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Landslide watch: Can experts predict collapse at Washington's Rattlesnake Ridge?

Sandi Doughton, The Seattle Times on

Published in Science & Technology News

SEATTLE -- While Las Vegas bookies aren't laying odds -- yet -- on when the landslide at Rattlesnake Ridge will let loose, engineers and geologists tracking the treacherous slope near Yakima are placing their bets on mid-March to early April.

But what are the chances they'll be right? And is it even possible to predict when a 1,800-foot-tall ridge will come tumbling down?

There's a lot riding on the answers. The impending, 8 million-ton slide looms over Interstate 82, a rail corridor and the Yakima River. Residents of a cluster of homes at the base of the ridge have evacuated to motels, with no idea when they will be able to return.

Since cracks were spotted on the hillside near a quarry in October, the area has been swarming with state, local and tribal officials, as well as consultants hired by the pit operator.

Their landslide forecasts are cautious, hedged with warnings that the collapse could come earlier. But the approach being used has a solid track record, particularly in the mining industry. It's based on blanketing a hillside with instruments to measure the rate of slip, then extrapolating from the trend.

"This is a blunt instrument, but it's tested and it has worked on slides before," said University of Washington geomorphologist David Montgomery, who has been following the situation at Rattlesnake Ridge but isn't directly involved.

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One of the method's biggest successes came on April 10, 2013, when the operators of Kennecott's Bingham Canyon mine near Salt Lake City hustled their crews out of the pit in the morning and announced that a slide was imminent. Hours later, a towering wall collapsed in one of the biggest nonvolcanic landslides in North American history.

In Switzerland, years of monitoring in a steep valley paid off in 2012 with several days' warning before a series of catastrophic slope failures. In another mining incident, the failure date was accurately forecast three months in advance.

The method works because of the way many landslides unfold, said University of Utah engineering geologist Jeffrey Moore. As a slope cracks and begins to slide, the forces holding the mass of soil and rocks in place weaken and the underground surface the mass is sliding on -- usually a weak layer of soil -- gets slicker. As a result, the mass gains momentum and starts to slide even faster. Gravity eventually overcomes friction, and the slope collapses.

"There's enough case histories behind the method that when the data look right and you are able to identify trends, you can accurately predict the time of failure," Moore said.

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