PORTLAND, Ore. -- Heavy rains activated rifts in a slope that had collapsed before, causing the December landslide that shut down U.S. 30 in northwest Oregon, expert reviews have concluded.

The reviews found no evidence that clear-cuts by the Oregon State College of Forestry in 1992 and 2004 triggered the slides.

Rainfall in the area approached or set records, loosening deep sections of earth that had slid long ago, said Gunnar Schlieder, an engineering geologist with Geoscience Inc. in Eugene.

"We just had more rain than the slope could handle," he said.

Evidence of the ancient slides would have been visible only to a trained eye, he said.

SLIDESHOWS:

Slide buries Hwy 30

Aerial view of mudslide

The highway runs northwest from Portland along the route of the Columbia River.

Clear-cutting has been found to increase the risk of rapidly moving landslides, called debris flows, and the state enacted rules to limit logging on slide-prone slopes. The OSU land above the settlement Woodson was not steep enough for those rules.

A separate administrative review by the Oregon Department of Forestry says the state should have better recognized the history of landslides in the area.

"Clearly we didn't capture that -- our tools weren't strong enough," said Mike Cafferata, policy unit manager at the Department of Forestry.

More Video: Aerial view of slide

Simulation of how slide happened

The two studies noted the effect of an old railroad crossing that blocked a stream, multiplying the force and damage of the landslide. Water and debris from the OSU land plugged a culvert under the crossing, allowing water, mud and logs to back up in a small lake that finally broke loose and deluged the homes below.

Schlieder said the state should identify other embankments similar to the railroad crossing do more to keep people from living in areas in danger of severe landslides.

The state launched an initiative to reduce landslide safety risks after people were killed by slides in 1996, but some elements of the plan were never carried out. Maps of landslide hazard areas were never officially disseminated.

State officials have given the U.S. 30 corridor a high priority for laser-based mapping that can better identify landslide-prone areas, but the initiative needs funding, said Ted Lorenson, assistant state forester.

Schlieder said there is a misperception that landslides can be avoided by controlling logging and other activities on the slopes where they start, when sometimes it's better to keep homes out of the danger zone.

 Background: Slide took out 2 homes

In the case of Woodson, there is a long history of landslides barreling down the drainage where the December slide hit, he said.

"We have this problem where some of the houses at the bottom were rentals, and you probably have people there who were clueless as to what happened in the past," he said.