New Roads Falling Prey To Old Problem -- Landslide-Damage Repairs Costing Billions Each Year

PORTLAND - Development and mountain highways are increasing the landslide danger during the rainy season in Oregon, costing about $30 million in repairs in the past four years.

"Landslides are part of the natural history of Oregon," said Scott F. Burns, associate professor of geology at Portland State University.

"Many of the major lakes in Oregon are the result of landslides damming up streams," Burns said. "Triangle Lake in Lane County and Loon Lake near Scottsburg in Douglas County are examples of that."

Because highways pass through steep terrain, rockfalls are a major problem for the state Transportation Department. The agency has identified more than 3,000 potential rockfall sites along Oregon highways.

Slides in coastal area

The Oregon Coast is a major problem area.

On the south coast, work continues on parts of U.S. 101 damaged by two slides totaling 55 million cubic yards of earth. Sections of the scenic highway were ripped away by a slide south of Gold Beach earlier this year and another slide north of Gold Beach in 1993.

On the north coast, Arch Cape Tunnel on U.S. 101 reopened last week after being closed for three weeks because of damage caused by shifting rock. Nearby, repair work will begin soon to fix the continuing problems with rockfalls on the highway from Neahkahnie Mountain.

Resulting injuries

Injuries or death are uncommon, but a man was killed by falling rock in the magnitude 5.9 and 6.0 earthquakes in Klamath Falls in 1993.

A woman was killed in Coos County by a falling boulder in 1989, and two children were killed by falling rocks on Interstate 84 near Cascade Locks in 1984.

In perhaps the worst Oregon disaster involving a landslide, nine men were killed by a rock and mud slide near Canyonville in 1974 while working on a broken telephone cable.

Nationally, slope failures and rockfalls take about 25 lives and cause $1.5 billion in property damage each year.

"Landslides are everywhere," said William M. Brown of the U.S. Geological Survey's geologic-hazards team in Denver.

"They occur on a very regular, almost daily basis somewhere in the world," Brown said. "But they don't get much attention compared to earthquakes and volcanoes - they're more like the auto accidents of the disaster world as opposed to the airline crashes."

The Columbia River Gorge is notorious for landslides and rockfalls.

Last month, a 400-ton boulder plunged from the cliff face behind Multnomah Falls, injuring 22 people.

Of Northwest fame

Historically, the most famous Northwest landslide was the Bonneville landslide near Cascade Locks, which dammed the Columbia River about 800 years ago.

Covering 14 square miles and forming a temporary land bridge, the landslide is believed to be the origin of the Indian legend about the Bridge of the Gods.

Brown said a common occurrence in the United States is the reactivation of ancient landslides by people.

"Some of these landslides have been sitting around for centuries," he said. "But these get disrupted by various human activities - mining, logging, road building and urbanization."

Still, rainstorms are the primary triggers, said Brown, who developed the National Landslide Information Center in 1990.

"When water accumulates and reaches a certain level in a slope, the water pressure begins to push the soil particles apart and the slope loses strength. And when it does, it collapses."

Susanne Lee D'Agnese, a geologist with the Oregon Department of Transportation in Roseburg, said the mechanics of a rockfall or landslide involve water, gravity and the strength of the soil or rock.

"A lot of times you hear people say that water lubricates or adds weight, but that's not really the effect at work," D'Agnese said. "Water actually works as a buoyant force. It makes the grain particles lighter and there's not as much frictional force between them."

D'Agnese witnessed those effects at the Wilson River landslide April 4, 1991, when a half-million cubic yards of water-soaked mountainside slid onto Oregon 6 about 31 miles east of Tillamook.

She and Transportation Department geologists Amy Pfieffer and Bob VanVickle were standing on the road when slide started.

"There had been a tremendous rainfall - there was 9 inches of rainfall within a 12-hour period just prior to the failure," said D'Agnese, who had found the potential slide three weeks earlier.

"We predicted that it was going to fail, but we just didn't know when," she said. "It was exciting watching that fail - it was a bit too exciting, because we had to run."