Characteristics of a Large-Scale Deep Foundation Pit Excavated by the Central-Island Technique in Shanghai Soft Clay. I: Bottom-Up Construction of the Central Cylindrical Shaft
Publication: Journal of Geotechnical and Geoenvironmental Engineering
Volume 139, Issue 11
Abstract
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 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 artesian water was one of the key factors affecting the excavation behaviors. The influence zone of basal heave caused by soil removal (stress relief) reached very deep soil layers, far beyond the depths reported in literature. Column movement and stress were shown to be reliable indicators of the relative movement between basal soils, diaphragm walls, and interior columns. Finally, the comparisons of this study with other bottom-up excavations in Shanghai, including 11 cylindrical excavations, 92 building basement excavations, and eight metro station excavations, demonstrate that self-supported (unpropped) cylindrical excavations had much stronger capabilities of resisting deformation than both multipropped regular basement excavations (in terms of deformation magnitudes) and long and narrow metro station excavations (in terms of scale of influence zones). For cylindrical shafts, their diameters, and not wall penetration ratios, played a dominant role in the determination of excavation behaviors. However, when the shaft diameter reached 90 m or more, the advantage of cylindrical shafts over rectangular pits in constraining wall deflection was no longer obvious.
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Acknowledgments
Many organizations and people contributed to the success of this research project and special thanks are due to Mr. Hongliang Lan and Mr. Linkang Xuan of Shanghai Geotechnical Investigations and Design Institute Company Limited for providing the detailed information about site conditions and field instrumentation. The financial support provided by the Innovation Program of Shanghai Municipal Education Commission (No. 13ZZ027) is gratefully acknowledged. Dr. Ye Lu from Shanghai University is appreciated for her contribution to improve the quality of this paper. Finally, the insightful comments and suggestions from the anonymous reviewers, the Associate Editor, and the Editor-in-Chief Dr. Patrick J. Fox are sincerely appreciated.
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Published In
Journal of Geotechnical and Geoenvironmental Engineering
Volume 139 • Issue 11 • November 2013
Pages: 1875 - 1893
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© 2013 American Society of Civil Engineers.
History
Received: Jan 27, 2012
Accepted: Mar 20, 2013
Published online: Mar 22, 2013
Published in print: Nov 1, 2013
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