Fairbanks Permafrost
Most who have spent some time in Fairbanks know that the Golden Heart City lies within a region of permafrost. Permafrost is any soil or rock which remains at or below the freezing point of water for a period of at least two consecutive winters (and the intervening summer), though most permafrost is much older than this, as evidenced by the now-extinct Ice-Age animal remains that it sometimes preserves. For permafrost to grow or to persist, the average annual air temperature must be below freezing. Historically, the average annual air temperature in Fairbanks has hovered near 27 degrees Fahrenheit.
When permafrost thaws, the ice it contains changes to water and the water drains away. If the soil is sufficiently rich in ground ice (present either as pore ice or as massive deposits of nearly clear ice), the thaw can result in a large volume change (loss) which causes the ground to subside and sink. Depending on the magnitude of the subsidence, damage to a building founded upon the thawing soil can be minor, resulting in some unsightly cracking, sticking doors and windows, and uneven floors, or it can be catastrophic damage rendering the building unsafe and/or uninhabitable. Depending on the kind of permafrost and its depth below the warm building floor, thaw related settlement and damage can occur rapidly or it can take many years, even decades, for its effects to be seen. The deeper the permafrost exists below the warm floor, the longer it takes heat from the floor to reach it and to cause thaw and thaw-settlement related damage.
The permafrost of the Interior of Alaska (including Fairbanks and North Pole) is discontinuous, meaning it is not found everywhere. It is often found where exposure to sunlight is limited such as north, northeast, or northwest facing slopes. It can also be found where sunlight is plentiful but where water has historically collected (and frozen) and cooler temperatures have prevailed, such as the valley bottoms of south facing slopes.
It is often encountered on north-facing exposures such as the north side of College Hill (Goldfinch Road or Skyline Drive) and Chena Ridge (Chena Ridge Road) and is also common in low, boggy terrain, even with a southerly exposure such as the base of Chena Ridge (Chena Pump Road) and the base of College Hill (Farmers Loop Road), Permafrost is pervasive, or nearly continuous, across the broad Goldstream Valley. Mid-to-high south-facing hillside exposures can often be permafrost free but lobes (fingers) or isolated masses of permafrost are sometimes encountered in these areas as well. Because of the sporadic nature of permafrost, subsurface exploration is critical during the planning phase, prior to building construction.
For reasons that should be increasingly clear, some form of subsurface exploration should be done prior to clearing and before foundation selection and construction. Because permafrost-related hazards often lie below a depth that is easily reachable by conventional backhoes (and because of the disturbance created by excavating deep pits with standard excavation equipment), the use of a geotechnical drill rig is usually preferred. The depth explored should be at least the width of the building.
This website is intended to help folks gain an understanding of the permafrost related hazards in the Interior, the process of a subsurface investigation, and the methods used to help identify and reduce risks.
