The Changing Face of Fairbanks and the Interior
Most who have spent some time in Fairbanks know that the Golden Heart City lies within a region of permafrost. The permafrost is discontinuous. It is not found everywhere. It is found most frequenty where exposure to sunlight is limited or even in sunny areas where standing water has historically collected. It is often encountered on north-facing exposures such as the north side of College Hill (Ballaine Road) or on low, boggy terrain with a southerly exposure such as just north of College Road. Mid-to-high south-facing hillside exposures are often permafrost free. Some low-lying boggy areas are also permafrost free. Because of the sporadic nature of the hazard, drilling is a very good idea prior to construction.
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In the early days of Fairbanks, at at time before the effects of building on permafrost were fully understood; soil testing was not common and as a result many buildings were unwarily constructed on permafrost. Occasionally, builders actually had full knowledge of permafrost but overestimated the ability of buried insulation to prevent the thaw. The results were often disastrous.
As experience and knowledge of permafrost grew, testing became more commonplace, problems were reduced, and a knowledge-base was gained. Areas that were generally permafrost-free were identified and have since been heavily developed. As development continues, the amount of permafrost-free land is reduced and this has forced home-builders, determined to build houses in marketable locations, toward the boundaries between "good ground" and frozen ground. These in-between areas are especially dangerous. A small deposit of ice rich frozen soil under a single corner of the building is more than enough to cause considerable damage along with the associated headache and grief to the property owner.
In the safer areas, one or two test holes for a building was the accepted standard for many years. But as development continued toward questionable areas, a increasing number of distressed foundations made it clear that the areasd bounding permafrost areas could contain mostly non-frozen ground with localized pockets of frozen material that were themselves large enough to seriously damage a home. In other words, drilling two building corners left two corners up to chance (not to mention the building center). It was seen that conditions in boundary areas actually changed that fast. Increasing awareness of this fact prompted some engineeers (like the author) to suggest additional test holes help to reduce the risks of localized pockets of frozen ground in many known permafrost "boundary areas."
Today, a residential investigation can consist of any number of soil borings, depending on the size of the building and its location. While no practical number of borings can rule out all risk, two borings may be wholly inadequate. In summary, the owner or builder must be aware that the level of risk associated with building conventional warm foundations (basements, slabs on grade, crawlspaces, etc . . . ) in boundary areas is directly related to the number of boreholes drilled.