TM 5-818-1 / AFM 88-3, Chap. 7
(3) Engineering problems may also arise
for a permafrost area having a mean annual temperature
from such factors as the difficulty of excavating and
of 26F (Fairbanks, Alaska).
handling ground when it is frozen; soft and wet ground
(1) For the computation of seasonal freeze
or thaw penetration, freezing-and-thawing indexes are
conditions during thaw periods; surface and subsurface
used based upon degree-days relative to 32 F. For the
drainage problems; special behavior and handling
requirements for natural and manufactured materials at
average permanent structure, the design indexes should
low temperatures and under freeze-thaw action; possible
be those for the coldest winter and the warmest summer
ice uplift and thrust action on foundations; condensation
in 30 years of record. This criterion is more conservative
on cold floors; adverse conditions of weather, cost, and
than that used for pavements because buildings and
sometimes accessibility; in the more remote locations,
other structures are less tolerant of movement than
limited local availability of materials, support facilities,
pavements. It is important to note that indexes found
and labor; and reduced labor efficiency at low
from weather records are for air about 4.5 feet above the
temperatures.
ground; the values at ground surface, which determine
(4) Progressive freezing and frost heave
freeze-and-thaw effects, are usually different, being
of foundations may also develop under refrigerated
generally smaller for freezer conditions and larger for
ware- houses and other facilities where sustained interior
thawing where surfaces are exposed to the sun. The
below-freezing temperatures are maintained.
The
surface index, which is the index determined for
design procedures and technical guidance outlined in
temperature immediately below the surface, is n times
this chapter may be adapted to the solution of these
the air index, where n is the correction factor. Turf,
design problems.
moss, other vegetative cover, and snow will reduce the n
value for temperatures at the soil surface in relation to air
18-2.
Factors affecting design of foundations.
temperatures and hence give less freeze or thaw
a. Physiography and geology. Physiographic
penetration for the same air freezing or thawing index.
and geologic details in the area of the proposed
Values of n for a variety of conditions are given in TM 5-
construction are a major factor determining the degree of
852-4/AFM 88-19, Chapter 4.
difficulty that may be encountered in achieving a stable
(2) More detailed information on indexes
foundation. For example, pervious layers in fine-grained
and their computation is presented in TM 5-852-6/AFM
alluvial deposits in combination with copious
88-19, Chapter 6. Maps showing distribution of index
groundwater supplies from adjacent higher terrain may
values are presented in TM 5-852-1/AFM 88-19, Chapter
produce very high frost-heave potential, but clean, free-
1, and TM 5-818-2/AFM 88-6, Chapter 4.
c. Foundation materials.
The foundation
draining sand and gravel terrace formations of great
depth, free of excess ice, can provide virtually trouble-
design decisions may be critically affected by the
free foundation conditions.
foundation soil, ice, and rock conditions.
b. Temperature. The most important factors
(1) Soils.
(a) The most important properties of
contributing to the existence of adverse foundation
conditions in seasonal frost and permafrost regions are
soils affecting the performance of engineering structures
cold air temperatures and the continual changes of
under seasonal freeze-thaw action are their frost-heaving
temperature between summer and winter. Mean annual
characteristics and their shear strengths on thawing.
air temperatures usually have to be 2 to 8F below
Criteria for frost susceptibility based on percentage by
freezing for permafrost to be present, although
weight finer than 0.02 millimeter are presented in TM 5-
exceptions may be encountered both above and below
818-2/AFM 88-6, Chapter 4. These criteria have also
this range. Ground temperatures, depths of freeze and
been developed for pavements. Heave potential at the
thaw, and thickness of permafrost are the product of
lower limits of frost susceptibility determined by these
many variables including weather, radiation, surface
criteria is not zero, although it is generally low to
conditions, exposure, snow and vegetative cover, and
negligible from the point of view of pavement
insulating or other special courses. The properties of
applications. Applicability of these criteria to foundation
earth materials that determine the depths to which
design will vary, depending upon the nature and
freezing-and-thawing temperatures will penetrate below
requirements of the particular construction. Relative
the ground surface under given temperature differentials
frost-heaving qualities of various soils are shown in TM
over a given time are the thermal conductivity, the
5-818-2/AFM 88-6, Chapter 4.
volumetric specific heat capacity, and the volumetric
(b) Permafrost soils cover the entire
latent heat of fusion. These factors in turn vary with the
range of types from very coarse, bouldery glacial drift to
type of material, density, and moisture content. Figure
clays and organic soils. Strength properties of frozen
18-2 shows how ground temperatures vary during the
soils
freezing season in an area of substantial seasonal
freezing having a mean annual temperature of 37F
(Limestone, Maine), and figure 18-3 shows similar data
18-3
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