Planning for a permafrost investigation begins with a basic understanding of the general area and the expected subsurface conditions. This understanding is based in part upon past experience in the general area, and in part upon the observable surface features that are present at the specific site of interest.
Of the methods locally available to perform a permafrost investigation in Interior Alaska, given the depth that must often be explored for most structures, boreholes advanced using drilling equipment are most practical. Of the drilling techniques available to drill the holes, boreholes drilled using an auger rig (see image above) are usually preferred. The total depth, number, and spacing of the boreholes varies with each specific situation.
For a modestly-sized (<2,000 square feet) single-family dwelling located in an area that is generally understood to be permafrost free, which does NOT lie close to an area of known permafrost (e.g., a permafrost boundary), at least two boreholes are good practice to begin to characterize subsurface conditions at the site of interest. Depending on the conditions encountered and the wishes of the Client, additional borings may be suggested. For larger structures, naturally, a greater number of boreholes is justified.
In areas of known permafrost, or in transitional areas (areas separating known permafrost areas from areas that are generally permafrost free), a greater number and tighter spacing between boreholes is always recommended by SYNGEN for any structure, regardless of size. In such permafrost boundary areas, subsurface conditions can change rapidly and such changes can include unidentified lobes or isolated masses of permafrost that are difficult to detect with too few borings or with many borings that are too widely spaced. In such locations a greater number and tight spacing of boreholes reduces the risk of unidentified hazards. To further characterize subsurface conditions across a building footprint in high risk areas, subsurface temperatures can be measured (see image below). Departures (towards freezing) in, say, one borehole from temperatures measured in the four others where temperatures were measured to be unfavorable for large masses of permafrost may indicate an isolated mass of permafrost nearby that borehole and such information could be used to locate additional boreholes in an effort to identify the source of the cooler temperatures.
