Permafrost
Although many people living in more southerly regions have never
heard of permafrost, it is not at all uncommon.
In fact, it has been estimated permafrost underlies one fifth of
the world's land surface. It is a potential problem for gardeners
and builders in most of Alaska, much of Canada, and even in some
parts of the northwestern continental United States.
Alaskan Permafrost
In Alaska, permafrost occurs as a continuous sheet north of the
Brooks Range, extending from a few inches below the surface down to
as deep as 1,000 feet. As one goes south, however, it gets
progressively thinner, the melted layer on top gets thicker, and
holes or gaps begin to appear in it.
Permafrost in Fairbanks
In Interior and Western Alaska, the permafrost is often less than
100 feet thick and the thawed layer on top may be as much as several
feet in depth. It may be completely absent in places, particularly
on the sun-warmed southern slopes of hills and along the inner sides
of riverbeds.
One homesteader in the Fairbanks area found permafrost beginning
35 feet down. His neighbor a few miles away had to drill his well
through 90 feet of permafrost. When the neighbor later sold his home
and rebuilt 500 feet away, he encountered no permafrost whatsoever!
Permafrost Farther South
Farther south, in Southcentral and Southeastern Alaska, permafrost
occurs only sporadically, in isolated and often widely separated
masses. In the Panhandle and along the Aleutian Chain, and on other
islands, it is usually completely absent, but may occur in any
region in which the average annual temperature is freezing or below.
Of course, average temperature means just that. Summer temperatures
may be quite high -- 90° to 100° F in Interior Alaska, for example
-- as long as the winters are long enough or cold enough to bring
the average down.
The key factor to look for in a known area of discontinuous or
sporadic permafrost is insulation of the ground surface -- something
that can keep the ground from thawing out completely during the
summer. Sphagnum or peat moss is the most efficient insulator,
usually helped out by heavy spruce woods or thick underbrush. Often
a good indicator of frozen ground is surface moisture. A boggy or
swampy surface is almost a sure sign that the ground is too frozen
to permit drainage.
Thawing Permafrost
The good news is that if the surface insulation can be removed or
cleared away, the permafrost can then melt down to a level which
will permit good natural drainage and which will cause no problems
to, for example, gardeners. Of course, this may take a couple of
years, but the result could be excellent garden soil. Much of the
Agricultural Experiment Station land in Fairbanks is underlain by
permafrost which by the early 1980's had melted down to 16 feet and
which has caused no trouble for many years.
On the other hand, it is sometimes better, on poorly drained
soils, to avoid thawing the permafrost at all, as when building
roads, as discussed in the following section.
Ground Ice
Of particular importance in the north is the ground ice often found
in permafrost. Of different origins, there are about five types of
ground ice, but what is important to know is that the ground can
hold more ice than it can unfrozen water. When the ice melts it
causes the ground to sink. This can result in deep pits and
hummocks.
About one quarter of the Fairbanks area is underlain with
permafrost with large ground ice masses, and some fields show pits
and heaves. This heaving is not seen as often on the actual flood
plain, since course-grained sediments without ground ice are there.
When Chena Hot Springs Road was rebuilt and straightened some
years ago, the contractor was unaccustomed to working with
permafrost. According to Dr. W. E. Romanovsky of the Geophysical
Institute, they dug down deep, but ended up laying fabric (used to
hold the road together) directly on ice. The fabric was covered with
sand and gravel and finally asphalt, but the damage had been done.
Since the ice had been exposed to the warmth of summer, the
permafrost began melting, and of course, the road began dipping in
places, while deep sinkholes formed in others.
Even though frozen ground may exist three feet below the surface
after clearing and extend two or three hundred feet down, except for
roadways, there will normally be few settling problems But
permafrost does cause other problems for gardeners -- cold soil and
poor drainage being the most critical.
[This article excerpted from Alaska
Gardening Guide, © 2000 by Ann D. Roberts, all rights reserved]
