UFC 3-220-01N
15 AUGUST 2005
that may be encountered in achieving a stable foundation. For example, pervious
layers in fine-grained alluvial deposits in combination with copious groundwater supplies
from adjacent higher terra-in may produce very high frost-heave potential, but clean,
free-draining sand and gravel terrace formations of great depth, free of excess ice, can
provide virtually trouble-free foundation conditions.
11-2.2
Temperature. The most important factors contributing to the existence of
adverse foundation conditions in seasonal frost and permafrost regions are cold air
temperatures and the continual changes of temperature between summer and winter,
Mean annual air temperatures usually have to be -17 to -13 C (2 to 8F) below
freezing for permafrost to be present, although exceptions may be encountered both
above and below this range. Ground temperatures, depths of freeze and thaw, and
thickness of permafrost are the product of many variables including weather, radiation,
surface conditions, exposure, snow and vegetative cover, and insulating or other special
courses. The properties of earth materials that determine the depths to which
freezing-and-thawing temperatures will penetrate below the ground surface under given
temperature differentials over a given time are the thermal conductivity, the volumetric
specific heat capacity, and the volumetric latent heat of fusion. These factors in turn
vary with the type of material, density, and moisture content. Figure 11-2 shows how
ground temperatures vary during the freezing season in an area of substantial seasonal
freezing having a mean annual temperature of 3 C (37F) (Limestone, Maine), and
Figure 11-3 shows similar data for a permafrost area having a mean annual
temperature of -3 C (26F) (Fairbanks, Alaska).
For the computation of seasonal freeze or thaw penetration, freezing-and-
thawing indexes are used based upon degree-days relative to 0 C (32F). For the
average permanent structure, the design indexes should be those for the coldest winter
and the warmest summer in 30 years of record. This criterion is more conservative than
that used for pavements because buildings and other structures are less tolerant of
movement than pavements. It is important to note that indexes found from weather
records are for air about 4.5 ft above the ground; the values at ground surface, which
determine freeze-and-thaw effects, are usually different, being generally smaller for
freezer conditions and larger for thawing where surfaces are exposed to the sun. The
surface index, which is the index determined for temperature immediately below the
surface, is n times the air index, where n is the correction factor. Turf, moss, other
vegetative cover, and snow will reduce the n value for temperatures at the soil surface
in relation to air temperatures and hence give less freeze or thaw penetration for the
same air freezing or thawing index. Values of n for a variety of conditions are given in
UFC 3-130-01.
More detailed information on indexes and their computation, including
maps showing distribution of index values, is presented in UFC 3-130-01.
11-2.3
Foundation Materials. The foundation design decisions may be critically
affected by the foundation soil, ice, and rock conditions.
11-2.3.1
Soils. The most important properties of soils affecting the performance of
engineering structures under seasonal freeze-thaw action are their frost-heaving
11-4
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