Thursday, December 26, 2019

How we work to prevent and respond to unstable slopes and landslides

By WSDOT staff

A lot of people think of pavement when they hear our name, but we're often just as focused on the land around – and above and below – our roadways. Given Washington's abundant rain and topography there are hundreds of slopes statewide that affect our roadways and travelers – and that's where our local and specialized crews come in.

We work hard to prevent slides when we can and to quickly clear roadways if debris does close a highway. We also need the public's help whenever they encounter a slide area or road closure.

Do not get out and attempt to clear a slide, leave the area and call 911 to notify authorities. Please, never drive around a closure sign – they're there for your protection and we need everyone's help to ensure the road can be cleared as quickly as possible.
A massive slide in January 2016 blocked US 2 in Pine Canyon near Waterville and
took more than two weeks to fully clear the roadway.

Before a slide

We're often able to start monitoring and planning a response to an area in danger of sliding before large amounts of a slope slide. Often one of our crews, residents or one of our partner agencies notice something amiss above or below a roadway.

That's what happened in early October 2017 when a slow-moving landslide east of Yakima on Rattlesnake Ridge threatened residents and travelers on Interstate 82. Several agencies worked together to research and monitor the risk while also planning for a worst-case catastrophic slide – including placing barriers near the roadway and installing monitoring equipment to track and better understand the land movement.
During the initial Rattlesnake Ridge slide monitoring, large freight containers were placed along
Thorp Road block any rockfall debris from reaching I-82.

The Rattlesnake Ridge movement has slowed since it was initially discovered and, based on on-going monitoring data, it now appears it will continue to move slowly to the south and fall into a quarry pit until it stabilizes.

Slide response and reopening roads

Mother Nature is often the ultimate determining factor of when and where a slope gives way. When that happens, our crews work to clear the road as soon as it's safe to do so – which sometimes requires assessment by our specialized Geo Tech crews to determine if the slope has stopped moving.

In July on US 97, the forecast had crews out on the roadway looking for trouble areas, making them first on the scene when mudflow topped the roadway and spilled on to adjacent train tracks. The slide was still moving as night drew near, so we closed the road overnight between Wenatchee and Chelan rather than put crews in a dangerous situation. We know these closures are disruptive, but our first priority is the safety of our crews and travelers.

More than mud, our maintenance crews chase rock, particularly in cold weather. When there's been a stretch of below-freezing temperatures and a Chinook blows in (unseasonably warm weather up to 45 degrees), that sets the stage for rocks to fall. Sometimes it's just little spatters of gravel and pebbles, sometimes rocks as large as basketballs.

Sometimes the situation calls for an emergency contract to scale the slope – remove loose or potentially unstable material – before a road can reopen. A few years ago, a large rockfall in Pine Canyon closed US 2 for several weeks in late winter, leading to a large scaling project in which crews removed loose rock and debris to prevent further slides.

Different conditions closed SR 20 near Loup Loup this spring, where thousands of yards of material fell on the downslope of the roadway, undermining the pavement nearly to the centerline. Cleaning that up and rebuilding the roadway required constructing an access road to the bottom of the drainage and lasted more than a month.
Crews begin clearing rocks that fell on the roadway along US 97 near Knapps Tunnel in the Chelan area.

Many of the routes where slides are common are rural highways through mountain passes, canyons and along rivers with few options for detour – which makes closures particularly disruptive. We know access is vital and an expedited reopening of the highway is always the goal, but safety remains our top priority when determining when and how to reopen the road to the public.

Long-term slide repair

Slides, particularly repeat events, often require work beyond cleanup to proactively stabilize the slope against future landslides. Because each slope is unique, there is no one-size-fits-all approach and often a number of factors come into play when prioritizing work in our slope stabilization program. We create a plan for work each year, but severe weather also plays a factor or alters our schedules.

In winter 2015, for example, after several days of heavy rain, the slope above northbound I-5 near Woodland gave way, closing the roadway with dirt, debris and rocks across all three lanes. The road was closed for two days while it was cleared and one lane remained closed for two months while crews worked around the clock to stabilize the slope with a series of rock anchors.
After a 2015 slide along I-5 near Woodland, crews drilled holes and placed and grouted steel rods
50 feet into the hillside to stabilize the slope.

In late December 2017, heavy rain caused the slope above SR 4 near Stella to give way, closing all lanes. In that case, crews determined there was an ongoing threat of additional slides and placed shipping containers to block debris from getting onto the roadway short term until a more permanent fix could be made. Then, in the summer, the slopes were excavated and stabilized and a damaged portion of debris flow fence was repaired.

While the repairs and timing may be different – and timing may not always be as quick as we'd hope – the end goal of all slope stabilization work is the same. We're working to keep the roads clear and people and goods moving.

1 comment:

Unknown said...

Ty for all your hard labor to help us on the roads as we travel

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