Characteristics of a Large-Scale Deep Foundation Pit Excavated by the Central-Island Technique in Shanghai Soft Clay. II: Top-Down Construction of the Peripheral Rectangular Pit

TLDR

The 492‑m‑high Shanghai World Finance Center’s 17.85‑to‑25.89‑m‑deep, 30,000‑m² foundation pit was excavated by a central‑island technique, first constructing a bottom‑up central shaft and then top‑down the peripheral rectangular pit. This study investigates peripheral pit behavior by measuring lateral and vertical wall deflections, ground movements, axial forces in floor slabs and braced struts, lateral earth pressures, pore pressure variations, settlements, basal heaves, column movements, and stresses. Field data from 33 additional top‑down excavations in Shanghai were incorporated to assess how pit size influences excavation behavior. Comparisons show that plan area dominates pit behavior, with large pits (30,000–50,000 m²) exhibiting three to five times greater wall deflections and ground settlements, wider influence zones, smaller trapezoidal earth pressure envelopes peaking deeper, deeper basal heave zones than conventional models, and floor slabs bearing loads from the full retaining wall exposure.

Abstract

Because of its large size (30,000 m2 in plan), the 17.85- to 25.89-m-deep foundation pit of the 492-m-high Shanghai World Finance Center building was excavated by the central-island technique, i.e., bottom-up construction of the central cylindrical shaft first and then top-down construction of the peripheral rectangular pit. As part of the comprehensive study on the characteristics of this large-scale foundation pit, this study mainly focuses on the behaviors of the peripheral pit via the following investigated items: (1) lateral wall deflections; (2) vertical wall movements; (3) lateral ground movements; (4) axial forces in the cast floor slabs and braced struts; (5) lateral earth pressures on both sides of retaining walls; (6) variation of pore pressures along depth and deep artesian water levels; (7) ground settlements; (8) subsurface settlements; (9) basal heaves; (10) vertical column movements; and (11) column stresses. To explore the potential effects of pit sizes on the excavation behaviors, field data from another 33 top-down excavations in Shanghai were also included for comparison. The comprehensive comparisons show that in addition to the well-known factors (e.g., excavation depths, supporting system stiffness, and factor of safety against basal heave), pit sizes in plan played a key role in determination of the pit behaviors. The large-scale pits with the areas in plan of 30,000–50,000 m2 experienced wall deflections and ground settlements three to five times those of regular building basement and metro station excavations with the sizes in plan no more than 6,000 m2, and the corresponding influence zones behind the large-sized pits were also much wider. Different from the braced struts that just carried the load because of soil removal in the proximity, the floor slabs sustained the load induced by exposure of the entire retaining wall along the depth. For the top-down excavations in Shanghai soft clay, the lateral earth pressure envelopes behind the retaining walls were trapezoidal, but their magnitudes were significantly smaller than those predicted by the methods available in the literature, and the corresponding peak values occurred at a greater depth. The influence zones of basal heave caused by soil removal (stress relief) extended much deeper below the excavation bases than those assumed in the conventional slip circle basal stability models.