Vaughn Concrete Products Featured in Precast Solutions Magazine

Precast Gets Tolerant

Tight tolerance requirements on troughs at a Denver water treatment plant were best handled by precast concrete.

As one of Colorado’s primary water utilities, Denver Water can’t afford to have any of its water treatment plants out of commission for any length of time. With 2,499 miles of water pipelines, 17 pumping stations and 34 underground reservoirs located throughout the city of Denver, the utility is responsible for the collection, storage, quality control and distribution of drinking water to nearly one-quarter of all Coloradoans. More than one million people and more than 14,000 fire hydrants in the Denver Metro area rely on its services on a daily basis.

Denver Water’s treatment plants employ conventional process designs consisting of coagulation/sedimentation, filtration and disinfection processes. The plants must meet all the standards set by the state of Colorado and the Federal Safe Drinking Water Act, according to Martin Garcia, the utility’s project engineer. But some of the plants are old – built back when the utility started operations in the 1920s.

For Denver Water, opening a new plant and closing an old one requires meticulous attention to detail and years of planning. Four years ago, Denver Water knew its Marston Water Treatment Plant needed a major overhaul to bring it up to speed. The plant, with a capacity of 180 million gallons of water per day, was built back in 1924. After careful consideration, the utility took the plunge and decided to spend $36 million to replace it with a new facility.

“Marston was getting old and replacement parts were getting hard to find,” says Garcia, “so we decided to tear it down and start from scratch.” Between 1999 and 2001, Denver Water designed its new plant, then put the engineering and construction out for bid in 2001. Some of the key components were 96 precast concrete water troughs measuring 2-feet by 23-feet 10-inches by 2 feet 4 inches. Construction started on the new plant in 2001 and was wrapped up this September.

Steve Price, senior project manager for CDM, an engineering firm based in Cambridge, Mass., says his company won the bid to design the project. “They had an aging infrastructure and wanted to expand and be able to treat more water,” says Price. “It was time for a replacement.”

Careful Consideration
Getting Denver Water’s new plant from concept to completion meant running the existing plant while building the new plant to avoid having to shut down Marston’s 180-million-gallon production stream. Most critical, says Garcia, were the stringent tolerances that the project required. The top edge of the troughs couldn’t have more than 1/16-inch deflection in their 23-feet 10-inch lengths.

Garcia says the tight tolerances were necessary because of the water that rises up in the plant’s filter during the backwash process. “We want it to fill evenly into the trough so that we don’t get more water in one section than in another,” he explains. “If we do get more in one section, it can upset the media.”

Price says precast concrete was the right choice based on the stringent tolerances and the need for structural longevity – after all, the new plant would replace one that lasted for 79 years. “We debated between fiberglass and precast,” he recalls. “After several discussions with Denver Water, we decided to use precast, mainly because of the longevity factor.”

According to Garcia, precast concrete has been Denver Water’s “material of choice” for many years. “Fiberglass was ruled out early because concrete is much more durable,” he says, adding that the “ready to go” aspect of precast concrete made it much more attractive than the poured-in-place option.

“When they were delivered to the site, the troughs were prepped and ready for installation with no extra work required,” adds Garcia. “That cut down on the time involved with, say, poured-in-place troughs that would need forms, time to cure and everything else. Using another option would have definitely added more time to the schedule.”

Mike Vaughn, president of Vaughn Concrete Projects Inc. in Henderson, Colo., says Denver Water chose precast not only for its permanence, but for its ease of installation as well. “If they had installed fiberglass troughs, I’m sure they would have had difficulty in supporting the troughs to minimize distortion, deflection and anchorage problems,” he says.

According to Vaughn, the original Marston water treatment plant was constructed with concrete wash-water troughs that probably wouldn’t have met Denver Water’s new tolerance specifications. Nor could they have expedited the project the way the precast troughs did. “Doing it the original way would have easily doubled the installation time,” says Vaughn.

Pizzagalli Construction Co. of South Burlington, Vt., and Construction Consultants Inc. (CCI) of Falls Church, Va., were joint-venture partners who handled the general contracting for the Marston project.

Pizzagalli and CCI got involved with the project in 2001 after winning the bid to complete the work. Bryan Rufer, Pizzagalli’s mechanical project manager, says that for the troughs, precast concrete worked well because of the project’s tight tolerance requirements. “We could have fabbed them on site,” he says, “but we chose to have them precast based on both the fabrication tolerances and the installation tolerances.”

Jumping Hurdles
If designing and manufacturing 96 precast concrete troughs and hauling them out to a job site for installation sounds like a routine job, think again. Between the tight tolerances and the fact that an existing plant had to continue running adjacent to the new plant, the Marston Water Treatment Plant was no ordinary job for the engineers, contractors or the manufacturers involved.

For starters, working at a busy job site over a two-year period where a water treatment plant is still running while a new one is under construction is hard enough. Vaughn says his company was “always the first in the gate every morning when the site opened.” The punctuality allowed the precaster to back its truck up to a large crane and unload the troughs before the site got too congested. “The site being busy didn’t cause much of a problem for us,” says Vaughn, “but I know it did for the contractor.”

From the engineering perspective, Price says the design challenges included the ability to maintain the existing plant’s land operations during construction. “We worked it out with a consensus-type process with the existing plant operations,” says Price.

Adding to the challenges, says Garcia, is the fact that the two plants were connected by pipes, making it difficult to get the new plant up and running while maintaining the existing plant. To circumvent the challenge,they shut down both plants at two different intervals to do all the necessary maintenance on the old plant. Garcia says that overall, the installation of the 96 troughs went smoothly and that no extra work was necessary to get them in place.

One change that took place during the manufacturing process, says Garcia, was a request from the precast manufacturer to build flat-bottomed troughs as opposed to the original V-shaped bottom troughs, although the internal part of the trough would retain its V-shape. Denver Water agreed to let Vaughn Concrete Products Inc. make the design change, based on the fact that the troughs would be easier to form and easier to stack.

“We’ve been testing and backwashing the troughs and haven’t seen any problems as a result of that design change,” says Garcia.

Precise Measures
Vaughn says his company got involved with the Marston project in January 2002 and wrapped up its portion of the project early this year. The project itself was a tricky one, mainly because of the specified geometry and the precision with which the tops of each trough had to be manufactured.

“There couldn’t be more than 1/16 of an inch of distortion in 24 feet because the units were acting as a weir for the water to run over,” Vaughn says.

Vaughn and a representative from Denver Water sat down together at 10:30 one evening to nail down the details and come to terms on the price. After the meeting, Vaughn and his team reviewed the structural design, taking into consideration both the long-term and short-term deflections to assure that the structures wouldn’t sag with time, and that they wouldn’t exceed the strict tolerances.

“We then went ahead and built the forms for the troughs,” says Vaughn, adding that another challenging aspect of the project was the fact that the structures were going to be fully engulfed with water. Because of this, the structures had to be designed in a way that would cradle or support one end while the other end was cast into a wall structure.

“That certainly created a bit of a challenge during the manufacturing process,” Vaughn says, “but we worked around that with a good, structurally sound design.”

Once the project was underway, Denver Water made another request of Vaughn Concrete: a smoother finish on the troughs. Not part of the project’s original specifications, Vaughn says his company altered its production and vibration techniques to ensure that the resultant finish was satisfactory.

“They wanted a considerably smoother finish than what was specified,” he says. “After some experimenting, we were able to accommodate their request and everything worked out OK.”

Satisfaction Guaranteed
After four years of careful planning, design and installation, the new and improved Marston Water Treatment Plant began full-scale operations in September, much to the delight of the utility itself and the numerous companies that played key roles in replacing the 79-year-old plant with a more modern, functional version.

According to Rufer, all 96 troughs are serving their intended purpose, and all were backwashed multiple times before use. Garcia says Vaughn Concrete did a “very good job,” particularly when it came to meeting the stringent tolerances that the project required.

Vaughn toured the job site with the contractor after the installation was completed. “Everyone seemed very happy with the outcome,” he says. “Denver Water’s expectations were definitely met and everything is working well.”

Project Profile
Project: Marston Water Treatment Plant Upgrade
Owner: Denver Water
Architectural/Engineering Company: CDM, Cambridge, Mass.
Contractor/Installer: Pizzagalli Construction Co., South Burlington, Vt., and Construction Consultants Inc., Falls Church, Va.
Precast Manufacturer: Vaughn Concrete Products, Henderson, Colo. *
* Vaughn Concrete Products is a certified plant under NPCA’s Quality Assurance/Plant Certification program.

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