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On Tuesday morning, Chuckanut Drive was shut down due to a twenty-foot rockslide, blocking both lanes of traffic. Road closures due to landslides occur regularly on the highway, with the latest closure happening only a few months ago in November 2024.  

The closure comes on the heels of the 11th anniversary of the Oso landslide in March, which claimed the lives of 43 people in 2014.  

We asked �������� Professor of Geology Bob Mitchell: What makes areas like Chuckanut Drive prone to landslides, and what are some signs or weather conditions we might look for when trying to avoid potential hazards? 

“Landslides occur in steep terrain in either unconsolidated materials such as soils and glacial deposits and/or in rock formations. For example, the Oso Landslide occurred in glacial deposits whereas the failures that occur along Chuckanut Drive happen in the Chuckanut Formation—a folded and broken up rock sequence of sandstone, siltstone and coal. Both types of landslides pose great risk to people and property, including roads and highways in Washington State because transportation corridors are sometimes engineered through these suspect terrains, e.g., Chuckanut Drive.  

The mechanisms that cause landslides are complex, but in general, rock failures along Chuckanut Drive occur when the force of gravity overwhelms the mechanical resistance offered by the friction between tilted rock surfaces, somewhat like a block sliding down an inclined plane.   

As you drive down Chuckanut Drive you will notice broken blocks of sandstone forming steep cliffs. Over time the friction forces holding the rock blocks in place can be made weaker by groundwater pressures, or by forces generated by freeze-thaw events, or by vibrational forces resulting from earthquakes. The friction resisting forces between blocks can weaken to the point where gravity wins. The pressures induced by groundwater usually play a role in weakening the mechanical properties, that is why most landslides occur in the rainy winter months in the Puget Sound region.” 

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Mikayla King (‘17) covers the College of Science and Engineering and Woodring College of Education for University Communications. Reach out to her with story ideas at kingm24@wwu.edu.