TM 5-818-1 / AFM 88-3, Chap. 7
CHAPTER 18
FOUNDATIONS IN AREAS OF SIGNIFICANT FROST PENETRATION
temperature conditions are sufficiently low, including
18-1.
Introduction.
some mountains in the contiguous 48 States. In areas of
a. Types of areas. For purposes of this
continuous permafrost, perennially frozen ground is
manual, areas of significant frost penetration may be
absent only at a few widely scattered locations, as at the
defined as those in which freezing temperatures occur in
bottoms of rivers and lakes. In areas of discontinuous
the ground to sufficient depth to be a significant factor in
permafrost, permafrost is found intermittently in various
foundation design. Detailed requirements of engineering
degrees. There may be discontinuities in both horizontal
design in such areas are given in TM 5-818-2/AFM 88-6,
and vertical extent. Sporadic permafrost is permafrost
Chapter 4, and the Arctic and Subarctic Construction
series, TM 5-852-1 through 9/AFM 88-19, Chapters 1
occurring in the form of scattered permafrost islands. In
the coldest parts of the Arctic, the ground may be frozen
through 9, respectively.
Areas of significant frost
as deep as 2000 feet.
penetration may be subdivided as follows:
(b) The geographical boundaries
(1) Seasonal frost areas.
between zones of continuous permafrost, discontinuous
(a) Significant ground freezing occurs
permafrost, and seasonal frost without permafrost are
in these areas during the winter season, but without
1
poorly de- fined but are represented approximately in
In northern Texas,
development of permafrost.
figure 18-1.
significant seasonal frost occurs about 1 year in 10. A
b. General nature of design problems.
little farther north it is experienced every year. Depth of
Generally, the design of foundations in areas of only
seasonal freezing increases northward with decreasing
seasonal frost follows the same procedure as where
mean annual and winter air temperatures until
frost is in- significant or absent, except that precautions
permafrost is encountered. With still further decrease of
are taken to avoid winter damage from frost heave or
air temperatures, the depth of annual freezing and
thrust. In the spring, thaw and settlement of frost-
thawing becomes progressively thinner.
heaved material in the annual frost zone may occur
(b) The layer extending through both
differentially, and a very wet, poorly drained ground
seasonal frost and permafrost areas in which annual
condition with temporary but substantial loss of shear
freeze-thaw cycles occur is called the annual frost zone.
strength is typical.
In permafrost areas, it is also called the active layer. It is
(1) In permafrost areas, the same annual
usually not more than 10 feet thick, but it may exceed 20
frost zone phenomena occur, but the presence of the
feet. Under conditions of natural cover in very cold
underlying permafrost introduces additional potentially
permafrost areas, it may be as little as 1 foot thick. Its
complex problems. In permafrost areas, heat flow from
thickness may vary over a wide range even within a
buildings is a fundamental consideration, complicating
small area. Seasonal changes in soil properties in this
the design of all but the simplest buildings. Any change
layer are caused principally by the freezing and thawing
from natural conditions that results in a warming of the
of water contained in the soil. The water may be
ground beneath a structure can result in progressive
permanently in the annual frost zone or may be drawn
lowering of the permafrost table over a period of years
into it during the freezing process and released during
that is known as degradation. If the permafrost contains
thawing. Seasonal changes are also produced by
ice in excess of the natural void or fissure space of the
shrinkage and expansion caused by temperature
material when unfrozen, progressive downward thaw
changes.
may result in extreme settlements or overlying soil and
(2) Permafrost areas.
structures. This condition can be very serious because
(a) In these areas, perennially frozen
such subsidence is almost invariably differential and
ground is found below the annual frost zone. In North
hence very damaging to a structure. Degradation may
America, permafrost is found principally north of latitudes
occur not only from building heat but also from solar
55 to 65 degrees, although patches of permafrost are
heating, as under pavements, from surface water and
found much farther south on mountains where the
groundwater flow, and from underground utility lines.
1
Proper insulation will prevent degradation in some
Specialized terms relating to frozen ground
situations, but where a con
areas are defined in TM 5-818-2/AFM 88-6, Chapter 4,
and TM 5-852-1/AFM 88-19, Chapter 1.
18-1
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