Can experts predict 8M-ton collapse?
But what are the chances they’ll be right? And is it even possible to predict when a 1,800-foot 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.
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.
On Rattlesnake Ridge, data is pouring in from multiple types of measurements that including GPS readings, automated laser surveying stations, seismometers, groundbased radar and a laser scanning technique called LIDAR. More than 100 instruments are sitting on, or aimed at, the hillside.
When monitoring started, the 20-acre slide mass was moving an average of a foot a week. It has since sped up to about 1.6 feet per week, but the acceleration hasn’t been constant. The rate of movement in December pointed to a major slide by mid-January. The date was pushed back to midMarch when the acceleration slowed this month.
The target date could well shift again if the slope speeds up or slows down.
SEATTLE»Engineers and geologists tracking the treacherous Rattlesnake Ridge slope near Yakima, Wash., are predicting a landslide in mid-March to early April.