15 AUGUST 2005
FOUNDATIONS IN AREAS OF SIGNIFICANT FROST PENETRATION
Types of Areas. For purposes of this UFC, areas of significant frost
penetration may be defined as those in which freezing temperatures occur in the ground
to sufficient depth to be a significant factor in foundation design. Detailed requirements
of engineering design in such areas are given in the UFC series, Arctic and Subarctic
Construction (UFC 3-130-01 through 3-130-07.) Areas of significant frost penetration
may be subdivided as follows.
Seasonal Frost Areas. Significant ground freezing occurs in these areas
during the winter season, but without development of permafrost. In northern Texas,
significant seasonal frost occurs about 1 year in 10. A little farther north it is
experienced every year. Depth of seasonal freezing increases northward with
decreasing mean annual and winter air temperatures until permafrost is encountered.
With still further decrease of air temperatures, the depth of annual freezing and thawing
becomes progressively thinner.
The layer extending through both seasonal frost and permafrost areas in
which annual freeze-thaw cycles occur is called the annual frost zone. In permafrost
areas, it is also called the active layer. It is usually not more than 3 m (10 ft) thick, but it
may exceed 6 m (20 ft). Under conditions of natural cover in very cold permafrost
areas, it may be as little as 305 mm (1 ft) thick. Its thickness may vary over a wide
range even within a small area. Seasonal changes in soil properties in this layer are
caused principally by the freezing and thawing of water contained in the soil. The water
may be permanently in the annual frost zone or may be drawn into it during the freezing
process and released during thawing. Seasonal changes are also produced by
shrinkage and expansion caused by temperature changes.
Permafrost Areas. In these areas, perennially frozen ground is found
below the annual frost zone. In North America, permafrost is found principally north of
latitudes 55 to 65 degrees, although patches of permafrost are found much farther south
on mountains where the temperature conditions are sufficiently low, including some
mountains in the contiguous 48 States. In areas of continuous permafrost, perennially
frozen ground is absent only at a few widely scattered locations, as at the bottoms of
rivers and lakes. In areas of discontinuous permafrost, permafrost is found
intermittently in various degrees. There may be discontinuities in both horizontal and
vertical extent. Sporadic permafrost is permafrost occurring in the form of scattered
permafrost islands. In the coldest parts of the Arctic, the ground may be frozen as deep
as 610 m (2000 ft).
The geographical boundaries between zones of continuous permafrost,
discontinuous permafrost, and seasonal frost without permafrost are poorly defined but
are represented approximately in Figure 11-1