A large amount of field and experimental data has been devoted to rectangular excavations thus far. In contrast, very few data were available for cylindrical excavations, especially those with large diameters in thick soft clay deposits. Via a comprehensive instrumentation program on a 100-m-wide×25.89-m-deep unpropped (self-supported) cylindrical excavation constructed by the bottom-up method in Shanghai soft clay, the characteristics of this large-sized circular excavation were extensively investigated. This cylindrical shaft was a part of a large-scale deep foundation pit excavated by the central-island technique. The investigated items included (1) lateral wall deflections, (2) vertical wall movements, (3) lateral ground movements, (4) ground settlements, (5) subsurface settlements, (6) basal heaves, (7) vertical column movements, (8) column stresses, and (9) variation of pore pressures along depth and artesian water levels. Analysis of the field data indicates that discharging of the deep art...

A 16–18 m deep excavation for a metro station with in-plane dimensions of approximately 20 m wide by 290 m long was constructed by using the cut-and-cover technique in soft clay in the Shanghai metropolitan area, in which many high-rise buildings and utilities exist. To investigate the performance of this deep excavation and the associated effect on the adjacent buildings and utility pipelines, a long-term comprehensive instrumentation program was conducted during construction. Field observations included deflections of diaphragm walls, vertical movements at wall tops, ground settlements, and settlements of surrounding buildings and utilities. Analyses of field data indicated that overexcavation (i.e., removing more soil than designed) and a long construction duration caused the diaphragm walls to develop substantial deflections. The base and middle floor slabs played dominant roles in suppressing postexcavation wall deflections and ground settlements. This excavation in general exhibited different behavi...

Via a long-term comprehensive instrumentation program, the performance of a 26 m deep metro station excavation constructed by the top-down method in downtown Shanghai was extensively examined. The measured excavation responses included diaphragm wall deflections, wall settlements, ground settlements, uplifts of interior steel columns, axial forces of propping struts, groundwater table levels, and settlements of adjacent buildings and utility pipelines. Based on the analyses of field data, the following major findings were obtained: (i) the concrete struts along with the floor slabs effectively suppressed later wall movements and consequently reduced the chance that the maximum wall deflections would occur above the excavation surfaces, (ii) with the progress of excavation to a lower depth, the diaphragm walls underwent a serrated settlement pattern over time, (iii) no significant post-excavation wall deflection occurred, (iv) the relationship between the interior column uplifts and the maximum wall deflections can be described by a linear equation, and (v) most system loads due to soil removal were carried by the concrete struts along with the floor slabs. The struts sustained mainly the released earth pressures due to the exposure of the adjacent portions of diaphragm walls, and the soil removal distant from the struts imposed limited effects on the strut axial forces.

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...

Construction of a subway station in Shanghai called for a 31.4–33.1–m-deep excavation, which extended through upper impermeable clayey strata into underlying erodible silt and sand aquifer layers. When the north section of this pit nearly bottomed out on July 23, 2013, a sudden outburst of groundwater took place and the pit base was flooded quickly. In the subsequent two days, the adjacent ground and building successively underwent 30–42-mm settlements in a plunging manner. To prevent aggravation of the situation, a contingency plan was initiated quickly in an attempt to seal off the water flow path. By constructing mixed-in-place and jet-grouting piles respectively along the outer and inner perimeters of the diaphragm wall where leakage occurred, flow channels were successfully blocked. Despite this, the building adjacent to the outburst of groundwater suffered long-term postexcavation settlement. Forensic diagnoses show that this through-wall leaking accident was related to two major factors: (1) ignorance of minor leakages emerging two weeks before without adopting any remedial measure; and (2) a flawed slab connector embedded in the confined aquifer layer. Based on visual inspection and retrospective analysis of field monitoring data, the possible leakage flow path and associated ground subsidence zone were mapped and a settlement history of the affected building was reconstructed. Both contractual and technical lessons learned from this accident will be useful for professionals to mitigate or prevent future incidence of through-wall leakage during excavation.