Cofferdam Failure - Geotechnical Lesson Learned

ABSTRACT: The existing Kabinakagami River Bridge located on Highway 11, approximately 32 km west of Hearst, Ontario, was replaced with a new bridge structure in 2008-2009. During construction of the West Abutment, the cofferdam failed and the construction was delayed. After the failure, the subsequent geotechnical investigation, commissioned by the Contractor was carried out in December 2008. Based on the results of that investigation, the Contractor contended that the borehole soil data shown in the Contract Documents was significantly different than what was actually encountered in the site during the sheet pile and H-pile installation. It was claimed that the existing soil conditions had contributed to problems encountered during the cofferdam installation and to its failure. Consequently, the third-party verification of the subsurface conditions in the area of interest was requested. To verify the subsurface conditions and clarify discrepancies in data collected during pre-failure and post-failure geotechnical investigations, the additional geotechnical investigation in the vicinity of the failed cofferdam was performed in September 2009, results of which are presented in this paper. The paper also includes the assessment of the original subsurface information available for the cofferdam design and suitability of that design as well as the assessment of installation procedures and sequences applied during the construction. The geotechnical lesson learned is discussed.