Published December 2002

County sidewalk helps earth, budget

By Kimberly Hilden
SCBJ Assistant Editor

As rain fell on one north-county neighborhood early in November, water pooled in driveways and ran along roadside curbs. But on the sidewalk where Darrell Ash and Vladimir Malinsky stood, there were no puddles, and there was no runoff.

Snohomish County Business Journal/ KIMBERLY HILDEN Darrell Ash, an engineering supervisor with Snohomish County Public Works, pours water onto a pervious concrete sidewalk in a residential neighborhood just north of the Marysville city limits. Instead of turning into a puddle, the water percolates through the material and into the ground below.

That’s because the sidewalk, a Snohomish County Public Works project, is made of pervious concrete, a “green building” material consisting of cement, coarse aggregate and some admixtures that enables water to percolate through the concrete — being filtered on its way down — and into the soil, said Malinsky, the project manager.

Once in the soil, the water reaches nearby streams naturally instead of being discharged at one point, reducing incidence of erosion, flooding and damage to habitat, added Ash, an engineering supervisor with Public Works.

The 5-foot-wide walkway, which stretches along the south side of 100th Street NE between 48th Drive NE and 51st Avenue NE just north of Marysville, is the first of its kind for the county, Malinsky said, and one of a select few in the Puget Sound region.

Originally, the sidewalk was designed with traditional concrete in mind, with off-site water detention ponds, curbs, gutters and underground catch basins necessary to mitigate the impact of the impervious structure. But Malinsky and Ash had looked at a pervious-concrete sidewalk in Seattle and saw an opportunity.

“This project was an ideal candidate for the pervious concrete because the underlying soils are sandy, with very high water-infiltration rates, and the ground is relatively flat,” Malinsky said.

“It eliminated a headache, and it saved money,” Ash said. “The final construction cost was around $130,000 — some $114,000 less than the original budget.”

Laid during the summer by Wilder Construction of Everett, the sidewalk will be monitored over the next few years by the county to see how it stands up to Northwest rainstorms, sleet and snow — not to mention the foot traffic as children make their way to Cascade Elementary School or shoppers head to Fred Meyer nearby.

In laboratory tests, the pervious concrete didn’t inhibit the water’s flow at all, Malinsky said, indicating that water should be able to percolate through the material before freezing during the cold winter months.

But the pervious material is not as strong as traditional concrete, and it creates a slightly rougher walking surface, he said, so the next few years will be telling ones.

If the sidewalk does hold up, pervious concrete could find its way into other county projects benefiting both the environment and the county’s budget, he and Ash said.

Already, they are studying the possibility of using the material to build the curb alongside the roadway, enabling water to seep through it into the planting strip that separates the walkway from the road instead of being directed into catch basins.

“We’ll hope for the best, and we’ll watch,” Ash said.

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