TM 5-852-4/AFM 88-19, Chap. 4
CHAPTER 2
BASIC CONSIDERATIONS AFFECTIVE FOUNDATION DESIGN
permafrost is not far below freezing. Here tangential
2-1. Thermal effects.
adfreeze and other strength values are low, and creep
a. As indicated in chapter 1, ground
rates are high. In far northern areas of continuous
temperatures and presence or absence of permafrost
permafrost and lower ground temperatures, strength
are the product of many interacting variables. In addition
values and creep factors are more favorable.
to reflecting the effects of such factors as snow cover, air
e. Ground temperatures and relative thermal
stability at sites in permafrost areas are important not
temperatures of a specific area can be related to the
only in determining the amount and rate of progression
recent history of the terrain. For example, in flood plains
of permafrost degradation which may be initiated by
of meandering streams permafrost may be absent in
construction, but also the rate of creep closure of
areas which were under water in recent decades and
underground openings and the extent and dimensions of
present elsewhere. Again, destruction of vegetation by
protective measures such as ventilated foundations or
forest fires in past years may have produced special
refrigeration, which are required for heat producing
subsurface temperature conditions.
facilities. Ground temperatures influence installation
b. The fundamental properties of soil or rock
procedures and the rate of freezeback of slurried piles.
which determine the depths to which freezing and
The dynamic response characteristics of foundations are
thawing temperatures will penetrate below the ground
also a function of ground temperatures.
surface under given temperature differentials over a
f.
Thaw of permafrost from below will result if
given time are the thermal conductivity, the volumetric
a long-duration increase occurs in the surface ground
specific heat capacity, and the volumetric latent heat of
30
temperature. This may be visualized by assuming that
fusion . These factors, defined in TM 5-852-6/AFM 88-
14
the entire temperature gradient curve in figure 1-4 is
19, Chapter 6 , vary in turn with type of material,
197
moved to the right. However, as Terzaghi has shown,
density, and moisture content. Figure 2-1 shown typical
the natural heat flow out of the earth can at most
values of thermal conductivity vs dry unit weight and
produce only quite slow upward thaw of permafrost, that
porosity. More detailed plots are presented in TM 5-852-
14
is, somewhat under 2 cm/yr for permafrost containing 30
6/AFM 88-19, Chapter 6 The specific heat of most dry
percent ice by volume. If a normally developed and
soils near the freezing point may be assumed to be 0.17
stable geothermal gradient exists in the permafrost and
Btu/lbF. Specific heats of other construction materials
14
the foundation is designed to maintain original
are given in TM 5-852-6/AFM 88-19, Chapter 6 .
permafrost temperatures, there will be no thaw from
c. Permafrost will thaw when sufficient heat is
below. If the permafrost is in process of warming from
transferred to it from the underside of a structure. The
prior colder climatic conditions or is even in an
rate of thaw depends on the temperature of the building,
isothermal condition at the thawing temperature, as may
the insulation in the floor, air space or ventilation
sometimes happen, thaw will occur from below at rates
provisions, layers of special material (such as gravel) on
up to the maximum possible under the geothermal
the ground, and the natural characteristics (such as
gradient existing in the sub-permafrost materials. In any
moisture content and temperature) in the annual frost
case, thaw of permafrost from below is not normally a
zone and permafrost. Insulation retards and reduces but
significant factor in design of foundations for structures,
cannot prevent heat flow from the structure. Thaw will
except that a flow of warm sub-permafrost groundwater
progress in permafrost below a heated structure unless
might rarely present a special situation.
provisions are made for removing the escaping heat
g. Designers must also keep in mind that both
such as by an air space beneath the building, natural or
manufactured and natural construction materials
forced circulation in ducts, or even artificial refrigeration.
experience significant linear and volumetric changes with
During the winter, such a system must provide sufficient
changes in temperature. Linear coefficients of thermal
cooling and refreezing of any foundation material
expansion for some common materials are shown in
warmed or thawed during the summer so that
table 2-1. Note particularly that values for asphalt and
progressive thermal changes and degradation will not
ice are mush higher than for soil or rock. The higher the
occur.
percentages of asphalt or ice the greater the degree of
d. Ground temperatures influence bearing and
adfreeze strengths and creep rates of the permafrost.
Normally, in areas of discontinuous permafrost, where
the most difficult foundation engineering problems are
encountered, the mean annual temperature of the
2-1
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