Challenges Associated with the Construction of Cofferdam

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This case study describes the construction of a sheet-pile cofferdam for a hydroelectric powerhouse in Northern Ontario. The cofferdam was designed with sheet piles extending 24 m below ground and one level of anchors, with dewatering planned using four wells, or six to provide redundancy. The design relied on geotechnical investigations indicating predominantly sand and gravel.<br /><br />During installation, sheet piles refused to reach their design depth at several locations, creating openings, or “windows,” in the perimeter. Excavation later confirmed gravel pockets, cobbles, and boulders that had not been identified in the original investigation. These openings, combined with the nearby Abitibi River and high groundwater, increased seepage into the cofferdam. Initial helical anchors also failed load testing and were replaced with cement-grouted anchors.<br /><br />The team responded by installing additional wells and developing a three-dimensional finite element groundwater model that incorporated the as-built sheet-pile profile, well configuration, and field measurements. The model was calibrated against observed groundwater levels and pumping rates and used to guide well placement and construction planning. Fourteen wells were ultimately included in the system.<br /><br />The final dewatering arrangement achieved groundwater levels of approximately 112–115 m across the excavation, enabling powerhouse construction to proceed in dry conditions. The case demonstrates the value of field monitoring, calibrated numerical modelling, and an observational approach to adapting designs when actual ground conditions differ from expectations. It also emphasizes the importance of designer involvement during construction.

Keywords

cofferdam

sheet piles

dewatering

hydroelectric powerhouse

Northern Ontario

groundwater modeling

finite element model

geotechnical investigation

helical anchors

construction monitoring