ments that could tear through dense gravel and pluck out boulders. Hazard potential When Block Dam 2 sustained slope movements without warning, part of the crest dropped 1.5 m, exposing internal earthfill materials and potentially compromising the integrity of the core. Maintaining the dam was crucial, as it retains Umfreville Lake. Ontario’s ministry of natural resources and forestry rates Block Dam 2 ‘extreme’ under its Hazard Potential Classification System. This classification, along with the unexplainable movement, compelled OPG to expedite the complete replacement of the dam. The foundation across the
entire site was dominated by a unique, altered type of Lake Agassiz clay. An inadequate understanding of the strength and mechanical behaviour of this material has historically been the root cause of instabilities for other water-retaining structures. To address this issue, KGS implemented a multi-pronged approach for stability and seepage control measures, including construction of the CB wall core, shear key, chimney drain, filter blanket and a toe berm. During construction, highly irregular bedrock with a fracture zone was encountered that could cause significant leakage. The team reacted immediately, optimizing the dam alignment and narrowing the core to avoid the irregular sur-
We care enough to do it right Dedicated to excellence in engineering and project management, our seasoned team has the exper tise to tackle almost any project.
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faces. A bedrock grouting program was also designed to make the fracture zone watertight. Every effort was made to prevent an uncontrolled release of water from Umfreville Lake. Such an event would have both safety impacts and catastrophic environmental consequences. OPG held regular review sessions with its dam safety expert panel of internationally recognized engineers, whose input was incorporated at key stages of the project, from preliminary design through construction. The replacement dam was designed to meet modern standards, augmented with instrumentation and telemetry to support OPG’s dam safety monitoring and sur-
veillance program. Goals accomplished The project was completed a month ahead of schedule, allowing OPG to proceed with raising the reservoir back to normal operating levels, restoring the generation station’s power production capacity.
Caribou Falls Block Dam 2 Reconstruction, Caribou Falls, Ont. Award-winning firm (prime consultant): KGS Group, Winnipeg, Man. (Dami Adedapo, P.Eng.; Jon Friesen, P.Eng.; David Kurz, P.Eng.; Kelly Fordyce, P.Eng.; Jacqueline MacLennan, P.Eng.; Colin Mohr, P. Eng.; Moises Alfaro). Owner: Ontario Power Generation. Other key players: Peter Kiewit Sons (contractor).
