Crews Friday poured the city’s first section of “pervious concrete” sidewalk as part of a project to test its strength and its ability to reduce storm water runoff.
The pilot project is located on the north side of Northup Way, between 165th and 168th avenues Northeast, in front of Ivanhoe Park. It includes approximately 675 feet of pervious sidewalk; the rest will be conventional sidewalk so the two surfaces can be easily compared.
Pervious concrete uses less water and less sand than conventional concrete to create a sidewalk with substantially more voids, or empty spaces, allowing water to drain rapidly through it to a bed of gravel below.
Pervious sidewalks reduce the amount of storm water that drains into rivers, lakes and Puget Sound, carrying with it motor oil, fertilizer and other pollutants. A better alternative is for rain water to soak into the soil, which acts as natural filtration system. This helps protect salmon and other wildlife from the impacts of runoff due to impervious surfaces.
Also, instead of creating a separate detention area, catch basin or vault, a pervious concrete sidewalk utilizes the space below the sidewalk itself. A 1- to 2-foot layer of course gravel beneath the concrete acts as a mini-drain field, holding storm water until it can seep into the soil below. This system saves space that otherwise would be devoted to drainage.
The pervious concrete sidewalk in Bellevue is about 5.5 inches thick, slightly thicker than a conventional 5-inch thick sidewalk. Another difference from conventional sidewalks is that the contractor for the pilot project, Riverton Contractors Inc., is installing an overflow pipe under the concrete to prevent excess rain water from bubbling back up through the sidewalk.
At about $60 per square yard, pervious sidewalks cost about twice as much as conventional sidewalks to construct, though the pervious concrete work is a relatively small part of the pilot project’s total budget of $262,000. Most of the cost consists of design, engineering, landscaping and construction of curbs, gutters and non-pervious sidewalk.
Bellevue’s effort to test pervious sidewalk follows a pilot project last year to test the effectiveness of rubber sidewalk, made from recycled tires, as a way to prevent heaving and cracking due to tree roots.
Like rubber sidewalks, the pervious sidewalk pilot project attempts to meet an objective of the city’s Environmental Stewardship Initiative: to create a sustainable urban habitat with clean air and water.
