Structural Behaviors of Large Underground Earth-Retaining Systems in Shanghai. I: Unpropped Circular Diaphragm Wall

Abstract

The excavation of a large-sized foundation pit for one skyscraper in Shanghai required installation of a 100-m diameter unpropped circular diaphragm wall at the pit center first, followed by installation of a multipropped rectangular diaphragm wall at the periphery. Due to the limited paper length, this study just focuses on the structural behaviors of the inner circular wall and those of the outer rectangular wall will be presented in a companion paper. The circular diaphragm wall exhibited apparent spatial arching effects during excavation and only slight differences were observed in its maximum lateral movements along the circumference. Like braced struts, waler beams mainly carried the load due to soil removal in the proximity. In contrast, the circular diaphragm wall carried the hoop stresses due to soil removal throughout the excavation. The vertical bending stresses in diaphragm wall panels were symmetric about their neutral axes. Although the adopted remedial measure effectively mitigated their potential adverse effects on wall deformations, structural deficiencies (e.g., diaphragm wall panels encountered the existing long driven piles at several locations) resulted in some detrimental effects (e.g., tension stress development in concrete and uneven stress distributions in retaining structures). Throughout the construction, the stress development patterns in concrete corresponded well with those in reinforcing rebars, which indicated that the retaining structures were not overstressed. The comparisons between the design analysis results and the field measurements show that the adopted beam-on-elastic-foundation method can make relatively reasonable estimation on excavation-induced deflections of the unpropped circular retaining wall.