Response of building adjacent to stiff excavation support system in soft clay

A three-story school supported by shallow foundations was affected by an adjacent 12.2-m-deep excavation in soft clay which the excavation support system was a 0.9-m-wide secant pile wall braced by both cross-lot struts and tiebacks. The school is reinforced concrete frame structure with exterior reinforced concrete foundation walls. This paper summarizes the conditions at the and presents correlations among construction activities, measured deformations and distortions, and attendant damage in the school. lateral ground movements associated with the excavation were monitored with four inclinometers placed around the school. The movements were monitored with optical survey points established on interior columns, exterior walls and on the roof, and with installed on the exterior foundation walls. The damage to the school mainly consisted of 300 to 500-mm-long hairline cracks in bearing walls. Only a few cracks had widths greater than 6 mm. The school deformed such that the portion closest to the sagged and the remainder hogged. Damage was first observed in the area of sagging when angular distortions reached 1/940 and excavation was approximately 5.5-m deep. Angular distortions as large as 1/300 were observed at the end of the project. The data that angular distortions had to be less than 1/1000 to preclude any damage to the school.