A Subsurface Investigation

Of the available methods used to explore subsurface conditions at a proposed building site in the Interior of Alaska or other permafrost areas, boreholes made with auger drilling equipment are by far the most common given the depth that must be explored.  The number and spacing of the boreholes vary but are mainly dependent on the size of the structure and its proximity to areas of known permafrost. 

For example, for a modest structure located in an area that is generally held (and mapped) to be permafrost free, which does not lie near an area of known permafrost, at least two boreholes are recommended by SYNGEN to begin to get a grip on subsurface conditions across the proposed building footprint.

For larger structures, or for modest structures in areas lying near known permafrost (near permafrost boundaries), a greater number of boreholes increases knowledge of subsurface conditions and reduces the risks of unidentified lobes or isolated masses of permafrost which could lie between widely-spaced boreholes.   In such locations, measuring subsurface temperatures in the boreholes helps to further characterize subsurface conditions and reduce risks.  While no practical number of boreholes can rule out all risk, a minimum of five boreholes (the four building corners and the building center) are recommended for modest sized buildings located in or near areas identified as permafrost boundaries.

SYNGEN's methodology used to evaluate a given building site generally includes the following:

• Review of mapped soil and geologic conditions;
• Review of SYNGEN's own database of subsurface conditions made from previous explorations over the years;
• Review of site aerial imagery;
• Drilling and sampling program;
• Instrumentation program (thermistors), and;
• Review of available hydrogeologic information (nearby water well logs).

Upon completion of the above work, a geotechnical foundation evaluation report is issued which transmits the findings, conclusions and recommendations regarding:

• Suitable foundation type;
• Conventional
• Permafrost-resistant
• Geotechnical foundation design information;
• Allowable bearing pressure (for footing design)
• Lateral earth pressure coefficients (for basement walls)
• Soil modulus (for floor slabs)
• Frost protection (to guard against frost heave)
• General construction considerations;
  
• Water supply considerations, and;
• Wastewater disposal considerations.

The process normally takes three to four weeks from the time of drilling to the completion of the final report.  However, verbal results are usually available the day that the drilling is completed.  

THE CULPRIT: MASSIVE GROUND ICE:  Shown is a photo of massive ice buried deep in the ground.  Such deposits are often not visible from the surface and do not show themselves until heat from the building reaches them.  This particular image was taken from a tunnel bored into permafrost in an area near Fox, Alaska.  It is representative of the types of ground ice deposits that can be found in Fairbanks permafrost and are responsible for the distress to the cabin above.