Shikwamkwa – re-planted on solid ground

Hatch Energy’s involvement with this project dates back to 1979 with performance monitoring and analysis of the original dam. In the years following, we monitored the dam closely as its condition worsened. In 2003, based on state-of-the-art instrumentation monitoring and computer-aided assessment techniques, as well as old-fashioned engineering judgment, we advised Brookfield Power that the structure had reached a state where ongoing repairs and monitoring would no longer be an appropriate measure to mitigate the long-term risk posed by the dam. This resulted in the implementation of an accelerated response plan that included full-time monitoring of the structure during critical periods and restrictions on the operation of the reservoir, as well as a mandate to replace the dam as soon as possible.

Implementation of the Shikwamkwa Replacement Dam quickly became Brookfield’s first priority and an experienced engineering team from Hatch Energy was assembled to undertake the designs and assessments needed to deal with the complex foundation conditions that exist at the site. To accommodate a fast-track construction approach, a team including staff from Hatch Energy, Peter Kiewit Sons and Chant Construction, was assembled to plan and execute the designs.

The Original Shikwamkwa

The original Shikwamkwa Dam was constructed in 1958 as part of the Hollingsworth Generating Station Project near Wawa, Ontario. It was a zoned earth-fill structure built on a highly pervious and complex overburden foundation consisting predominantly of interbedded glaciofluvial and glaciolacustrine deposits ranging from silts to boulders.

The structure’s primary defence against seepage was a relatively short (by today’s standards) impervious blanket that connected to a central impervious silty core and extended upstream along the bottom of the reservoir. While this type of seepage defence was considered an adequate measure based on technology of the time, problems started to develop almost immediately following impounding of the dam. These incidents included excessive seepage, deep sinkholes in the head pond, boiling downstream of the dam and the migration of fine-particles through the pervious foundation.

Seepage and related incidents gradually worsened in the years following construction which led to the implementation of a phased subsurface investigative performance monitoring and remedial work program that was first implemented in the late 1970s. This program was stepped up in 1994 with the implementation of an extensive downstream filter blanket and the installation of an automated instrumentation-monitoring system that permitted the continuous real-time collection of weir flows, piezometric pressures, groundwater temperature and a device that measured the turbidity of the seepage flows, allowing immediate identification of the onset of fine particle migration.

This phased approach to remediation of the dam was expanded by the Shikwamkwa Dam Accelerated Response Plan in 2003. As part of this work, Hatch Energy developed an advanced method for detailed mapping and monitoring of sinkhole development by means of precise sonar profiling that allowed these features to be monitored during the planning and implementation of the replacement dam project.

Emergency-Response Design and Construction

In early 2003, Brookfield Power accepted Hatch Energy’s recommendation to replace the dam – a recommendation that was endorsed by an internationally recognized panel of experts convened by Brookfield Power as part of its accelerated response plan. By mid 2003, Hatch Energy started the site investigations that were needed to define a location for the replacement dam and preliminary engineering designs commenced. Construction of the replacement dam began in late 2004 with a limited program that extended through the harsh northern Ontario winter to seal the bedrock by means of cement grouting. This involved drilling through up to 65 m of overburden to reach the deep bedrock surface that was to serve as the impervious base for the plastic concrete cut-off wall. With the completion of this task by April 2005, full production commenced involving the placement and compaction of approximately one million cubic meters of fill materials and the installation of one of the world’s deepest cut-off walls. At its deepest point, the Shikwamkwa replacement dam and foundation cut-off extends 100 m above the bedrock surface.

The work was performed following a fast-track construction approach using a number of innovative and highly successful construction techniques designed to reduce construction risk. This approach allowed impounding against the replacement dam to commence in late November with the entire project being completed a few days before Christmas 2005 – five months ahead of what the Advisory Panel had considered to be a very aggressive schedule. Overall, just over two years after the accelerated response plan was implemented and investigations began, the project was complete.

Hatch Energy performed the designs and provided construction quality monitoring including the provision of a sophisticated site laboratory performing triaxial testing of plastic concrete, and the installation of numerous electronic instruments for long-term performance monitoring. Engineering work continues in 2006 with Hatch Energy performing regular assessments of the monitoring data to confirm the new structure meets the design requirements and that its anticipated behaviour is in accordance with Brookfield’s dam-safety policies.

The replacement dam, located immediately downstream from the original dam, is founded on the same deep, complex overburden deposit as the original structure. The successful designs implemented for the new dam highlight the additional half century of technology advances that have occurred in the geotechnical engineering profession, as well as Hatch Energy’s expertise in dealing with even the most challenging dam foundation issues.

The replacement dam is a zoned embankment dam, standing 35 m above the original river bed with a crest length of approximately 700 m and a fill volume of 1.1 million cubic meters. The central impervious core straddles a 21,000-square-meter plastic concrete cut-off wall, 65 m deep, which extends from the base of the dam through the complex overburden to bedrock.

Another interesting feature is a flow-compensation facility that conveys reservoir water through the embankment dam in a controlled and regulated manner in order to maintain flow to an existing downstream fishery. The downstream pond and flow channel was enhanced by the construction of a fish refuge, spawning sites and various flow-disturbance features.

The dam is also equipped with extensive automated instrumentation and set alarm levels. Remote radio links are in place for continuous monitoring by Hatch Energy while data is also transmitted via SCADA to Brookfield’s System Control.

The Shikwamkwa Replacement Dam was successfully commissioned, incident free, and the old dam safely breached and taken out of service in December 2005.

The new dam is performing as designed, was completed below budget and ahead of schedule, thereby permitting Brookfield to resume normal reservoir operation six months earlier than planned.

For further information, please contact:

Darren G. Protulipac, MASc., P. Eng., Hatch Energy Senior Geotechnical Engineer
Resident Engineer, Shikwamkwa Replacement Dam Project
Hatch Energy, Niagara Falls, Ontario, Canada.
905-374-5200
DProtulipac@hatchenergy.com

Steve Rigbey MASc., P.Eng., Hatch Energy Project Manager
Design Manager, Shikwamkwa Replacement Dam Project
Hatch Energy, Vancouver, British Columbia, Canada.
604-683-9141
SRigbey@hatchenergy.com

Geoff Rigby, M.Eng., M.I.C.E., P.Eng., Hatch Energy Project Manager
Project Manager, Shikwamkwa Replacement Dam Project
Hatch Energy, Niagara Falls, Ontario, Canada.
905-374-5200
GRigby@hatchenergy.com

Rick Donnelly, M.ASc., P. Eng., Hatch Energy Director Water and Wind Power
Project Sponsor/Geotechnical Quality Assurance, Shikwamkwa Replacement
Dam Project
Hatch Energy, Niagara Falls, Ontario, Canada.
905-374-5200
RDonnelly@hatchenergy.com