The city puts a little gravel in the travel of Seattle drivers
It sounds strange and looks odd.
Drivers hear a clinking sound as their vehicles crawl over new road surfaces throughout North Seattle.
Small rocks crunch under tires and hit the underside of cars.
Yellow caution signs warn motorists to keep their speed at 10 miles per hour because of “loose gravel.”
What looks like a good, old fashioned gravel road is actually a chip-sealed surface. Seattle is finishing work this week on chip sealing 36 lane miles of residential streets in the Crown Hill and Greenwood neighborhoods.
“This is very durable,” says contractor Tim Carter.
It’s also called “seal and chip” or “tar and chip” in other parts of the country.
“All it is are small pieces of stone that are placed in an asphalt emulsion that’s very sticky and sticks to the existing road surface,” he says.
A thin layer of asphalt is applied to an existing pavement and then small rocks are distributed over the seal spray.
The Seattle Department of Transportation (SDOT) says for the first time, this year they tried another process called “micro surfacing” on 12 miles in the Wedgwood neighborhood.
That’s similar to chip seal, but instead of first applying emulsion, dropping gravel into the emulsion and pressing it in, the gravel is blended with the emulsion and the two are applied to the street simultaneously. The result is less loose gravel.
SDOT says chip sealing costs about one-tenth as much as putting down blacktop.
“Regular black top uses the same stones, it uses the same asphalt basically – the same sticky asphalt that’s the cement that holds everything together – but the only other thing it uses is a little bit of fine sand that they mix in with it,” Carter says.
“After a few years, that wears off and basically both pavement surfaces look the same. That’s why when you look at brand new blacktop it is smoother, and the blackness comes from the asphalt cement that is coating all those stones.”
Chip sealing is also a faster process. Crews can resurface several miles of roadway in a day’s time.
It does have some drawbacks.
Loose rocks that are not removed can cause safety and environmental problems such as cracked windshields, and could lead to crashes for motorcyclists and bicyclists.
Those rocks can end up in storm drains, which is why the city sweeps the roads several days after the emulsion sets.
It needs to be done every seven years or so, while asphalt can last for between 12 and 15 years. Also, it only works well on a surface that’s already in decent shape.
While the gravel side roads look novel in an urban area, about 25 percent of Seattle’s roads are chip sealed adding up to 550 lane miles.
Pierce County has 140 lane miles are treated that way. Snohomish County chip sealed 85 lane miles this summer alone.
By LINDA THOMAS – curious about this because I grew up in a rural area where we had real gravel roads with crushed limestone rocks that left long dust trails behind my dad’s yellow Ford Galaxie 500.