OSO — A new study of the Oso mudslide released Friday sheds fresh light on the hillside’s collapse last year. The study raises more questions than it answers, however, and researchers are still far from being able to point to a definitive cause.
The hillside, which geologists knew by the name Hazel, collapsed March 22, 2014. The mudslide killed 43 people and destroyed the Steelhead Haven neighborhood nestled below the hill.
The slide followed weeks of intense rainfall, and scientific research done to date points to extreme saturation of the soil as the most likely reason why a historic landslide zone collapsed in such a deadly fashion.
Understanding how the hillside got that way is the subject of continued investigation, including the most recent report from the state Department of Transportation.
Lawsuits filed in King County Superior Court by those harmed by the slide seek to prove that the state was negligent in allowing logging on Whitman Bench behind the slide. The plaintiffs contend the logging increased the amount of rainwater in the soil and contributed to the slide’s deadliness.
However, the new report provides evidence that heavy rain did not appear to significantly affect aquifers detected deep underground.
The report concludes: “While the initial findings of this study contribute to improved understanding of the subsurface conditions within and around a few of the large volume, high mobility landslides present on both sides of the valley, more questions of importance have arisen and uncertainties with the available information remain.”
The field work began in October by WSDOT engineers working in concert with the state Department of Natural Resources, Snohomish County, the U.S. Geological Survey and the Tulalip and Stillaguamish tribes. Crews drilled two bore holes immediately behind the headscarp of the slide on the Whitman Bench and a third one on a historic slide across the valley near Skaglund Hill.
They installed underground monitors that measured subsurface water during the rainy season between December 2014 and May 2015.
The drilling revealed that the deadly March slide contained two distinct masses of approximately the same size: a weaker deposit that covered the hillside and had been the source of historic landslide activity, and a stronger 650-foot-thick underlying layer of earth that had been undisturbed since it was compressed beneath ancient glaciers.
The slide carved out a chunk of the hillside through both those masses. Understanding the differences is important to determine how groundwater has contributed to slides, the study says.
Monitoring done for this study found that deep aquifers did not appear to be affected significantly by winter rainfall.
On the south side of the valley under the Skaglund Hill slide, however, monitoring found shallow aquifers affected by rainfall.
The report says that while it was likely such shallow aquifers existed in the landslide deposits of the historic Hazel slide, it is impossible to know for sure because those layers collapsed in the March 22 slide.
Another finding of interest was a pressurized aquifer geologists found when they drilled 625 feet below the Whitman Bench. The report says water shot 14 feet into the air for five minutes before it tapered off.
When the second hole was drilled 50 feet away it did not encounter those same conditions, so the significance of that deep aquifer, and whether it played a role in the landslide, remains a mystery, the report says.
The report concludes that more study, including drilling and core sampling, is still needed to get a better picture of subsurface water conditions.
The King County judge presiding over the mudslide lawsuits plans next week to consider whether to delay the trial, now scheduled for October, so more drilling and other tests can be conducted.
State attorneys general say that work could push the trial back to June 2016. Experts they’ve retained point out that much of what is believed about the ways groundwater moved through the hill before the slide is based on assumptions, not actual data.
The state says data is necessary to respond to the plaintiffs’ claims that forest practices on the hillside, particularly a 7.5-acre harvest on the Whitman Bench a decade ago, set the stage for the disaster by increasing the amount of water that could seep into the ground.
In a May 18 order, King County Superior Court Judge Roger Rogoff wrote that “pursuit of this proposed testing is worthwhile and reasonable” but he wasn’t ready to reschedule the trial. He urged state officials to pursue the work and promised to take that into account when he’s ready to rule on the trial’s timing.
Lawyers for the plaintiffs on Tuesday asked the judge to order the state to share details about the protocols it will follow during testing, to allow them to monitor the work, and to require that any data be shared as it becomes available.
“If truth is what the State is after, there should be no problem with producing data simultaneously so that Plaintiffs have it ‘in sufficient time for all parties to use at trial,’” attorneys John Phillips and Corrie Yackulic wrote.
Chris Winters: 425-374-4165; cwinters@heraldnet.com.
Twitter: @Chris_At_Herald.
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