Colorado's Pine Brook Dam was completed a year ahead of schedule and $1 million under budget, proving how effective design-build can be for dam construction. The Class 1 dam holds 100 acre-feet under normal conditions and up to 137 acre-feet at flood stage. The column in the center is the overflow outlet leading to conduit beneath the dam. Photos: Bob Olliver
John Bowen, project manager and ASI Constructors president, says the Pine Brook Dam's final cost came within 10% of the conceptual estimate. He credits the Pine Brook Water District's board with keeping the project on track by making timely business decisions.
"We delivered on time despite challenges in the permitting and regulatory process," he says. "Because this was the first design-build dam project in Colorado, it was high-profile in the eyes of state regulators."
The design-build team cut costs further by reducing conventional concrete quantities.
According to Rodney Eisenbraun, PE, TCB vice president, the design-build team came to the key realization that if the dam was designed and built to survive a probable maximum flood (PMF: a rainfall event greater than the Big Thompson Flood of 1976), the spillway wouldn't have to handle the full amount of a flood. This meant the spillway could be significantly smaller, reducing the overall amount of conventional concrete necessary to complete the job.
The team also reversed the usual roller-compacted concrete (RCC) material-specification process by first determining the strength of RCC that could be produced when maximizing use of onsite materials, and then adjusting the dam's thickness to limit the stresses to what the RCC could safely resist. While this approach increased the quantity of RCC somewhat, it significantly reduced the unit cost of RCC by minimizing hauling cost. It also reduced construction traffic that would have inconvenienced residents.
Phasing the project to avoid a winter shutdown kept construction on schedule. When winter temperatures fell too low for placing RCC, ASI Constructors excavated, crushed rock, or placed conventional concrete for the spillway using winter protection for curing.
ASI extended the width of RCC on the downstream face an extra foot instead of building formwork to place conventional concrete on this side. Waste soil from the excavation will be placed on the downstream face of the dam and seeded this year to enhance the overall aesthetics of the downstream dam face.
"While a flood that overtops the dam would wash away the soil, the structural integrity of the dam would not be affected," says Krista Nordback, TCB onsite engineer.
Because the Colorado State Engineer requires that the RCC face be exposed for six months for leak observation after filling the dam, the water district had to stockpile 25,000 cu. yds. of dirt in this tight mountainous site until this fall. Residents have affectionately named this stockpile "Mount Bob" and have dedicated the dam to de Haas and his family in recognition of his 40 years of service.Beneficial Lessons
The Genesee Water & Sanitation District, whose new reservoir and dam will be functioning by summer, used some of the cost-saving techniques pioneered by the Pine Brook Water District.
The low bid on the original design for Genesee's 125-acre-foot reservoir and dam was $10.5 million from ASI/Flatiron, a joint venture of ASI Constructors and Flatiron Constructors Inc. of Longmont, Colo., which specializes in building roads, bridges, and dams.
"When the bids came in everybody had shell shock because cement costs had gone up significantly after Hurricane Katrina," says Scott Jones, Genesee Water &Sanitation District manager. "So we started talking with ASI/Flatiron about what we could eliminate from the project."
Genesee worked the design changes as a change order because it had already obtained approval for the original design from the state engineer and didn't want to start over. The value engineering saved concrete costs by replacing the structural stair step concrete on the downstream face with a soil cover. Additional excavation will replace the acre-feet lost by lowering the dam 20 feet. Like Pine Brook Dam, the Genesee dam was also redesigned to survive a probable maximum flood with overtopping that resulted in a smaller spillway requiring less conventional concrete.
In the end, Genesee will build its Class 1 dam at $6.5 million, a $4 million reduction in construction costs from the original estimate.
— Carol Carder is a Denver-based freelance writer specializing in construction and engineering.
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