safety of the project. Based on the massive field data, the performance of the ground settlement, retaining structures and adjacent building deflection were investigated. Field data indicate that the influence zone of ground settlement in depth was above the stiff clay layer. The maximum lateral displacement (δhm) of the retaining structures occurred at 0.4 He (He is the excavation depth), and δhm decreased when the excavation depth was over 0.5 He. The maximum ground settlement δvm occurred at 0.2 He which gradually decreased with d (d is the distance from the monitoring point to the excavation edge). This large discrepancy of the ground settlement induced 3.13‰ tilting of the adjacent buildings. The ground settlement influence zone was 1.7 He away from the pit edge (d ≤ 1.7 He). A half-3D numerical model was established to investigate the influence of different excavation sequences. The results show that the maximum principal stress (σm) of the cover plate increased to peak value due to the excavation of subsoil, which indicated that the sidewall construction and subsoil excavation should be carried out simultaneously to reduce the stress concentration of the cover plate. TD excavation can effectively minimize the ground and building settlement, while the excavation may induce uneven displacement of the interior supporting columns. Push in failure of the sidewalls may occur during the construction which was the combined result of unloading and increase in the excavation depth.