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Think of it as a giant burrito.
Actually, think of it as five giant burritos stacked on top of each other, stretching more than 600 feet and holding a combined 5,200 cubic yards of gravel. Then put a highway on top of them.
That's the recipe the Clark County Public Works department hopes will stabilize a stretch of Northwest Pacific Highway outside La Center, a roadway long plagued by slides and erosion during the wet season. The rebuilt foundation may keep the highway sound for 10 years or more, said Michael Meyer, an engineer with Portland-based Landslide Technology, helping coordinate the $1.7 million project.
But long-term stability isn't guaranteed in an historically unstable location.
"It may settle it down," Meyer said. "Or at least slow it down."
The "deep patch" method follows a process not used before in Clark County. Crews began by digging out the existing highway and excavating down several feet. After putting down an initial layer of base rock, workers began wrapping subsequent layers of gravel in a flexible "geogrid" fabric designed to withstand some movement without compromising stability. Lead contractor Nutter Corp. of Vancouver will eventually put in five such layers, each about one foot deep, wrapped like a gravel burrito under the highway. Driven posts will help hold the foundation in place.
Meyer's company has helped with other projects using the deep patch technique, including three in Oregon, he said. Other sites have used wood chips or shredded tire bits to fill foundation -- alternatives that haven't always worked as well, he said.
Clark County's project shut down a half-mile section of Pacific Highway, just west of Northwest Bolen Street north of La Center, on July 6. The county hopes to have the rebuilt corridor open to traffic again by the end of August, said county project manager Robin Washington.
That section of Pacific Highway was built on a slope near Jenny Creek that's prone to sinking and shifting. Earlier excavation revealed just how much -- where the roadway had sunk and cracked, crews had previously just laid down more asphalt to level it out. The result? A layer of asphalt that ended up measuring eight feet thick after decades of quick fixes.
All that pavement may have evened out the road, but it didn't help stabilize the foundation, Meyer said.
"You're adding more and more weight to this," he said, "which is kind of chasing your tail."
Crews also removed the original concrete that carried Pacific Highway before
Interstate 5 became the region's main transportation artery. Workers were surprised to find square rebar built into it, Washington said.
"You don't know until you start uncovering stuff," she said.
New stormwater pipes and infrastructure should better direct flow and prevent damage under the highway, Meyer said. The finished product will widen Pacific Highway to two 12-foot lanes with 2-foot shoulders and guard rails, according to the county. After that, monitoring devices will track how the landscape -- and the highway -- hold up.
"It all depends on what Mother Nature does," Meyer said.