What do I need to be aware of when building on permafrost?
Permafrost has long bedeviled those building homes and infrastructure in Alaska. Loosely defined as soil or rock that remains frozen for more than two years, it underlays roughly 80% of Alaska. Building on frozen ground is risky because it disrupts the normal conditions of the soil and makes it more likely to thaw, which can lead to settling and damage to any structure sitting on it. As the climate warms and soil temperatures rise, it’s only getting riskier.
With permafrost, the safest bet is to avoid it altogether and move to another piece of land. In Interior Alaska, permafrost tends to be discontinuous and primarily concentrated on north-sloping hills and in lower elevations with heavy ground cover. While big trees may be an indicator of ground type, they do not guarantee the absence of permafrost but may simply mean that permanently frozen ground or ice is down far enough that the those soils can support a larger root system. The only way to be certain of your ground content is to have a soils test drilling done.
While avoidance is the best strategy, sometimes there is no other option but to build in an area with permafrost. In this case, be as strategic and cautious as possible. Smaller and simpler structures will tend to fare better than larger, more complicated ones.
Minimizing site disturbance is paramount. Trees and ground cover are your best friends. They protect and insulate the ground from the heat of the summer. A great example is the green moss that you find on many of the shaded low-level areas in Fairbanks. Moss has a high insulating value, and in many cases, if you dig down a couple of feet the ground may still be frozen in the middle of summer.
Strategies for construction on permafrost include:
- As a general rule, the organic layer of ground cover provides insulation and should not be removed, as this will increase the risk of thawing any frozen ground underneath.
- Keep the ground frozen! Elevate and properly insulate the bottom of your house to prevent heat loss through the floor system from reaching the ground underneath, which can lead to thawing.
- In post and pad construction, use a thick gravel pad that is significantly wider than the house itself (also insulated if possible) in order to stabilize the ground and spread building loads.
- If wood or steel piles or helical piers are used, they must be installed to a depth that will both support the structure and resist frost jacking due to seasonal ground movement.
- Make your foundation adjustable, either with a screw jack integrated into the piers or some other system that can accommodate at least minor movement. You will very likely need it.
- Cut trees sparingly to maximize site shading (while permitting for a fire break).
- Build a wrap-around porch, which will help shade the ground around and underneath the house.
- Incorporate large roof overhangs to shed water away from the house and provide shade.
- Install gutters and manage site drainage well away from the house.
- Hire an engineer familiar with local soils conditions to assist in designing a foundation system that will adequately and safely support your home on the soils specific to your site.
- Septic systems must also be engineered to function on permafrost, and remember that conventional systems may risk thawing the ground.
As Alaska’s climate warms, more and more foundations are failing and homes are needing to be adjusted. This reality needs to be incorporated in the planning of any future home.