Permafrost is defined as a soil layer that stay frozen for at least two consecutive years. The layer above the permafrost starts thawing every spring and refreezes in fall. This layer is called the active layer. In this layer, plants can root and grow. The deeper this layer is, the taller vegetation can grow.
Permafrost is a pain in the neck for any construction. I have written about the damage of houses built on permafrost to the permafrost and the house some time ago. Permafrost can cause problems when drilling for drink water and can be the source of artesian wells that sprout water even in the middle of winter.
Today’s post is about an annual annoyance by permafrost in the back parking lot of the Geophysical Institute at the University of Alaska Fairbanks. Nearly each year, students, staff and faculty receive an email from the operations manager that part of the parking lot is closed due to permafrost damage. One year the damage was even so large that you could have easily hide a car in the whole caused by the permafrost damage. How does this work?
The Fairbanks area is an area of discontinuous permafrost. This means there are areas with and without permafrost underneath. Typically south-facing slopes have no or only few small areas with permafrost, while north slopes have a lot or are totally underlain by permafrost. This distribution is due to the amount of insolation an area receives. As you have experienced yourself at the beach, once there is shadow it’s your skin heats up less than when there is no shadow. The heat from the sunshine is conducted into the ground. Heat conduction is the transfer of heat by the molecules of the material. You well know this physical process from your iron pan’s iron handle that gets hot when you fry your breakfast eggs.
In case of the parking lot, it is located at the top of a north facing slope. Prior to the area’s becoming a parking lot, it was covered by trees and vegetation. When the lot was paved it was cut off from water supply from the top. This means that now evaporative cooling occurs anymore at the top of the soil layer in summer. Furthermore, the pavement heats up much stronger than wet soil and/or vegetation. This means during summer more heat is transferred into the ground underneath the paved parking lot than it was when the lot was still in its natural condition.
Over time, this increased heat transfer increases the depth of the active layer underneath the pavement. The water in the active layer can’t evaporate and stays there until it finds a way to flow away. In this case, the soil layer with a sudden looses part of its mass. Large soil grains remain, while the water takes away small grains (silt). Once the water flows away, soil matrix collapses and the remaining grains and stones sink into the space that formerly was filled by a mixture of frozen water, soil material, and stones. In that process, the pavement, of course, sinks down too.
The next thing is the closure of the area as shown in the photos and said email. In the following, the parking lot is repaired by filling the sink-whole and repaving the area. However, depending on the size of the permafrost lens underneath, it is just a matter of time when the next permafrost damage will occur due to heat conduction into the ground.
Thus, why didn’t the area collapse with the natural vegetation? I don’t know whether it didn’t and couldn’t find any information about it. However, in general, the shadows of trees reduce the potential heating of the ground. You surely have experienced as a kid when running around bare feet that grass doesn’t get as hot as pavement. For these reasons, the heat from sunshine does not penetrate so deep into the ground by conduction as it does under the same sunshine conditions in the paved parking lot.
Of course, you can find places where a permafrost lens collapsed in a hot summer underneath vegetated areas as well (see photo above). These sink-wholes are the reason why many lawns in the Fairbanks area are not plain as the lawn in a German soccer stadium, but bumpy up and down.
Aren’t these sink-whole amazing? What damages does nature cause to constructions in the area you live? Let me know, I am curious.
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Photos: G. Kramm
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