Geologists working to unravel one of the most destructive landslides in state history still haven't found a smoking gun — but they're beginning to uncover some pretty hot clues.

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Geologists working to unravel one of the most destructive landslides in state history still haven’t found a smoking gun — but they’re beginning to dig up some pretty hot clues.

Chief among them is that the seemingly solid Yakima County hillside that collapsed onto Highway 410 near Naches was underlain by layers of rock as soft as putty and packed with so much water that crews unleashed geysers when they drilled into the debris.

Those soft, saturated layers, 200 to 250 feet below the surface, are the main suspects behind the spectacular collapse the morning of Oct. 11. But there’s still a mountain of data to sort through to reconstruct what happened — and figure out where to rebuild the highway, said Steve Lowell, chief of the Washington Department of Transportation’s (WSDOT) Geotechnical Division.

The question of whether the slide was triggered by gravel mining at the base of the slope is being tackled by Washington’s Department of Natural Resources (DNR), which regulates mine reclamation. Drawing from the same mountain of data, DNR geologists plan to re-create the mine’s operations with a computer model, to determine if the excavation could have undermined the slope.

“It’s like being a detective,” Lowell said. “We’ve got all these pieces of the puzzle, and now we’re trying to put them together.”

The detective analogy is apt, because landslide investigations are a type of forensic geology, said University of Washington landslide expert David Montgomery. But instead of analyzing dirt from a murder victim’s shoes, landslide investigators seek to understand why a slope let loose and how to reduce the risk of a repeat.

Highway buried

The arid cliffs along Highway 410 northwest of Yakima were sculpted by giant landslides 1 million or more years ago. But no one was prepared for the scene that unfolded on that clear Sunday last fall.

Forty million cubic yards of rock and dirt — enough to fill Safeco Field more than 10 times — came roaring down over the course of several hours. The slide buried nearly one-half mile of highway and shoved the Naches River out of its banks.

No one was hurt, but several homes were destroyed or damaged beyond repair.

The disaster’s price tag includes more than $13 million to reroute the river channel and build a detour road across former pastureland.

But the new roadway isn’t designed to handle the traffic that streams between the west and east sides of the state once snowy Chinook Pass opens in the spring, said Brian White, WSDOT assistant regional administrator for development. Nor do local residents want all those cars whizzing past their homes.

So WSDOT intends to rebuild the stretch of Highway 410. The question: Where?

The answer depends on the results of the landslide investigation.

If the landslide is stable or could be shored up with barriers, crews may be able to build around or over it, at an estimated cost of $15 million to $25 million. But if it’s likely to keep sliding, WSDOT may have to move the road to the other side of the Naches River — an option that could cost up to $50 million.

Cores reveal structure

Almost as soon as the dust settled from the landslide, WSDOT crews were drilling cores in the field.

“We’re trying to figure out what it looks like below the surface, and where it failed,” Lowell said.

Near the top of the slide, where it was impossible to punch in roads, drill rigs were lowered by helicopter. Crews extracted nearly a mile of core from 23 boreholes, the deepest extending 500 feet below the surface.

The cores provide narrow windows into the earth, revealing a structure reminiscent of a layer cake: Thick slabs of basalt, interspersed with thinner layers of sedimentary rock. Eons of tectonic shove has tilted the layers so they dip directly toward the road. While the basalt “cake” is solid, some of the “icing” layers are soft and weak.

WSDOT engineering geologist Tom Badger opened a box of core at WSDOT’s Materials Laboratory in Tumwater and pressed his fingers into a gooey section. “We have zones that feel like that at 230 feet below the ground surface,” he said. “That certainly attracts our attention.”

Water trapped in the soft layers weakens the structure even more.

“You’ve got a situation where the slope is predisposed to fail,” Badger said.

But simply identifying weak layers doesn’t prove that’s where the ground gave way.

To zero in on the failure zone, geologists run instruments called inclinometers up and down the bore holes. When the ground moves, it deforms the pipe that lines the holes — and the instruments can pinpoint and measure those shifts to about a tenth of an inch.

Crews continue to take measurements about every two weeks. But the ground so far hasn’t shifted enough to nail down the failure zone.

3-D models

Rain-triggered landslides are common in soggy Western Washington, and relatively simple to figure out, Lowell said. “There’s a lot more to this one.”

It’s even possible the ground failed in multiple zones.

To help build a three-dimensional picture, scientists at Washington State University identified each basalt layer through chemical analysis. Workers at WSDOT’s lab are analyzing the strength of each layer of earth.

That information will be fed into a computer model that allows geologists to essentially rewind time: to simulate conditions before the landslide, then see what could have caused the slope to come crashing down.

The models also look forward, predicting what conditions could cause the landslide to shift and whether it can be stabilized with retaining walls or other structures.

“If it lurches again and blocks the Naches River [again], that’s going to be pretty bad for everyone,” DNR geologist Isabelle Sarikhan said.

Did quarry play role?

In her quest to tease out the gravel pit’s role, Sarikhan is hampered by the lack of cores from near the quarry. It’s possible to fill in those blanks by extrapolating from other bore holes, but the resulting analysis will be more uncertain, she said.

Aerial photos show the landslide started slipping as early as 1998, Sarikhan said. DNR inspectors noticed large cracks above the Simmons & Son quarry, and warned the operators in 2005 that they might be destabilizing the slope.

After the slide, which swallowed up the quarry’s rock crusher but stopped just short of a house on the site, DNR ordered a halt to all mining until the owners conduct a stability analysis, which would include core drilling.

The mine owners appealed the order.

On a frigid day last month, co-owner Robin Simmons watched an excavator claw at the crumbly slope, scraping out an access road for the stability studies. “I think it’s going to work out,” she said. “We just have to determine it’s safe.”

If the hillside failed 200 feet or more underground, the quarry couldn’t be responsible, Simmons argues.

But Montgomery, the UW professor, said removing material that buttresses an unstable slope can contribute to its failure, regardless of depth.

However, even a field called forensic geology can’t prove cause-and-effect beyond a shadow of a doubt, he said. “Whether the mine caused the slide is always going to be a matter of professional judgment.”

Sandi Doughton: 206-464-2491 or