Permafrost
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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]

 

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