Slope stabilization – using proactive strategies in a constant battle of nature

Our hearts are with those affected by the SR 530 Slide, and thoughts with those who continue to toil through the debris.

Photo of debris field on SR 530 near Oso.
The extreme reach of this landslide was catastrophic, and it’s not an event that WSDOT would have expected to impact the highway, given that the slope is on the opposite side of the valley.

WSDOT tracks known unstable slopes adjacent to state highways and Interstates. With the recent slide near Oso being more than a mile away and across the North Fork of the Stillaguamish, it’s not one that was included in our slope database. Prior to 1995, unstable slopes were stabilized reactively after they had failed. Since then, we’ve been proactively tracking problem slopes next to our roadways – slopes that have the potential to threaten the highway and its users.  Typical slope hazards that we manage include landslides, debris flows, rockfall, settlement, and erosion. Currently, we’re keeping tabs on about 3,300 locations which can be found on the map below and in this list (pdf 648 KB).
Map image of unstable slopes by rating (pdf 3.6 MB)

Each identified location receives a score ranging from 33 (lowest) to 891 (highest), which is based on the potential risk factors to the usability of the highway if a slope failed. This is helpful so we can strategically determine which slopes should be addressed with the limited funding available. With this management system, we’ve worked to stabilize about 244 slopes adjacent to state highways and Interstates since 1995 at a cost of $180 million, using both federal and state funds.

One example is I-90 over Snoqualmie Pass.

Photo of nail wall being installed along I-90 at Snoqualmie Pass.

As part of the five mile project to widen I-90 from four to six lanes, crews are working to significantly reduce the rockfall hazards that are present by constructing more stable cut slopes, heavily reinforcing these cuts with rock anchors, and providing more catchment area for the natural, ongoing weathering of these new slopes.

Another is a stretch of SR 20 near Rockport that is constantly buffeted by the Skagit River, especially during flooding. When the river runs high, it eats at the riverbank and, at times, has threatened to even wash out the road. Right now we have a project in construction that is building logjams with something called dolos to protect the riverbank and our highway from continued erosion.

Photo of Skagit River - Permanent restoration using engineered logjams
combined with concrete dolos.

The geology of our state is complex and dynamic, and conditions are subject to change. With potential problem areas identified, our maintenance staff who drive these areas every day are on the lookout for developing problems, such as new rocks in a ditch or dirty water that wasn’t there the day before. This points our geologist to a spot that may be changing and is worth a closer look. If it’s determined that a highway is unsafe for travel, we’ll close it until it can be made safe.