Something light on the menu

Aug. 19, 2002

You can't always count on a deep freeze in Alaska.

All of the roadwork in the state is done between the months of May and September, with most of the crews working 12-hour shifts. But even in the summer the Alaska DOT struggles with the soil. When the temperature hovers around 32ºF, embankments, which are a necessity, fall victim to the thawing permafrost.

You can't always count on a deep freeze in Alaska.

All of the roadwork in the state is done between the months of May and September, with most of the crews working 12-hour shifts. But even in the summer the Alaska DOT struggles with the soil. When the temperature hovers around 32ºF, embankments, which are a necessity, fall victim to the thawing permafrost.

"If you want to build a new stretch of road (during the summer) the ground is barely frozen, and that?s very problematic," Steve Saboundjian, research implementation engineer for ADOT, told Roads & Bridges. "If it was really, really frozen it would be better."?

Permafrost is soil that survives below freezing for more than two years. Throwing down a bunch of heavy aggregate might kill it, but the remains will inflict the most damage--creep. Ice gradually deforms under the weight, resulting in settlement and cracking of the embankment. Researchers, however, believe there?s an easier way to prevent ice creep. Light-weight aggregate could be the solution Alaska is banking on.

"Whenever you place an embankment on permafrost its starts melting. When it thaws basically that embankment starts sinking into the ground. The only way to prevent that is to make that embankment lighter," said Saboundjian.

All roads in Alaska must be at least 3 to 4 ft above natural ground, which is where the value of embankments comes into play. Traditionally, ADOT has used self-refrigerating pipe units, a technique called thermo siphoning, to produce the chill needed underground. Another more passive method is using very coarse aggregate in the embankment.

"In winter the cold air penetrates that embankment, circulates and keeps the subgrade soil frozen," said Saboundjian.

The word on lightweight aggregate is that it has an "insulating" effect created by pores of air voids within the structure of the stone. Because it is porous water will drain instead of stand. And, as the name suggests, the lighter version is lighter than conventional fill so there is less stress applied to the permafrost.

There is one wallet-hitting disadvantage. Currently, the product is made in the continental U.S., meaning it would have to be shipped by truck to Seattle and by barge to Alaska. "The transportation costs are enormous," said Saboundjian. So ADOT is looking to produce its own brand of lightweight aggregate. Argillic shale and mudstone are readily available in the region and could serve as key ingredients. Using a portable plant, the shale or mudstone is fired in excess of 2,000ºC and crushed.

ADOT is looking at the feasibility of the project before moving on. A full-blown study, including experimenting with the aggregate on a test project, was initially suggested, but officials opted for a more conservative approach.

One unexpected drawback has been discovered during the research phase. ADOT first thought it could be used in asphalt surface treatment, where you "spray a layer of asphalt and place aggregate on top."

"But we found out it breaks," said Sabounjian. "So you need to place it far from the surface--at least 2 or 3 ft--so it's not subjected to high stresses and pressures."

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