A slump or a slide? Density decides

12:33pm, April 19, 2004
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In landslides, as in life, it's the little things that count. Using a full-scale simulator, researchers have shown that just a small difference in soil density determines whether a landslide becomes a fast-moving killer or merely one that slowly slumps downhill.

In each experiment, the scientists placed 12 tons of soil on a large concrete slab tilted at 31 degrees. That's a typical slope for hills at high risk of landslide, says Richard M. Iverson, a hydrologist with the U.S. Geological Survey in Vancouver, Wash., and lead author of the report in the Oct. 20 Science. The researchers controlled the amount of water in the soil with surface sprinklers and subsurface channels in the concrete.

In trials with soil that had been dumped and raked loosely into place, adding water eventually caused sudden slides that accelerated to speeds of more than 1 meter per second in less than a second. Instruments showed that as the material collapsed, water pressure increased in the pores between the grains in the soil. This reduced the friction between the grains and triggered the landslide, Iverson explains.

When researchers added water to soil that had been packed so that it was about 80 percent as porous as in the earlier test, the result was a slump of only 8 inches that took a minute and a half. Instruments showed that in the denser soil, the grains had to move apart to slide past each other, increasing pore size. Water pressure decreased within the soil pores, increasing friction and arresting the slide, Iverson says.

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