TM 5-852-1/AFR 88-19, Volume 1
CHAPTER 4
DEEP SEASONAL FROST AND PERMAFROST
4-1.
Distribution
4-2.
Characteristics of permafrost
a. Areas of deep seasonal frost penetration
The following summary of the characteristics of
may be defined arbitrarily for engineering as seasonal
permafrost should be considered merely introductory.
frost areas with design air freezing indexes of 1500
a. Structure. Depending on local conditions,
Fahrenheit degree-days or more. An air freezing index
permafrost may exist as (see figure 4-3):
of 1500 Fahrenheit degree-days corresponds to a depth
-A continuous layer with its upper surface at the
of seasonal frost penetration of about 7 feet into very well
bottom of the annual frost zone (active layer). This is
drained clean gravel under a bituminous surface kept
common in arctic regions.
clear of snow and ice. The actual frost penetration at a
-A continuous layer with its upper surface
given point will depend on soil and surface conditions,
separated from the annual frost zone (active layer) by a
and other factors as discussed in chapter 5. If the soil is
residual thaw layer. If the permafrost table is lowering
saturated silt, the frost penetration may be about half of
progressively, this is a degrading condition.
that in a very well drained clean gravel for otherwise
-Frozen layers separated by layers of unfrozen
identical conditions. Design freezing indexes of 1500
material.
Fahrenheit degree-days exist well south of the Subarctic
-Inclusions of remnants of permafrost in
in the northern temperate zone. However, there are also
unfrozen ground.
lower values in a few locations within the Subarctic, such
b. Depth to surface.
The depth to the
as in northern Iceland. As the freezing index increases
with increasing latitude, or increasingly cold geographic
permafrost table is dependent primarily on the magnitude
location or topographic position, the depth of seasonal
of the air thawing index, the amount of solar radiation
freezing increases; it may reach as much as about 20
that reaches the surface, the surface cover conditions
feet in very well drained clean gravel under a bituminous
that have existed for the previous several years, and the
surface kept clear of snow and ice, without development
water content and dry unit weight of the soil. See table
of permafrost. When a point is reached, however, where
4-1 for typical depths to the permafrost layer. For
the depth of winter freezing exceeds the depth of
comparison, in a subarctic area without permafrost, a
summer thaw, permafrost results if frozen ground
well-graded sandy gravel that is relatively dry (w = 5%)
persists over at least two consecutive winters and the
and seasonally frozen would have an annual thaw
intervening summer.
As one travels northward,
penetration of about 16 feet. Methods for estimating
permafrost, which in the more southerly part of the
depths of freeze and thaw penetration are discussed in
permafrost zone is discontinuous, becomes gradually
TM 5-852-4/AFM 88-19, Chap. 4 and TM 5-852-6/AFR
continuous, except under some large water bodies that
88-19, Volume 6.
c. Factors affecting existence and distribution.
are deep enough to prevent freezing temperatures from
reaching the bottom, and the depth of summer thaw
Under natural, undisturbed ground cover conditions, the
mean annual air temperature must usually be 21 to 30F
becomes progressively smaller. In areas of continuous
permafrost, lateral heat flow may cause permafrost to
for permafrost to exist, although exceptions are possible
extend partly or entirely under bodies of water, even
on either side of this range. If temperatures at the
though there is a layer of unfrozen soil immediately
ground surface could be used, more precise correlation
below the water. Along the seacoast, permafrost may
could be obtained. However, surface temperatures have
extend for a substantial distance offshore, either as relic
not been generally available in the past from
permafrost or as permafrost formed as a result of
meteorological records.
The existence of ground
seawater temperatures below 320F.
temperatures perennially below freezing is a function of
b. Figures 4-1 and 4-2 show the approximate
many factors other than air temperatures, as discussed
extent of permafrost areas in North America and Eurasia
in chapter 5, including solar radiation, surface cover,
respectively. Islands of permafrost are also found in
snow cover, wind, soil type, soil moisture content,
mountains
south
of
the
Subarctic.
groundwater flow and presence of stationary or moving
surface water. Forest fires or meandering of streams
may cause alterations of permafrost conditions extending
over many years.
4-1
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