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'Soil Nails' stabilize slide on Springwater Trail

by: DAVID F. ASHTON - The soil nail gun awaits repairs, while a GSI heavy-equipment operator carves the hillside contour below the Springwater Trail in the vicinity of Ross Island Sand and Gravel.Heavy rain and high Willamette River levels began to tear away the hillside supporting the Springwater Corridor Trail, beginning in March of last year, and have been taking a toll on the paved pathway.

“It’s here, below the Ross Island Sand and Gravel plant, between mileposts 1.5 and 1.75, that we started to see the hillside fall away,” reported Portland Parks & Recreation Construction Manager Dale Cook.

On-site to look at the progress of the repairs by GeoStabilization International (GSI), THE BEE learned from Cook that workers had narrowed the path to one lane, and covered the soil erosion area with plastic tarps to slow down the degradation of the asphalt pathway.

It took more than a year for the work to get underway. “The project started on October 7,” Cook said. “During that time, we were working on design solutions, and finding a contractor that can do this work.

“Another part of the job that took some time was obtaining permits from the City of Portland,” commented Cook. “We had to pull quite a few permits to get this work done.”

Calling the trail “Portland’s I-5 for bicyclists”, Cook pointed out that as many as 3,000 cyclists use the trail every day. “We’ve made a big effort to keep the trail open during morning and afternoon ‘commuting’ hours, as well as on the weekends.”

This meant that GSI workers had only between 9 am and 4 pm on weekdays to haul in and out soil, and to “nail” the hillside together.

First, the contractor constructed a “bench” level that is above ordinary high water. GSI contractors then use specialized equipment to install a “Launched SuperMicropile™” system down into the bank to resist landslide movement and to support a concrete cap.

“Then, on top of that cap, they start to form a bio-wall,” Cook explained. “They stack layers of gravel and dirt, held back by geo-textile fabric, which is tied back into the original slope. We then install plants in those ‘soil bags’, giving a green face to that stabilization project.”

Finally, to anchor everything into the slope, GSI workers will use a system provided by a branch of their firm called “Soil Nail Launcher, Inc.”

The soil nails are shot horizontally into the bank, under the trail, using air pressure. “In one stroke, the 18-foot-long feet ‘nail’ (it looks like a steel fencepost) is shot into the soil. Then, under high pressure, they pump grout into end of the perforated nail until the cement oozes into the surrounding earth.”

A GSI supervisor reported to Cook: “On the test nails, we’re measuring that they’re holding 300% over design capacity. This means we can expect them to hold very well.”

While the project had a construction deadline of November 15, more rain – and a broken part in the soil nail launcher – briefly delayed the project further, for at least an additional week, Cook said.

But, with completion, cyclists will find they have a safe, smooth ride through the slide area – and those viewing the zone from the Willamette River will simply see hillside foliage.