State transportation officials, meanwhile, were working on plans should Interstate 82 be shut down by rockfall or a landslide.

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Authorities monitoring a growing fissure high on Rattlesnake Ridge east of Union Gap warned nearby residents Friday to be prepared to evacuate at a moment’s notice.

State transportation officials meanwhile were working on plans should Interstate 82 be shut down by rockfall or a landslide.

Officials first began monitoring the fissure located uphill from the Columbia Ak Anderson Quarry last October. But concerns grew Thursday, when authorities closed Thorp Road at the base of the 1,758-foot ridge after geologists reported increased movement of the hillside.

Since monitoring began, the movement of an estimated 4 million cubic yards of land has sped up, increasing the immediacy of a landslide of any magnitude, said George Machan, a senior associate geotechnical engineer with Cornforth Consultants, a Portland-based firm hired by the quarry to monitor the crack.

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The largest fissure is now estimated to be some 250 feet deep.

Authorities are preparing for the worst-case scenario, but it’s unclear what that might be.

Officials with the state Department of Natural Resources, which is overseeing the monitoring, and Machan don’t expect a massive landslide.

“I just can’t visualize this whole mass coming off at once,” Machan said.

While noting the slippage has been accelerating in the past few weeks, Machan said the crack’s angle makes it unlikely rocks and dirt would build enough momentum to create a massive landslide.

Instead, he said it’s more likely that falling rocks will land in the quarry’s pit. Some rocks would also likely roll past it and fall down the hillside toward Thorp Road, causing little damage.

But Stephen Reidel, an adjunct geology professor at Washington State University’s Tri-Cities campus who has watched the fissure since October after learning about it from the Yakima Herald-Republic, suggests the potential danger could be greater.

After visiting the fissure Friday, Reidel said the movement seems similar to that which caused a massive landslide onto state Highway 410 west of Naches in 2009. That slide saw 40 million cubic yards of rocks and dirt bury roughly half a mile of the highway and dam part of Naches River. No one was hurt, but several homes in the Nile area were destroyed or damaged beyond repair. Both the Rattlesnake Ridge site and the Nile slide were located near quarries.

Reidel cautioned a slide as large as the one in 2009 is possible, but isn’t the most likely scenario.

Rocks and soil near the surface could slide, but the basalt that forms the base of Rattlesnake Ridge would have to crack further for a massive slide to occur, he said.

“I would be pretty surprised if the whole block slipped away. But I’ve been surprised before,” he said.

Reidel said he’s been surprised how quickly the crack has deepened since October.

“I would make sure to urge people to stay off Thorp Road and stay off the hillside,” Reidel said. “It’s just not safe. I would never go back up there again until I’ve had the opportunity to see if it’s safe or not.”

Because of the wide range of possibilities, local emergency responders started planning for the worst Thursday and Friday.

Members of Yakima Valley Office of Emergency Management told residents near 4650 Thorp Road, a collection of trailers and modest buildings, they were under a stage two evacuation — meaning they need to gather their belongings and be ready to leave.

The Red Cross is expected to open a shelter for these people when they are called to evacuate. Residents were advised they should also evacuate without an official call if they hear a “large amount of rocks falling” or “feel movement from the ground.”

State Department of Transportation officials worked on a plan to protect Interstate 82 — which passes within about 100 feet of the ridge’s base — in the event rocks fall on it.

Transportation Department spokeswoman Summer Derrey said the agency plans to install a cement barrier not far from the interstate.

“It’s the Department of Transportation’s job to protect the highway, and that’s what we’re doing,” she said.

The department’s traffic engineers are working with the Washington State Patrol in planning a detour route should a slide affect the interstate.

Machan said it’s best to plan now, rather than later.

“We would say the concern level is high,” Machan said. “So, it’s not like wait another month and we’ll worry about it. Now is the time to be prepared.”