In March 2014, the area around Oso, Washington had been experiencing soaking spring rains, setting up high groundwater pressures on the historically unstable plateau. It was under sunny midmorning skies on Saturday, March 22, that residents of the community of Steelhead suddenly felt the ground shake and soon saw a wall of mud and debris emerge from the forest. By the time the mud came to a halt, the neighborhood and a nearby highway were buried by up to 20 feet of earth.
A lawsuit against timber company Grandy Lake Forest Associates LLC and the State of Washington alleged that an 8-acre timber harvest on the Whitten Bench above the landslide had led to the deadly water pressures. Grandy Lake retained Exponent to conduct a geotechnical analysis, geologic engineering assessment, and groundwater evaluation of the Oso landslide.
Exponent's Multidisciplinary Solution
Assemble the Exponent team
We created a collaboration between licensed geohydrologists, professional engineers, civil engineers, and ecologists with extensive experience in landslide hazard investigations, geotechnical design failures, and complex mediations and arbitrations.
Study the geological record
Oso's location near the Strait of Juan de Fuca leaves it vulnerable to extreme weather conditions, including extensive rainfall and seasonal flooding. The region's topography was shaped by glaciers, which withdrew approximately 10,000 years ago, leaving behind layers of sediment cake. Analysis of these layers reveal that a series of similar slides occurred up and down the North Fork Stillaguamish River Valley since before the recorded history of the area. Since 1908 at least ten significant historical earth movements have been documented in the source area of the Oso Landslide.
Understand previous landslides
Within two months of the Oso landslide, our team was onsite reviewing reports compiled by experts after previous landslides. Reports focused on subsurface water and precipitation as significant contributors to landslide activity. Our team spent considerable time hiking, collecting samples, and analyzing data to understand the area's historic glacial movement, seismic activity, precipitation, vegetation, and weather patterns.
Reconstruct the landslide
Our experts set about to understand and model the mechanics of the landslide and determine what conditions contributed to its severity.
Analyze and install new borings
Our team joined experts from the State of Washington to develop a groundwater and soil borings program. We began by analyzing previously installed borings and installed eight new borings on the Whittman Bench and twelve borings on the slide. We also installed several wells.
Evaluate groundwater flow
Based on over a year's worth of data, LiDAR Imaging and the GEERs report, we identified erosional gully features on the eastern side and confirmed that most waters from the Whittman Bench flowed away from the Oso landslide area.
Conduct a detailed precipitation analysis
Arlington and Darrington Weather Stationsrevealed the highest 4-year cumulative precipitation with a reported 20 inches compared to a monthly March average of only six inches.
We evaluated rain gauges installed on clear-cut forest stands of nine, 27, and 80 years. However, we were able to demonstrate that the small 8-acre tract of harvestable land did not significantly reduce the vegetation area, nor had the tract, harvested since 1997, led to any prior landfalls.
Conduct LiDAR mapping to reveal subsurface geology
LiDAR data, combined with the subsurface investigations performed by Exponent, provided a good insight into the subsurface geology and groundwater conditions.
The LiDAR images revealed gullying near Headache Creek and much rougher ground than the south side of the landslide area, indicating regular seepage and erosion toward the north of the plateau and not to the south - the area of the landslide.
Create a model to demonstrate the impact of groundwater
We determined that the presence of confined aquifers deep within the layer-cake geology, in combination with toe erosion, caused the 2006 Hazel landslide to mobilize and not seepage from the Whittman Bench.
Study the seismic evidence
The landslide registered on nearby seismic equipment as two distinct, sequential events, confirming our geological data and verbal testimony from those in the area indicating there were two distinct stages of failure.
Model the water flow
We created a debris flow model showing that although significant precipitation occurred before the landslide, most of the rain to Whittman Bench flowed north, away from the area which would become the Oso landslide.
Although materials collected at the head of the scarp did contain some moisture, the body of the scarp was highly saturated, indicating that the slide region was impacted by precipitation falling and seeping directly into the scarp.
Model the debris flows
In the debris flow models, we demonstrated that confined aquifers created significant water pressure in the subsurface, triggering debris flow movement leading to a loss of support for the rest of the slope. The same water conditions led to the more significant earth-slide event that followed the debris flow in 2014, showing the seepage from Whittman Bench was not a contributing factor.
Our team concluded that the 8-acre tract of harvested timber from the Whittman Bench did not lead to increased saturation of the slide area. Through detailed modeling we demonstrated how a combination of natural factors led to the landslide, including increased underground water pressure caused by the swelling of underground reservoirs, direct rainfall to the hillside scarp, and erosion at the base of the hill.
Increased Landslide Awareness
The Oso landslide confirmed the need for continual monitoring with advanced technologies. In 2017, Washington state launched the Landslide Hazard and Inventory program to collect critical data on landslide events and vulnerable areas.
Continual investment in and use of advanced technologies
The use of advanced technologies, including global positioning systems, aerial photo analysis, LiDAR technology, and interferometric synthetic aperture radar technology, significantly aided our investigation, enabling detailed tracking and characterization of ground and structural movement with exceptional precision and accuracy.