TM 5-818-1 / AFM 88-3, Chap. 7
foundations and structures, commonly differentially, with
or piping, and other detrimental effects. Often, the
a variety of possible consequences.
results may be catastrophic. For permafrost soils and
(c) When thaw occurs, the ice within
rock containing excess ice, design should consider three
alternatives, as indicated in figure 18-4: maintenance of
the frostheaved soil is changed to water and escapes to
stable thermal regime, acceptance of thermal regime
the ground surface or into surrounding soil, allowing
changes, and modification of foundation conditions prior
overlying materials and structures to settle. If the water
to construction. These approaches are discussed in TM
is released by thaw more rapidly than it can be drained
5-852-4/AFM 88-19, Chapter 4. Choice of the specific
away or redistributed, substantial loss in soil strength
foundation type from among those indicated in figure 18-
occurs. In seasonal frost areas, a heaved foundation
4 can be made on the basis of cost and performance
may or may not return to its before-heave elevation.
requirements after the development of details to the
Friction on lateral surface or intrusion of softened soil
degree needed for resolution.
into the void space below the heaved foundation
b. Foundation freeze and thaw and techniques
members may prevent full return. Successive winter
for control. Detailed guidance for foundation thermal
seasons may produce progressive upward movement.
(d) Therefore, when the soils within
computations and for methods of controlling freeze-and-
the maximum depth of seasonal frost penetration are
thaw penetration is presented in TM 5-852-4 and TM 5-
frost-susceptible, foundations in seasonal frost areas
852-6/AFM 88-19, Chapters 4 and 6, respectively.
(1) Design depth of ordinary frost
should be supported below the annual frost zone, using
penetration.
conventional foundation elements protected against uplift
caused by adfreeze grip and against frost overturning or
(a) For
average
permanent
sliding forces, or the structure should be placed on
structures, the depth of frost penetration assumed for
compacted non-frost-susceptible fill designed to control
design, for situations not affected by heat from a
frost effects (fig 18-4).
structure, should be that which will occur in the coldest
(2) Foundations in permafrost areas.
year in 30. For a structure of a temporary nature or
Design on permafrost areas must cope with both the
otherwise tolerant of some foundation movement, the
annual frost zone phenomena described in paragraph
depth of frost penetration in the coldest year in 10 or
18-4a(l) and those peculiar to permafrost.
even that in the mean winter may be used, as may be
(a)
Permafrost
foundations
not
most applicable. The design depth should preferably be
adversely affected by thaw.
Whenever possible,
based on actual measurements, or on computations if
structures in permafrost areas should be located on
measurements are not available. When measurements
clean, non-frost-susceptible sand or gravel deposits or
are available, they will almost always need to be adjusted
rock that are free of ground ice or of excess interstitial
by computations to the equivalent of the freezing index
ice, which would make the foundation susceptible to
selected as the basis for design, as measurements will
settlement on thaw. Such sites are ideal and should be
seldom be available for a winter having a severity
sought whenever possible. Foundation design under
equivalent to that value.
(b) The frost penetration can be
these conditions can be basically identical with
temperate zone practices, even though the materials are
computed using the design freezing index and the
frozen below the foundation support level, as has' been
detailed guidance given in TM 5-852-6/AFM 88-19,
demonstrated in Corps of Engineers construction in
Chapter 6.
For paved areas kept free of snow,
interior Alaska. When conventional foundation designs
approximate depths of frost penetration may be
are used for such materials, heat from the structure will
estimated from TM 5-818-2/AFM 88-6, Chapter 4, or TM
gradually thaw the foundation to progressively greater
852-3/AFM 88-19, Chapter 3, entering the appropriate
depths over an indefinite period of years. In 5 years, for
chart with the air freezing index directly. A chart is also
example, thaw may reach a depth of 40 feet. However, if
presented in TM 5-852-4/AFM 88-19, Chapter 4, from
the foundation materials are not susceptible to
which approximate depths of frost penetration may be
settlement on thaw, there will be no effects on the
obtained for a variety of surface conditions, using the air
structure from such thaw.
The possible effect of
freezing index in combination with the appropriate
earthquakes or other dynamic forces after thawing
surface index/air correction factor (n-factor).
(c) In the more developed parts of
should be considered.
(b) Permafrost foundations adversely
the cold regions, the building codes of most cities specify
affected by thaw. When permafrost foundation materials
minimum footing depths, based on many years of local
containing excess ice are thawed, the consequences
experience; these depths are invariably less than the
may include differential settlement, slope instability,
maximum observed frost penetrations.
The code
development of water-filled surface depressions that
serve to intensity thaw, loss of strength of frostloosened
foundation materials under excess moisture conditions,
development of underground uncontrolled drainage
channels in permafrost materials susceptible to bridging
18-8
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