According to the calendar, winter ends this month, which is good for Multnomah County Road Engineer Greg Kirby because winter is landslide season. Like unexpected guests, they show up suddenly and demand attention and the wet winter of 2010-11 has been no exception.
“We’ve had two landslides on our roads this winter, which is about average,” Kirby said. Both were in the West Hills, an area that is prone to slides because of its steep slopes and weak soils. Add more rain and snowmelt than the ground can absorb and the result is often a landslide.
“Water is the main agent that impacts the stability of our slopes,” Kirby said. Rain runoff and the freezing and thawing cycles of winter can disturb soils that are normally stable.
“One of our slides this winter is on Newberry Road,” Kirby said. “The rock there is very fractured and held together by dirt. Once the water and freezing start to loosen the rock face, the cracks migrate and debris begins to fall.” One lane of Newberry Road has been kept open around the slide while staff plans repairs. A retaining wall is being evaluated as a fix.
County engineers work with geologists and geotechnical engineers to identify affordable repair strategies that will work. “We need to assess the soil, its strength and capabilities,” said Kirby. “Especially if we want to add material. You don’t want to add weight to an area that could be unstable.”
A slow moving landslide has closed NW Thompson Road this winter. A growing crack in the pavement signaled that the ground under the road was moving downhill. The repair could involve soil nails, long rods drilled into the slope below the road to reinforce and stabilize it. Soil nails avoid the need to dig out and rebuild the road, which can create erosion risks during the rainy season.
After 25 years (and winters) with Multnomah County Roads, Kirby has seen some odd landslides. “Gopher holes can create water channels that contribute to a landslide,” Kirby said. Rodents contributed to a 2006 landslide that closed Hewett Road for several months. It was one of the most destructive acts by a gopher since the movie “Caddyshack.”
The County Roads budget includes limited contingency funds to deal with unexpected repairs for things like landslides. In bad winters like 1996 the contingency funds run out, so repairs take years to complete. “The potential for a landslide is there all the time,” said Kirby.
To learn more about landslides in Oregon, visit www.oregongeology.com.