Why Excavation of a Small Air Shaft Caused Excessively Large Displacements: Forensic Investigation

Abstract

Any deficiency in excavation design or construction could lead to excessively large retaining wall and ground displacements, usually accompanied by damage to adjacent structures or facilities. This paper documents a case history of a small air shaft excavation in Shanghai soft clay. Its maximum lateral wall displacement and ground settlement were twice and three times the local deformation control criteria and 3 and 14 times those of one adjacent air shaft excavation featuring similar subsurface conditions, geometric size, and supporting system. To identify probable causes pertaining to its poor performance, a comprehensive investigation was carried out in both design (propping element, supporting system stiffness, wall penetration ratio, and jet grouting of basal soils) and construction (excavation duration, excavation width, exposed wall length, and management of heavy-duty truck and stockpile of construction materials). The investigations show that there were several major contributory factors, among them were adoption of cast concrete struts at deep excavation level, removal of underlying jet-grouted soil layer before fully curing of concrete strut, and passing over of trucks and stockpiling of construction materials nearby. On the basis of the analysis of its field instrumentation data and comparison with the adjacent air shaft excavation and another four subway station excavations, adverse effects of the identified contributory factors were quantified, valuable lessons were learned, and graphical solutions for estimating effects of excavation width and exposed wall length on lateral wall displacement and predicting diaphragm walling-induced ground settlement were derived.