ACE embankment Thermosyphon
ACE Embankment-ACE stands for Air Circulating Embankment. In terms of physics, this system is very similar to the air duct cooling system. In the ACE system, air is circulated passively through a void matrix in a layer of crushed rock, rather than ducts. Here's how it works. The road is built to conventional standards-a trapezoidal mass of soil. This embankment is encased in an approximately 10 foot thick layer of crushed rock. The crushed rock layer consists of rocks that are approximately 8 inches in diameter. This "rock blanket" is very porous- air can freely move through the pores. As with the air duct system, cold air is warmed by the relatively warm soil, it expands, rises, and removes heat from the soil. As with the air duct system, the laws of physics dictate that the convection process can not occur during the summer months. While the convection currents within the ace embankment are dormant, the stagnant air is trapped within the pores where it serves as an effective insulating blanket which limits the thermal contact between the permafrost and the outside air.
Two diferences make the ACE system more efficient than the air duct system. The cooling effects of the ACE system are even distributed over the entire soil area, rather than the more localized cooling provided by the air ducts. Also, the ACE system, because of its voids, is lighter than a conventional road embankment. Consequently the road imposes less of a load on the subsoil and this reduces the magnitude of potential thaw settlement.
Thermosyphons are also remotely similar to air duct cooling. They are a passive cooling system that utilizes fluid to carry off heat, rather than air. In this system, "loops" of conduit are embedded in the road embankment. These loops connect the "heated" permafrost with the "cold" road surface. A special coolant, that is capeable of holding more heat than air, is circulated through the circut, carrying heat away from the permafrost.
Under current experimentation, these two systems are often used together. Each system has its own limitations. It should be noted that the ACE system needs a certain vertical height to ensure proper air movement (this is totaly analogous to the "smokestack" effect in the air duct cooling system). If the embankment height is too shallow to properly utilize the ACE system, the thermosyphons will likely be used.
Both of these systems are currently being experimented with. Time will tell if these promising techniques provide an economical means of constructing roads over frozen ground.
