Caribou Falls Block Dam 2 Reconstruction

Ontario Power Generation (OPG) retained KGS Group to design and supervise the reconstruction of the Caribou Falls Generating Station Block Dam 2, which had sustained unexpected slope movements. It is one of four earth dams that contain the reservoir (Umfreville Lake) for the station.

Advanced techniques, innovative materials and sound engineering judgment were applied to construct a new earthfill dam. The dam and its generation capacity were restored within budget and schedule, while preserving the pristine natural surroundings.

Optimizing the solution

Situated in the rugged Canadian Shield in Northwestern Ontario, Block Dam 2 contains the reservoir for OPG’s Caribou Falls Generating Station. Built in 1958, the station and dams were operating reliably when, after 60 years of service, significant movement suddenly occurred on the upstream slope. For dam safety purposes, OPG partly lowered the reservoir level, reducing power generation capacity.

OPG retained KGS for investigations, design (conceptual through to detailed), contract administration and dam performance monitoring for the project’s construction and post-construction phases.

KGS used several investigation techniques to characterize the site, including a seismic refraction geophysical survey and seismic cone penetrometer testing (SCPTu). The latter allowed the firm to quickly obtain data from probes across the site, providing a dense spatial array of foundation soil information in a relatively short time frame.

A focused analysis of this data allowed the design team to optimize the new dam’s geometry to resist potential earthquake-induced instability, rather than incorporate traditional, expensive ground improvement methods.

The solution involved building a replacement cement-bentonite (CB) wall core dam on the downstream side of the original dam, while maintaining it as the construction cofferdam. Once the new embankment was rebuilt close to full height, the core trench was excavated while a slurry of water, ce - ment and bentonite were continuously added, to prevent the sand trench walls from collapsing.

Once cured, the slurry-filled trench became the new water-retaining core of the dam. The relative ease of the granular fill embankment construction followed by slurry trenching, which could be undertaken in almost any weather conditions, minimized the project’s vulnerability to seasonal restrictions and delays. Also, the CB wall’s slender footprint allowed for the new dam to be built immediately against the downstream slope of the old dam, to fit within the confined project boundary and minimize the amount of material needed.

To ensure the slurry trenching machines wouldn’t get stuck in the deepest, densest section of the foundation, the contractor equipped long-reach excavators with three custom interchangeable imple - 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 constr uction 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-