Thursday, March 30, 2017

The danger from above SR 503

by Tamara Greenwell

State Route 503, also known as Lewis River Road, closed on Monday, March 13, when a debris slide knocked trees and rocks down onto the highway near Speelyai Bay Road, about 21 miles east of Woodland in Cowlitz County. While the debris on the roadway does not look like much, the real danger is from up above. An estimated 20 million-pound rock slab has come loose from the hillside, making it unsafe for anyone below.

A rock slab estimated to weigh 20 million pounds has come loose about SR 503 in Ariel.


Typically, we would physically inspect the slope when it stops moving, but this one is unique and continues to move. It’s too dangerous to assess the slope from below, so we turned to some other methods. By using aerial drone technology, LiDAR (Light Detection and Ranging) and 3D imaging over the past two weeks, we’ve collected data that indicates we can access the hillside from on top. LiDAR is a surveying method using light pulses, which provides 3D images of the earth.
Photos do not do it justice! We usually see debris slides containing rocks, mud and trees, but the rock slab that has dislodged from the hillside is about the size of a basketball court, approximately 100 feet wide, 60 feet high and we are not sure yet how far back it goes into the hillside. Current estimates are about 10-20 feet.

We are putting an emergency contract in place with Pacific Blasting to remove the slab and stabilize the hillside as soon as safely possible. Meanwhile our geotechnical engineers will bring a drilling machine to collect core samples, by drilling approximately 200 feet down into the hillside from the top. Core sample data will tell us how dense the rock is and where the fracture points are in the hillside. That information helps us develop a removal plan, which will likely include trim blasting, rock scaling slopes, installing rock bolts and anchors as well as tree and debris removal. All of this work takes time and depends on what we find as well as ongoing weather conditions.

While the debris on SR 503 may not look like much, it’s the instability of the slope above,
including a massive rock slab, that has kept the highway closed.


Next steps
It is still too soon to tell when the highway will reopen. We have to remove the large rock slab, assess what is behind it and then stabilize the slope. Due to the current danger of the large rock slab breaking loose, the highway will remain closed. We know road closures are frustrating, but we simply cannot put our workers or the traveling public in danger by opening the road before we complete the necessary work.

We’ll keep you updated on our progress while this work continues. You can sign up to receive email alerts with updates about the work to stabilize the hillside and news about when the highway will reopen.

2 comments:

Unknown said...

Tough situation for us residents, but I appreciate WSDOT for the update.

Dan Jordan said...

That's one way from keeping those damn tourists from going upriver.

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