Grand Coulee Dam
New Deal for Some Old Power-House Roofs in Washington
by Alison Diegel, freelance writer for the construction industry
Overlooking the Columbia River stands one of the most well-recognized struchues in the western half of the United States, and quite possibly the world - the Grand Coulee Dam . As the largest freestanding concrete structure in the United States, the Grand Coulee Dam in Washington has captured the interest of thousands with its rich history.
During the height of the Great Depression, President Franklin D. Roosevelt called for his New Deal legislation as a way to create jobs and stimulate the declining economy. Largely funded by the Public Works Administration (PW At the New Deal aimed to create new projects that would improve the infrastructure of the country. In Washington State, the government identified the Grand Coulee Dam as a potential project that would benefit the residents living near the Columbia River by generating energy and providing flood control and irrigation.
Choosing the site of the Columbia River for the dam was an optimistic selection due to its enormous width and heavy water flow. Despite these potential roadblocks, the river was ultimately selected to be the site of the Grand Coulee Dam. Taking more than eight years to complete, the dam is made of nearly 12 million cubic yards of concrete and covers a span of more than one mile.
Two powerhouses needed to be built adjacent to the dam itself in order to transform the flowing Columbia River into electricity. The adjoining powerhouses are relatively small structures, but they are vital to the efficient and effective operations of the dam.
The U.S. Department of Interior and the Bonneville Power Administration, the local government entity responsible for the maintenance and oversight of the darn, undertook the replacement of the powerhouse roofs in the late 1990s as they began deteriorating beyond repair. The five-ply, coal tar pitch roofs were covered with 1" pea gravel set into bituminous mastic, and they were leaking so badly that they could have potentially compromised the integrity of the entire Grand Coulee Dam operation.
Over a ten-year period, Coastal Specified Products, the Carlisle SynTee representatives in the Pacific Northwest, worked with the dam's owner representative to understand the project scope, develop the roof system type, and provide a budget cost. Over that period, several roof system types were considered before the Department of Interior decided on a fully adhered, 80-mil thermoplastic polyolefin (TPO) membrane system, which was ultimately supplied by Carlisle SynTec. The budget was approved in 2007 for completion in 2008. When the Federal Government put the work out to bid, Pacific Tech Construction in Longview, Wash., was awarded the job. "The durability, easy installation, and strong, dependable heat-welded seams made Carlisle's TPO an easy choice for us," said Doug Yeaman, the project manager for Pacific Tech Construction.
Carlisle's Sure-Weld TPO membranes are composed of three layers: a TPO polymer base, a polyester-reinforced fabric, and a thermoplastic polyolefin compounded top ply. The top ply is one of the thickest and smoothest in the industry and provides enhanced resistance to dirt and biological growth. Every Carlisle TPO membrane also comes standard with the patented OctaGuard XT Weathering Package, a unique eightpart formulation that helps create one of the most dependable, long-lasting thermoplastic roofing materials.
The installation of the new Carlisle TPO single-ply membrane system required a complete tear-off of the old, worn out coal tar pitch roof assembly. Due to its viscosity and multiple layers, the tar pitch roof was difficult to remove, and the tear-off took multiple days to complete. As they neared the bottom layers of the worn out system, it was decided that it would be best if a small layer of the coal tar pitch was left adhered to the concrete substrate.
"The closer we got to the bottom layer, the more evident it became that the coal tar pitch was strongly adhered to the concrete t-beams," commented Yeaman. "The original specifications required shot blast to remove the tar, but this procedure would have removed a majority of the original caulking. This could have hurt the integrity of the roof, making it vulnerable to leaks, so it Grand Coulee Dam was decided that it would be best to leave a small amount of tar adhered to the roof to serve as a vapor barrier." After the deck was completely prepared, Yeaman and his crew from Pacific Tech began the installation of the new Carlisle Sure-Weld TPO roofing system. To initiate the process, a 2" layer of Carlisle polyiso insulation was adhered to the deck using hot asphalt. The combination of the insulation and the asphalt provided a redundant, efficient, and effective thermal and moisture barrier for the powerhouse roofs.
Two more layers of Carlisle polyiso were installed on top of the original layer, with the top layer being a polyiso composite nailbase. The last and most critical layer was an 80-mil gray Sure-Weld TPO membrane that was adhered using Sure-Weld bonding adhesive. With Carlisle's TPO roofing systems, a unique variety of sixteen colors are available. Gray TPO was chosen for the Grand Coulee Dam because of its ability to blend in with the natural rock surroundings.
The roof installation was completed with Carlisle's prefabricated accessories designed specifically for use around pipes, drains, and other rooftop penetrations, adding to the overall ease of installing the roofing system and helping increase roof system performance while reducing labor and installation costs.
Carlisle's Sure-Weld TPO membranes were also installed on the fan rooms and bus bay for each powerhouse. Given the history and longevity of the Grand Coulee Dam, it was important to install a roof system with proven performance. Because of the strength and durability of Sure-Weld membranes, a 20-year Total System warranty was available with the new roof.
Carlisle's Sure-Weld TPO membranes were an ideal choice for the Grand Coulee Dam because Sure-Weld membranes are 100% recyclable and are manufactured to withstand the long-term weathering effects experienced in the Pacific Northwest. These environmentally safe membranes are important when installing close to an important, historical natural resource. For years to come, tourists can continue to visit the Grand Coulee Dam and
enjoy one of the original great American projects from the New Deal Era.
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