Office Building Settlement and Remediation

A 3-story office building, with a footprint of 40 m x 28 m located within an industrial facility has undergone excessive settlement. Approximately 5 months after pouring the concrete for the roof slab (construction about 80% complete) the building started to suffer excessive settlement of up to 300mm. The office building is founded on a 0.6m thick raft foundation. The soil conditions at the site (based on two preconstruction bore holes located at the diagonal corners of the footprint) consist of 1m of structural fill, followed by 3m of soft clay, underlain by 3m of loose to medium sand, in turn underlain by 3m of soft clay followed by 3m of medium sand underlain by a 4m thickness of very stiff clay underlain by a very dens layer of sand which extends deep to the end of the bore hole. Groundwater was about 1m from ground surface. The measured settlement ranged from 170mm to 300 mm resulting in a differential settlement of 130mm. As a result, cracks began to show on the walls, flooring, and other building structural elements. In addition, the office building was constructed at the same time as an adjacent deep excavation, involving an extensive deep dewatering system, was being constructed. The authors became involved in the remediation alternative and recomended additional investigation during construction of the building to: (1) assess the subsurface conditions more accurately, (2) determine the root cause of the settlement, and (3) to develop the design for the remedial measures. This paper will discuss the settlement time history and analyze how the dewatering process played a role in increasing the office building settlement. In addition, the remedial measures taken to reduce additional settlement to the building will be described and discussed. Settlement monitoring following the remedial measures indicated that the settlement was controlled and the construction of the building was completed without experiencing additional movement.