Abstract: The Miaopu circular shaft foundation pit of a deep tunnel sewerage project in the Suzhou river in the Shanghai soft soil area has an inner diameter and excavation depth of 30 m and 56.3 m, respectively. To simplify the construction logistics and shorten the construction period, the feasibility of replacing the inner lining wall scheme of downtop ring beam by one using a topdown ring beam is investigated in this paper using numerical simulations. The Plaxis 3D software is employed to establish a threedimensional finite element model considering the soilstructure interaction. The HSSmall constitutive model considering small strains that develop in soil is used to simulate the entire excavation process of the foundation pit. The analytical results reveal that the deformations of the circular diaphragm wall increase with an increase in excavation depth and present a spindle type. The diaphragm wall is mainly subjected to circumferential axial compression supplemented by vertical bending. A comparison between the simulation results and monitoring data shows that the deformations and stresses predicted by the model agree well with the monitoring results, indicating a good applicability of the proposed model in analyzing the circular shaft foundation pit.

Key words: superdeep circular foundation pit, soft soil, threedimensional finite element simulation, HSSmall model, deformation behavior