by A. Smith

Created on: November 25, 2008   Last Updated: December 02, 2008

The rules for building on permafrost have changed. Permafrost used to be permanent. However, building on permafrost today requires footings that will survive even if the permafrost permanently melts.

Permafrost is defined as ground which has been frozen for two or more years. This condition is specific to northern arctic and subarctic regions, as well as some alpine areas. In the southern hemisphere, most areas which would otherwise have permafrost are occupied instead by ocean.

While permafrost can extend hundreds of feet deep, it is only the top section that concerns builders. This poses a problem, because permafrost usually does not begin until several feet below ground level. So long as it remains frozen, ground level permafrost is very strong and stable for building. Ground above the permafrost layer is the opposite. The freeze-thaw cycle allows the water between the soil particles to melt and freeze repeatedly, which weakens the integrity of the soil. Depending on the slope or other drainage, water may also flow away, leaving voids in the soil. Not all permafrost contains water, but any water will compromise the permafrost and surrounding soil.

For this reason, structures built on top of permafrost will gradually sink. Any heated areas will sink more rapidly. Over a year or two, this will cause the structure to tear itself apart.

There are several standard methods to determine existence of underlying permafrost. One is to look at vegetation and geological patterns. For example, black spruce usually indicates hidden permafrost, while birch, aspen, or white spruce indicate permafrost-free areas. Permafrost is more common on north-facing slopes than on south-facing ones.

Drilling into the soil gives better information. A standard hand auger will not bite into permafrost. In rodding, a 1/4 inch steel rod is driven into the soil until it can go no further, and then it is twisted. If it twists easily, it has hit a stone. If it springs back, it has hit wood or permafrost. If the rod is struck, it will give off a dull clang if it has contacted permafrost. Digging a pit reveals the depth and character of the soil and any permafrost, and the pit can be filled in with gravel afterward for a solid foundation. A drilled core is the best and most expensive way of analyzing soil. It can show a full log of the underlying soil, without committing to a pit.

There are three common ways to deal with permafrost. The first is to avoid it, but this is not always possible. For example,

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