On Rattlesnake Ridge, data is pouring in from multiple types of measurements that including GPS readings, automated laser surveying stations, seismometers, ground-based 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 mid-March when the acceleration slowed this month.
"The geologic foot is coming off the gas," Montgomery said. "Even though it's moving faster than it was, the rate of acceleration is declining."
The target date could well shift again if the slope speeds up or slows down.
"Slides are very dynamic environments and you have to constantly reassess them," said James Struthers, chief engineering geologist for the Washington State Department of Transportation (WSDOT).
The forecast method isn't foolproof, cautioned Rex Baum, a landslide expert at the U.S. Geological Survey. Sometimes, slopes that seem to be on the verge of collapse stall out instead.
"That can be a frustrating situation to deal with," he said. Eventually, the slide will start moving again, but it could be weeks, months or even years.
And being able to predict when a slide is likely to occur isn't the same thing as being to able to predict how a hillside will collapse, Baum said. The latter depends largely on the detailed geologic structure of the hill, which can be difficult to discern from superficial measurements.
"You have all this uncertainty from things that are unknown, and unknowable," Baum said. "We just can't get the information from the subsurface."
While Kennecott was able to pinpoint the time of the Bingham Canyon slide, the immensity was a shock. Millions of dollars' worth of equipment that had been moved to an area deemed safe was pulverized.