Permafrost and Frost Heaves

Traveling around in the north, we noticed the effects of ice on the way things are constructed. The Dempster Highway is often as much as 20 feet above the surrounding land. After some reading (Wikipedia) on the construction of the highway, I found that there were concerns that the road would heat and therefore melt the permafrost. 

Don’t drive off the edge of that!

Is simple terms, permafrost is ground that is saturated with water and stays frozen for at least two years. Permafrost can also stay frozen for centuries. It can be a few feet thick or thousands!

The ground above the permafrost gets warmed by the sun or constructed items so that the upper layer goes through freeze-thaw cycles. This upper layer is the "active layer." This layer will support plants that do not require a roots deeper than the active layer depth. This is generally 1 - 8 feet but varies by elevation, latitude, soil conditions, hours of sunlight received, etc.

Grasses, small shrubs, and trees where soil conditions allow

This limits the northern reach of trees or severely limits their size. Things like grasses and willows, which have shallow roots, grow all the way to the Arctic Ocean. Trees will grow in the narrow band below tree line but out of the swampy active layer which sits over permafrost.

The active layer acts as insulation keeping the permafrost frozen. If the active layer is disturbed by things like top soil removal for construction, fire, erosion, mining, or other activities it will reduce the insulating value causing the permafrost to melt.

Two Moose Lake, Tombstone Territorial Park

If the permafrost melts significantly in the disturbed area, the area will sink (since liquid water has less volume than ice) resulting in a flooded area like the lake above. 

Ice formation has other destructive actions.

Frost Heaves

If you live in a northern or cold part of the world you have probably encountered frost heaves on roads and perhaps even in a building. Frost heaves are caused by capillary action which draws water up toward the freeze surface. This in turn pushes the soil up unevenly. Over successive freeze-thaw cycles, the materials will be sorted by size and density with larger particles on top and fine (capillary action providing) particles below.

https://www.sciencedirect.com/topics/earth-and-planetary-sciences/frost-heave

It has been our observation that roads susceptible to frost heaves are best left unpaved that way they can be graded routinely to keep them flat, the material can then stay well mixed, and pot holes avoided. Pot holes in pavement seems worse that pot holes on a gravel road. 

Since fine material, like silt, allows more capillary action than gravel, the selection of building materials is key to good construction.

Typical Hill Top Gravel Supply

Due to the long distances between everything here, the common practice seems to be pick a hill, really any hill, because most of them are moraine (glacial till) and use that material for road construction. We see areas like the one pictured above every few miles. The road crews seem to take material from the closest pile. I will point out that we are driving through areas that are almost entirely public land.

This material has a high proportion of fines. So probably not the best material for constructing anything. We occasionally see a pile of large boulders that have been sifted from the finer material but there appears to still be lots of finer material used in road construction. Perhaps that accounts for the need to make the road so very high. The poor construction material and harsh environment accounts for the constant maintenance.

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