Contractors will dig nearly 600 shafts ranging from 30 to 60 feet deep and fill them with just shy of 29,000 tons of rock on the Cosi Hill on Highway 101 to stabilize the slide-prone section of roadway, according to Department of Transportation Project Manager John Romero.
The project is expected to start in June and last into the fall of 2020. A contract for construction has not yet been awarded but the state has earmarked just under $8 million for the project.
The project is similar to one that began on another slide-prone stretch of 101 in June of 2015 on Rock Crusher Hill to the south near Artic, but on a much larger scale. The Rock Crusher Hill site involved drilling 190 shafts, compared to the nearly 600 for Cosi Hill.
The state plans to keep one lane moving each direction throughout construction, but to do that, a five-day complete closure will be needed toward the beginning of construction some time in June to shift the roadway to the north of the slide, said Romero. During that closure, traffic will be diverted through Blue Slough Road.
“After that is complete then we’ll have one lane each direction with a speed limit of 25 mph,” said Romero.
“We ask for patience while we are working on it,” he said. “I know the areas are pretty tight. There’s not a lot of traffic, but there is traffic, and we and the contractors will do our best to work in a manner that is safe for our workers and the public.”
Each shaft is six feet in diameter and will be filled with 6-9-inch diameter rock. The majority of the shafts will be drilled on the side slope, with just a few under the roadway itself. Romero said most will be down the slope to stabilize the roadway. According to the Department of Transportation, these shafts will form a buttress to support the highway while addressing ongoing drainage concerns.
This short stretch of 101, between Cosmopolis and State Route 107 around mile post 79, sits atop a large, slow-moving landslide. The Department of Transportation has been performing ongoing maintenance, but this project will, according to geotechnical engineers working on the project, provide a more permanent solution.
SR 107 Bridge
Beginning this week, Rognlin’s, Inc. crews begin efforts to rehabilitate the 58-year-old State Route 107 Chehalis River Bridge located just south of Montesano. Crews will replace the timber on the south approach of the bridge, install modern safety rails, and paint the bridge’s steel structure.
Romero said the project includes stripping down the asphalt across the length of the bridge deck and replacing it with a modified concrete overlay deck “that will need less maintenance in the future.”
The speed limit through the bridge construction area will be reduced from 40 mph to 25 mph until construction is complete in the fall of 2020. The Department of Transportation notes that at times travelers can expect one-way alternating traffic directed by a temporary signal, and oversized and overweight vehicles may need to take an alternate route.
The total cost of the project will be $23.8 million paid for in Department of Transportation Connecting Washington funds.