Rattlesnake Ridge 'a different beast' when it comes to Washington landslides, professor says

UNION GAP, Wash. -- A major landslide feared at Rattlesnake Ridge near Yakima is a "different beast" than most landslides we see in Washington, according to state geologists.

The 8-million tons of dirt sitting above Interstate 82 near the Yakima River is moving about 2.5 inches a day. That's about a foot-and-a-half a week.

Or, quite a bit slower than most Washington landslides we've seen.

"This is really different from the tragedy we saw in Oso and the kind that are happening now in Southern California," University of Washington professor of geomorphology David R. Montgomery said.

"This is not one of those rainfall-driven landslides that is going to go during the peak rainfall," Montgomery continued. "This is a whole different beast."

As of recent measurements, the slide is expected to occur sometime between late January and early March.

Montgomery says this style of slide involves mostly rock; instead of rain and mud. Rattlesnake Ridge is made out of hard basalt, a common igneous rock seen in volcanic areas. The basalt has "inter beds" with relatively weak runs between hard basalt.

The slide seems to be breaking off from one of these inter beds, much different than the mud of Oso.

"The Oso slide mostly involved weak, loose glacial materials," Montgomery said.

Because of the basalt rock, it's hard to point to rainfall or earthquakes as potential contributors to the slide, Montgomery said. The water table is below the ridge, so it's likely not impacted by rainwater. And there haven't been any natural earthquakes big enough in the area to explain the massive shift in land, Montgomery says.

Still, the geomorphology professor stops short of saying a rock quarry right below the ridge is the cause of the slide. The de-buttressing of the hillside at the bottom of the slope is a potential cause, Montgomery said. But it's too early to tell.

"It's obviously too early to know for sure what role that might have played," Montgomery said.

About 70 people have been evacuated from the area as homes at the bottom of the slope remain in danger.  Earlier this month, officials came up with a plan to move big shipping containers filled with concrete barriers between the ridge and I-82, in an effort to keep rocks from falling on the roadway.

I-82 remains open, but detour routes have been set up just in case.

Despite some suggestions from Q13 News viewers, Montgomery said it's not likely they'll preempt the slide and use dynamite to spark it.

"I've been asked by a number of people why not use high explosives and trigger the slide," Montgomery said. "There are a couple of reasons."

Montgomery said the potential for "unintended consequences" is too high. Though the site is monitored with state-of-the-art equipment, it's hard to say exactly what the slide will do. Blowing it up could open too many unknowns, he said.

Running the risk of bringing high explosives in and changing the behavior of the slide is not advisable. Plus, with the slide being so slow moving, the "wait and see approach" is preferred.

"As tempting as it may be... what if that makes it worse," Montgomery said.