Fairbanks lies at around 63 degrees north latitude and the climate of the area is sub-arctic. The average annual air temperature hovers just a few Fahrenheit degrees below freezing. Fairbanks lies within a region of discontinuous permafrost. Permafrost in the area has been retreating since the last ice age and so permafrost boundaries are common.
The images below illustrate an actual residential construction site located near a permafrost boundary in Fairbanks. The site was located on the non-permafrost side of a boundary similar to the mapped example pictured above. The photos were taken from a site located in a different area than that which is depicted on the map above.
Owing to past experience in the area, the orientation of the site (located on a northwest-facing slope) and the close proximity of the site to a known permafrost area, nine boreholes were used to gather preliminary information about subsurface conditions on the property.
Three of the nine borings were located downhill of the preferred construction site to investigate suspicious uneven surface terrain observed there. Permafrost consisting of frozen silt containing variable clear ground ice was encountered in each of these borings. The remaining boreholes were located upslope, in the area preferred for construction. Four of these six borings were located near the four corners of a rectangular building footprint with the remaining two borings being drilled at the mid-point of each wall on the long dimension. Permafrost was not identified in any of these boreholes.
As can be seen below in the first photo (on the left), the predominant soil type at the site was loess (windblown silt) and the predominant vegetation was mature birch forest. While permafrost was not identified in any of the six borings drilled within the building footprint, isolated masses of permafrost were a concern between boreholes and careful observations were recommended during foundation construction. Excavation for a daylight basement was begun and was nearly complete at the time of the first site visit.
During the first site visit, careful observations were made of the exposed soil conditions in the excavation. As expected, the excavation revealed mainly Fairbanks loess (windblown silt) as indicated by the borings. This material is typically massive (meaning that individual depositional layering is not easy to see) and homogenous (meaning that it's material consistency is uniform in every direction). The silt is tan to buff in color and is usually naturally weakly cemented such that it holds its shape, and steep excavation cuts are possible.
During the visit, some staining was observed in the wall of the excavation near one corner of the building footprint which suggested a historic presence of water (see second photo from the left). The staining was observed in an area of the excavation that was adjacent to an apparent drainage gulley which ran across the property from higher elevations of the hill. During the observations, the author walked the excavation boundary and probed the soil at the base of the excavation with a 48 inch long surveyor's stake. At the footing location at the base of the excavation immediately adjacent to the staining, the stake could easily be pushed by hand to its full length into the soil. For the remainder of the foundation location around the building footprint, the stake could only be pushed by hand one or two inches.
A test pit was recommended at the location of the probe and revealed massive ground ice. The deposit was located between the two borings drilled at the building corners. The boreholes were spaced around 30 feet apart. The property is surrounded by residences supported by conventional concrete foundations.
A careful study of the ground ice and its surroundings indicated that the ice was associated with the nearby drainage and was likely the result of ground water that had seeped into a cast in the silt made by a previous ground ice deposit which had thawed and drained away during a previous warming period, leaving the open void (cave) behind. The climate cooled again and shallow surface water trickled into the cave and re-froze. Evidence supporting this hypothesis was seen in cores of ground ice (sixth photo from left) taken from a borehole located downslope of the preferred building site as well as the massive ice exposed in the excavation shown (third, fourth, and fifth photos from left).