This is what the Rattlesnake landslide will mean for I-82, Yakima River

A geologist hired to study the growing cracks on Rattlesnake Ridge estimates up to 1 million cubic yards of rock and soil could slide down the ridge, but most would be trapped in a quarry and not reach Interstate 82 or the Yakima River.

George Machan, a senior associate geotechnical engineer with Cornforth Consultants, a Portland-based firm hired by the Columbia AK Anderson Quarry to monitor the slide, also said this week that a landslide is projected to occur sometime between now and the end of February.

One million cubic yards is enough material to fill the U.S. Capitol Rotunda more than twice.

On Wednesday, state Department of Transportation crews continued placing what will eventually be 600 feet of shipping containers between Thorp Road, at the base of the ridge, and nearby Interstate 82.

The containers, each filled with 6 tons of concrete, are intended to defend the interstate against moderate rockfall, but won’t protect it from a major landslide, said Transportation Department spokeswoman Summer Derry.

“At the first sign of excessive movement or increased frequency of rockfall, our crews will close the interstate as a precaution,” she said. Potential detours would include U.S. Highway 97, west of the Yakima River and more than 3,000 feet west of the interstate.

Machan called the shipping containers a necessary and important precaution, but said the risk to the Yakima River and the interstate is low.

“I just don’t see it based on the characteristics of this slide,” he said. “There’s an extremely remote possibility that it’s going to get into the river.”

Instead, Machan said the majority of the soil expected to break off the ridge — about 500,000 to 1 million cubic yards — will fall into the quarry pit, where it will likely be trapped and begin to stabilize.

However, as the slow-moving slide — which is moving at an average of 2.5 inches per day — nears a cliff above the quarry, the surrounding areas could see increased rockfall, he said.

About 50 residents living in a collection of trailers and modest buildings near the base of the ridge were warned last week to evacuate.

About 35 had evacuated by Wednesday morning, officials said. Officials with Yakima Valley Office of Emergency Management were expected to visit the area in an effort to persuade residents to leave.

Residents will be offered hotel rooms paid for by the quarry, said Horace Ward, senior emergency planner with the county agency.

The Red Cross reopened an emergency shelter Wednesday afternoon at the Summitview Church of Christ on 72nd Avenue for residents warned to evacuate their homes. The shelter was initially opened last week, but closed when no one arrived.

This story was originally published January 4, 2018 at 10:10 AM with the headline "This is what the Rattlesnake landslide will mean for I-82, Yakima River."