Building on permafrost: beware!
The #1 piece of advice for building on permafrost: Don’t do it! That’s because homes are designed with the assumption that the foundation is both permanent and stable, and permafrost is neither. Especially not with the rapid climate change that is affecting Alaska. When permafrost thaws and settles, it causes foundations to move, which can create small problems like cracks in the drywall or major structural problems like sinking buildings, both of which can be seen in places like Interior Alaska where permafrost is widespread.
In cases where you cannot avoid permafrost, there are certain steps that will give you the best chance of success. First, it’s absolutely imperative to test the soil in your region. This starts with looking for localized signs such as:
- Are there any existing heated structures nearby?
- Are these foundations in good shape?
- Is the terrain sloped, open, and south-facing?
- Is the terrain flat? Are there marshes or standing water nearby?
- Do you see signs of thermokarsts or ice wedges?
- Is property an early-succession boreal forest?
- Is a drunken forest present?
If you decide to build on a site, it’s important to test the subsurface conditions to look at the occurrence, extent, and characteristics of permafrost, ground ice, soil, and rock. Borehole drilling is the most common and cost-effective method for evaluating permafrost, which can be performed by a soil engineer.
CCHRC has designed many homes in areas with highly unstable permafrost, which has forced us to come up with innovative solutions for building on ground that is likely to shift and thaw. What you do depends on your site and your budget, however, the conventional options include driven pilings and adjustable post-on-pad foundations.
Many homes in Interior Alaska have steel pilings driven deep into the ground to elevate the floor several feet high. You drill to a certain point based on resistance (often 30 feet or deeper), to ensure the surrounding ground freezes to the pilings. Once they freeze in place they are less likely to jack out of the ground. This foundation can be adjustable and allowing air flow under the building keeps the ground colder. This system only works on stable permafrost.
The lower-cost alternative is to build an adjustable post-on-pad system on top of a thick gravel pad in order to isolate the permafrost from the heated building. This allows cold air to get under the house and pull heat from the permafrost, making it more stable. The best post-on-pad homes are on adjustable posts. While this allows the house to respond to changes in the ground, it does not have as much adjustability built into it as piles.