Precast Stormwater Vault Serves Redmond, Wash.

Complex stormwater treatment is a challenge facing many major cities. For the City of Redmond, Wash., it was creating a financial and environmental catch-22. The city was forced to choose between expensive clean-up procedures and stiff penalties if they failed to meet state and federal regulations.

Oldcastle Precast in Auburn, Wash., an NPCA Certified Plant, offered the city the best solution for both challenges engineering and manufacturing a cost-effective product that also met the Washington General Use Level Designation requirements for flow rates and phosphorus removal.

Construction crews carefully place filter cartridges inside the precast concrete vault. (Photo courtesy of Oldcastle Precast)

The job site was located next to a busy thoroughfare adjacent to a paved parking area. The general contractor set up a delivery and installation schedule for the precast concrete to be transported two pieces at a time by truck. There were no delays and the contractor did not have to designate a special area for material storage.

“Precast concrete provided for a quick installation freeing up an already congested site for finish work and allowing businesses to stay open,” Dave Swanson, sales manager, said.

Swanson said ultimately the city wanted to build a regional water filtration unit capable of managing a 9,000-gallons-per-minute water flow rate. The precast vault design needed to incorporate a filter cartridge system with overflow and bypass technology.

“One primary purpose of a regional system was to mitigate the need of having multiple onsite filtration units, maximizing the foot prints of each site and providing a cost-effective solution,” said Swanson.

The precast concrete vault design allowed for easy maintenance and cleaning, and expansion if needed. (Photo courtesy of Oldcastle Precast)

Swanson said from quote to completion the project took approximately nine months. The vault – 14 feet wide by 214 feet long by 6 feet deep –  included 107 pieces total with 23 bases, two end walls, nine weirs cast in stainless steel frames to accept adjustable baffles, 25 divider walls and 48 false bottoms.

“The false bottoms are concrete slabs dropped into the vault that have a custom voiding to receive the filter cartridges,” Swanson said. “Each slab would have up to 16 cartridges some stacked two cartridges high.”

Tight tolerances in manufacturing required construction crews to place expansion anchors throughout the vault. (Photo courtesy of Oldcastle Precast)

Building the vault was straight forward, Swanson said, but having various bolt-in or cast-in frames and adjustable weirs required precise placement and strict quality control measures.

Scott Sertich, P.E., solutions engineer at Oldcastle Stormwater Solutions, said the media filtration system is the largest Oldcastle has manufactured. On average, the company builds precast vaults that hold 31 filter cartridges; this project had 1,092.

“When water goes through, it sounds like a jet engine,” Sertich said. “We’re happy to build a vault that has one or two filters, but when we get one with a thousand filters, it’s mind blowing. A year’s worth of projects compressed into one.”

The media filtration project went live January 2015. (Photo courtesy of Oldcastle Precast)

In the end, a lot of clear communication between Oldcastle Precast, the City of Redmond, the State of Washington, as well as close work with the contractor, ensured the system was successful.

“It felt great to see this thing finally come online – we were really jazzed,” Sertich said. “We worked on this as part of a team. That’s what made this product work.”