Measured behaviors of an oversized irregular basement excavation and its surrounding responses in thick soft clay

Abstract

An oversized 10.9–15.6-m-deep irregular pit (70,500 m2) supported by auger cast-in-place pile walls and abundant concrete struts in Shanghai soft clay was excavated by the bottom-up method. The excavation behaviors and responses of surroundings were measured with efforts. Investigations of the well-documented field data disclosed that: (1) owing to long construction duration and outsized pit scale characterized by large width, the maximum wall deflection and ground subsidence exceeded 0.86% of the final excavation depth (He) when the project was completed while the influence zone reached about 4.25He, which were much larger than those of other excavations in Shanghai and violated the allowable deformation criteria; (2) basal heave had a dome-shaped pattern, and the distortions between the adjacent columns were small while the maximum distortion between the columns and the retaining walls (i.e., 1/600) was close to the allowed limit (0.2%); (3) tremendous axial force discrepancy arose (e.g., quantitatively, the maximum axial strut force was more than three times the minimum at the same level) due to spatial effect of inner support and the axial forces in some diagonal struts were 15% larger than designed value; and (4) the surrounding pipelines exhibited three-dimensional deformation pattern and noticeable settlement (i.e., the maximum subsidence was 56 mm).