Frozen soil without water is just cold dirt. But add enough water to the right soil and freeze it in the right way and bizarre things begin to happen.
Permafrost in interior Alaska by nature usually contains some component of frozen water. Occasionally permafrost in the Fairbanks area can be quite dry, even bone-dry. But such permafrost is uncommon and its existence requires unique conditions.
Frozen water can be present in a soil matrix as pore ice or as segregated ground ice. Pore ice is frozen water that is contained within the soil intergranular (pore) spaces (the tiny air spaces between grains of sand, for example). In contrast, segregated ground ice is clear or nearly clear ground ice that is concentrated outside of the soil pore spaces. Segregated ground ice can exist as stratified seams or lenses of ice up to a few inches thick and it can exist as masses that are tens of feet thick or more.
Lens Ice
Ground ice occurring in the form of thin seams or stratified lenses or layers (lens ice) is usually the result of the capillarity of the soil. The capillarity of a soil is its tendency to draw up water against gravity (wick) in the same way that oil is drawn into to a wick in an old-fashioned oil lamp (or the way water creeps up your pant legs when you're walking through wet snow . . . don't ya hate that?). Capillarity is strongest in fine grained soils. This is because of 1) a neat property of water called surface tension (you may know that this property causes a metal sewing needle, if placed carefully, to float in a cup of water), and 2) because fine-grained soils have a high surface area (there are lots of grains in fine-grained soil and each grain has a surface area . . . you just add them up). So fine grained soils tend to have a high capillarity and can draw up lots of water. And when the water reaches the freezing front. Poof! You have lens ice!
Oh, and bumpy roads.
Why bumpy roads?
Growth of these ice formations is responsible for a phenomenon called frost-heave. Frost-heave causes the ground surface to swell this swell pushes up Fairbanks road pavements and makes them bumpy and uneven. How big are the bumps? Well. Count the ice lenses and add them up. Each individual lens can be a fraction of an inch, or, up to a few inches thick and these lenses add up. The thickness of each lens depends on the speed of the advancing freezing front (the colder the air, the greater the speed) and the amount of groundwater available to feed the hungry ice lens that is growing. Now the growth of these ice layers is parallel to the freezing front, which penetrates downward away from the ground surface. Freezing results in the upward expansion of the soil as the freezing line penetrates deeper into the ground (and water expands to ice). For lens ice to grow thick, there must be a ready source of groundwater and a soil that can wick the groundwater upward, towards the freezing front (against gravity). So roads that are constructed using soils containing a lot of fine grains that cross swampy terrain are usually are the most bumpy.
As you can see, lens ice is a form of segregated ground ice that forms under a very unique set of conditions and therefore should not be confused with other forms of ground ice which, as we will see, form in very different ways.
Massive Ice
As mentioned earlier, in contrast to segregated ground ice occurring as thin lenses of ice, it can also occur as masses of ground ice tens of feet thick, or more. Such ice deposits are also common in fine-grained soils, and some often cut across, or in a direction roughly perpendicular to, the horizontal lenses of ice described above. These are generally known as ice wedges. One of these bad boys is pictured below. These things eat buildings.
