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Explaining and exploiting a winter worry.

Explaining and exploiting a winter worry

Every spring, farmers in frigid climes reap an irksome harvest of rocks in their fields. Driven up by ice and water, once-buried boulders emerge during winter with enough force to crack roads and foundations, dismaying engineers and homeowners as well.

Though researchers have proposed many theories to explain this "frost heave," the mechanism has remained unclear. Now, a physicist has come up with seven brief equations that strongly support one explanation, suggesting ways to both limit the damage and harness the underlying force.

Every frozen substance has a thin coating of melted liquid, even when the surrounding temperature is below its melting point, notes J. Gregory Dash of the University of Washington in Seattle. His calculations, described in the Dec. 22 SCIENCE, indicate that when one part of an ice chunk is colder than another, the temperature difference sets up a suction, drawing the liquid toward the colder region.

If the ice lies within a wet, spongy environment such as soil, the suction pulls in outlying liquid as well. If the ice also lies under a rock, the drawn-in water takes up enough space that it forces the rock upward to make room, Dash calculates.

The new equations apply to any pure, frozen substance and thus appear to rule out the notion that frost heave results from water's unusual habit of expanding as it freezes, Dash says. He adds that geologists have observed water flowing toward ice in frozen ground and have found that the force pushing the rock up increases with the size of the temperature difference -- just as he calculates.

"This is a link between the condensed-matter physicists and the earth scientists," comments geologist Bernard Hallet of the University of Washington's Quarternary Research Center. While noting that Dash's calculations involve pure materials rather than the complicated mixtures of real soils and water, he says the theory makes specific predictions that experimenters can now test under various conditions in the laboratory and in the field. "It provides a new framework to view this phenomenon," Hallet says.

Since the calculations indicate that frost heave produces more thrust when it occurs in coarser substances, Hallet suggests using finer-grained concrete to build heave-resistant foundations. And Dash has an idea for exploiting the phenomenon in underground repositories of chemical and nuclear wastes. Buried refrigerator units, he says, could attract contaminated groundwater and prevent toxic wastes from leaching out.
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Title Annotation:rocks pushed up by cold weather
Author:McKenzie, A.
Publication:Science News
Date:Dec 23, 1989
Words:401
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