Air Duct Cooling Systems An air duct cooling system is a passive cooling system that utilizes metal conduit embedded in the road embankment. Here's how it works. When the road is built, a series of small (about 8 inches) diameter metal pipes are burried in the road bed. One end of the pipe system is an intake that protudes horizontaly from the road embankment. The other end is a "chimney" that extends horizontaly from the road shoulder. Cold air (that is, cold relative to the soil temperature) enters the intake. The air absorbs heat from the soil (to put things in perspective: a typical situation would entail air at a temperature of -20 F being "warmed" by the 31 F permafrost). As the air is warmed it expands, loses density, becomes lighter, and flows up through the chimney. As the warmed air ascends through the chimney, a vacume is created that pulls more cold air into the intake. By this process, heat is continualy pulled out of the soil, making it colder and thus more resistant to melting. It was found through experimentation that this system worked more efficiently if the outflow stack was taller rather than shorter. The taller stack created a stronger draft that pulled a greater volume of air through the system, resulting in greater cooling. This occurence makes sense intuitively. Think of a wood stove; it often occurs when setting up a wood stove that the chimney is not tall enough and consequently the fire does not burn properly. By increasing the chimney height, a greater vacume is created, pulling more air and accelerating the combustion process. The two situations are anologous. It is important to note that this system only works if the ambient air (that enters the duct system intake) is colder than the soil through wich it flows. Because of this, the duct cooling system is dormant in the summer months. The convection process simply will not work unless the air flowing through the ducts is warmed. The air duct cooling system is used in conjunction with a design detail known as a stabilizing berm. The stabilizing berm is basicaly a pile of soil directly next to and adjoining the road embankment. Cooling air ducts are routed through this berm. This berm serves as a sort of blanket. It creates a zone of protected permafrost that is substantialy wider than the road. It also butresses the road. Remember, when a road embankment fails from thaw settlement, it is a rotational failure. Hunks of the road embankment shear off and spread lateraly, creating gaping lateral cracks in the road surface. The stabilizing berm confines the road embankment and prevents it from spreading lateraly. This system has been used as an experimental feature in several locations in Alaska. To the best of my knowledge this system is no longer being actively persued. It is however similar in concept to the ACE embankment system of road construction. Go to the next page to check this out.