Characteristics of a Large-Scale Deep Foundation Pit Excavated by the Central-Island Technique in Chengdu Soft Clay

A large-scale foundation pit, 25,720 m2 (127 m × 213 m), was excavated to a depth of approximately 20 m by the central-island technique from 2012 to 2013 in Chengdu, China. This paper aims to comprehensively study the deformation behaviours of the foundation pit during excavation and investigate the primary factors affecting the deformation behaviour of the foundation pit. The deformation characteristics monitored during excavation included 1) horizontal column movement, 2) vertical column movement, 3) lateral column deflection, 4) internal stress in the column, 5) axial force in anchor cables, 6) ground settlement, and 7) artesian water levels. To further explore the pit sizes on the excavation behaviour, field data from another ten excavations in Chengdu were also included in the analysis. The results showed the maximum deflection of the column was mostly detected at the top of the column. The vertical column movement, δcu, undulates during excavation. After the desired depth was reached, the increase in the vertical column movement was negligible. The internal force in the anchor cable varied between 20–30 kN during the excavation and between 30–70 kN when the expected depth was excavated. A relatively small ground settlement in the studied excavation sites was observed due to the mudstone at the bottom of the excavation and the installation of anchor cables. In addition, it was noticed that the aspect ratio and excavation depth have a significant effect on the maximum deflection at the top of the column. This paper enriches the database and improves the understanding of the deformation behaviour of large-scale foundation pits during excavation.