Soil may signal imminent landslide.

Soil may signal imminent landslide

Loosened by rain or melting snow, ordinary soil on a steep hillside can suddenly turn into a lethal wave sweeping downward at speeds of more than 30 miles per hour. Now a team of geologists may have discovered a way of determining when hillsides are about to give way.

According to one theory about soil slides--a particularly form of landslide -- precipitation can free dirt and rocks by increasing the water pressure inside pores within the soil. As the water table rises and pore pressure climbs, friction holding the top layer of soil to the hillside begins to drop until gravity's pull overcomes it. While such a general theory would explain why the flow starts, geologists have not paid much attention to the details of the process, says Edwin L. Harp of the U.S. Geological Survey in Menlo Park, Calif.

By outfitting a hillside with pressure meters and then setting artificial landslides, Harp and his colleagues discovered that pore pressure did increase as expected when the water level rose. However, immediately before the soil started sliding, pore pressure took a nosedive. On a test slope in Utah, the pressure dropped 40 minutes before the slide. In an experimental California forest, the warning was shorter. The researchers believe the pressure drop signaled that soil was beginning to expand just before it started to slide.

Harp says some researchers have found this pressure drop in natural soil slides as well, but it will require much more work to determine whether the signal is common in many kinds of landslides. If so, it may be possible to set instruments into known slide areas -- providing a short-term warning for an impending flow.