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‘Lego’ cells buried beneath West Central sidewalks as part of stormwater project

In West Central, there’s a great future in plastics.

That’s the hope of Spokane’s public works department, which has spent the summer burying a network of plastic chambers on block corners throughout the historic neighborhood. When finished, the structures – called Silva Cells – will trap tens of thousands of gallons of stormwater runoff beneath the sidewalks and prevent it from entering Spokane’s combined sewer system. When that system is inundated, untreated sewage flows into the Spokane River, occurrences that must be limited under discharge agreements between the city and environmental regulators.

It’s the city’s first experience with the technology, which was developed by an urban landscaping firm called DeepRoot based in San Francisco. The appearance of the plastic framing, which is buried and covered with wheelchair-accessible sidewalks, grasses, shrubs and trees, has prompted more than a few analogies from the city’s engineering team.

“Legos,” said Kyle Twohig, director of engineering for the public works department.

“Glorified milk crates,” said Lindsay Duvall, the inspector for the project with the city’s engineering department.

The modular building blocks arrive in stacks of boxes ordered from DeepRoot. Crews from Halme Construction, the contractor who won a $3.4 million bid to complete the work, chop up existing sidewalks and dig several feet down, where the plastic pieces are put together and filled with a type of soil designed to promote filtration of pollutants in water running down storm drains. The chambers, which are topped with a plastic deck that is covered with concrete, allow the soil to remain loose, promoting the growth of root systems in the 96 trees that crews are planting above the stormwater chambers.

The chamber is not completely filled with soil, allowing space for stormwater. That water flows into large concrete chambers and is dispersed throughout the Silva Cell using another plastic pipe slitted at 6-inch intervals to allow the release of captured rainwater.

“It is really helping us both with our stormwater treatment, as well as giving us a neighborhood aesthetic,” Twohig said.

The approach is an alternative to the many massive subterranean tanks that are being built around town to hold stormwater runoff during heavy rains, but the end goal is the same: Less water in the pipes means fewer pollutant discharges into the river.

“This was the basin, of all the basins in our system, that penciled out the best for what we call a green solution,” Twohig said. “It would have taken multiple tanks, and we couldn’t easily locate a site where we needed the tanks.”

The work is taking runoff in the West Central neighborhood out of the combined system. Rather than heavy runoff filling up the pipes and causing discharge into the river, any backups in the system would occur in the gutters and along curbs in the neighborhood while the underground chambers catch up to the heavy rainfall or snowmelt. The system has been designed to withstand a 10-year storm, which means backups should only happen about once a decade.

“It would really just run down to the next cell, or the next storm drain,” Twohig said.

To date, the city has spent close to $70 million on the tank system, according to payment vouchers with firms that include Halme, Garco Construction, Clearwater Construction and others who have signed contracts for the projects. Another $70 million is expected to be spent before the tanks are completed, with part of the construction being funded by green bonds the city sold in 2014 as well as other sources, including grants from the state Department of Ecology.

Halme is under city orders to limit construction in the neighborhood to corridors, and not to work on more than five chamber locations at any one time, Duvall said. While neighbors said they’ve been pleased with the look of the replaced sidewalks and the speed of construction in most cases, Duvall acknowledged there had been some signs of discontent.

“There’s been some vandalism, half a dozen windows have been broken out and minimal theft,” Duvall said. “You can have that anywhere.”

A sign on Boone Avenue indicating work is underway has been spray-painted with a profane condemnation of “rich people.”

West Central residents, lured outside by cooling temperatures late this week, said there were some disruptions to traffic and parking as a result of the work, and that construction noise had been an issue during the day. But the end result has been worth it.

“Just the parking, basically,” said Jake Runke, a recent Gonzaga University graduate who’s renting a two-story home originally built in 1906 with fellow Bulldog alumnus Jonathan Osborn. The two watched from their porch Thursday as crews began filling the trench that will house the new, underground rain garden.

“It’s been a bit noisy,” Runke said.

Runke and Osborn said crews have been outside their front door for about a week. Across Boone Avenue, Dale Readus – an Army and Air National Guard veteran who’s lived in a duplex on the corner of Cochran Street and Gardner Avenue for about two years – said workers were outside his door for much longer this spring.

“It looks a little classier, if you know what I mean,” said Readus, casting his eyes across the street where new trees, sidewalk ramps and mulched planters have been installed. Readus said workers caused some damage to a chain-link fence, but repaired it for his neighbor.

To the east on Gardner Avenue, Shirley Larson watered new grape plant starts in front of a rancher she moved into a year ago. She said parking is a constant issue in the neighborhood, especially on blocks like hers where residents operate businesses out of their homes, and it was made worse by some of the construction work.

“When they got done, they did a great job,” Larson said. “Everybody got their front yard cleaned up.”

Twohig said crews had learned a lot from their first few weeks installing the cells and are now working at a much faster clip. Work is expected to be completed within the next four weeks, Duvall said.

Those out walking their dogs or taking a trip to Doyle’s for some ice cream who stop to look at the subterranean cells may get a once-in-a-lifetime view. Crews aren’t expecting to have to replace any of the plastic for quite some time.

“Nobody has had them long enough to degrade to that level,” Twohig said. “I don’t know that we’ll find that life span in our lifetime.”


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