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."
