Thompson Drive - Air Circulation Helps Preserve Permafrost
Air Convection Embankments
At several locations along Thompson Drive
you will notice brown rock along the sides of
the embankment as shown in the photo to
the right. This rock is not merely landscaping,
but actually part of a system that is designed
to preserve the roadway by cooling permafrost.
Portions of Thompson Drive cross areas of
permafrost (permanently frozen ground). It
is important that this permafrost layer remain
frozen, but roadway construction normally
produces a warming effect which can thaw it
out. If thawing does take place, difficulties
will arise due to the large amount of ice found in the permafrost layer. When this ice melts, it produces very wet soils and voids that are no longer capable of supporting the road. As a result, the entire roadway structure will settle and become distorted and unusable. This type of thaw settlement¯ would be difficult to repair because Thompson Drive has other improvements, such as sidewalks, curbs, and the bridge over the Alaska Railroad tracks.
The rock layers seen along the sides of Thompson Drive are part of a system known as an Air Convection Embankment. This special type of roadway embankment uses large rock to allow air to circulate through the embankment. Because of the way the system is designed, the air circulates more vigorously during winter when air temperatures are low. The net effect is an enhanced cooling of the embankment and permafrost layer below. By supplying enhanced cooling during winter months, the permafrost is able to survive the warm summer period without melting.
 The diagram on the left shows a schematic diagram of the air circulation cells that form in the embankment and shoulders. Air currents are so strong that they will flow through the snow layers and circulate beneath the pavement as shown in the diagram. As warm air moves upward from the lower layers of the embankment it is replaced with colder air from above, providing the cooling required to preserve the permafrost.
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The photo on the lower
right shows in infrared image of one of the vent
holes. The bright orange color in the center of
the image indicates that the hole is much warmer
than the surrounding snow surface. This is
because the air is warmed as it circulates up
through the rock layer and exits at a much
higher temperature than it enters. Approximately
100 temperature sensors, buried in the embankment
are used to monitor the cooling effect of the air
convection embankments.