The original Shikwamkwa Dam was constructed in 1958 as part of the Hollingsworth Generating Station project, near
Wawa, Ontario. The dam was founded on permeable, highly variable, interbedded glaciofluvial and glaciolacustrine
overburden deposits ranging from silts to boulders. The primary defence against foundation seepage was a relatively short
impervious blanket that connected to the central core and extended upstream from the dam along the bottom of the
reservoir. These seepage defence measures proved to be insufficient and throughout the life of the dam serious incidents
involving sinkholes, migration of fine particles and a partial failure on the upstream slope occurred.
The replacement Shikwamkwa Dam is an earth-fill embankment with a central impervious core and a plastic concrete
cut-off wall (COW) that extends through the overburden foundation and is founded on bedrock. The upstream and
downstream shoulders of the embankment are founded on in situ overburden material.
The embankment has a maximum height of 35 m and a crest length of 700 m measured as the length of the foundation
grout curtain but excluding the crest length 125 m right abutment capping dyke. The maximum height of the COW is
65 m.
The replacement dam is extensively instrumentated with a state-of-the-art automated instrumentation system that can be
remotely read via a radio link and by system control via the SCADA system.
In 1994, Hatch was asked by Brookfield Power Corporation (BPC) to undertake a review of the performance of the original
Shikwamkwa Dam and recommend a course of action that would mitigate what BPC felt were increasing dam and public
safety risks. As a result of this review, BPC implemented a long-term phased program of remedial works for the dam as part
of their dam safety program.
In the spring of 2003, following the discovery of an additional sinkhole at the toe of the dam, and in response to the
increasing frequency of instrumentation anomalies, BPC formulated an accelerated response plan, which consisted of:
- The formation of an advisory panel of experts to review and comment on structure performance data and any other
technical information presented to them
- Installation of additional instrumentation and remote daily monitoring of the Shikwamkwa dam
- The introduction of 24/7 on-site surveillance during the spring freshet when the dam is subjected to the most rapid rise
in reservoir level
- An enhanced sonar survey program to monitor potential sinkhole activity
- The refinement of the Emergency Preparedness and Response Program
In December 2003, BPC concluded that the original Shikwamkwa Dam had reached the state where continued repairs and
monitoring would not be an appropriate response to the long-term risk of dam failure. In addition, the state of the dam was
believed to be such that construction of a cut-off through the dam would be too high of a risk. Construction of a new
structure downstream of the existing dam was deemed necessary and the advisory panel concurred with the
recommendation.
Hatch’s scope of services for the Shikwamkwa Dam replacement project included:
- Dam safety program, reservoir-level guidelines for use in the BPC Emergency Response Plan
- Dam and foundation assessments
- Geotechnical foundation and borrow material investigations
- Environmental assessment and permitting
- Instrument installation, monitoring and data assessment
- Reservoir sonar bathymetric surveys
- Sinkhole infilling and impervious blanket remediation
- Detailed engineering design and detailed geotechnical baseline reports
- Water management plans
- Construction quality assurance
- Construction supervision
- The fast-track construction activities were completed five months ahead of schedule and millions of dollars below budget
- State-of-the-art engineering was required to design and construct one of the largest embankment dams built in North
America in the last decade on a fast-track basis. For example, during the design phase, the use of finite element
analyses incorporating detailed information from the geotechnical investigation programs as it became available was
needed to establish the design requirements for the deep plastic concrete wall
- A 3D seepage model was used to provide site staff and the designers the benchmarks needed to track the performance
of the new dam as the wall progressively closed as well as the safety of the original dam that was retaining 30 m of
water during construction of the replacement dam
- The post-construction monitoring results showed the quality of the work met and exceeded expectations and the
dam’s performance is in line with the predictions made using the sophisticated 3D seepage analyses
The project was completed with no lost-time injuries, a remarkable achievement given the numbers of personnel and
equipment that were needed at the site to achieve the project goals.
- Award of Excellence, Canadian Consulting Engineer and Association of Consulting Engineers of
Canada, 2007
- Award of Excellence, Consulting Engineers of Ontario, 2007
