TM 5-852-4/AFM 88-19, Chap. 4
Under conditions of natural cover in very cold areas, its
conditions which may be anticipated for specific
thickness may be as little as 1 foot or less; thickness
situations, more detailed explanation of the factors
may vary over a wide range even in a relatively limited
influencing freeze an thaw penetration, and typical values
geographical area.
of n are presented in TM 5-852-6/AFM 88-19, Chapter 6
14
.
(1) Seasonal variations in properties and
(6) Because temperature inversions and
behavior of foundation materials are caused primarily by
steep temperature gradients are common in levels of the
the freezing, thawing, and redistribution of water
atmosphere
nearest-the
ground,
temperature
contained in the ground and by the variations of stress-
differentials of as must as 50F may be found at a given
strain characteristics and thermal properties with
time at different local topographical positions. Within the
temperature. The water may be present in the voids
range of ground elevations subject to temperature
before freezing or may be drawn to the freezing plane
inversions, mean temperatures may actually increase
during the freezing process and released during thawing.
rather than decrease with increasing elevation. For such
Seasonal changes are also produced by shrinkage and
reasons, it is important to determine design indices for
expansion caused by temperature changes.
the specific site topographic position.
(2) Below the zone of seasonal effects the
(7) Anything which is done in the course of
temperature gradient usually averages 1 F for 40 to 50
construction is likely to alter the temperature conditions
feet of depth, although it may range from about 1 F in
at the surface of the ground and, as a consequence, to
15 ft to 1 F in 135 feet. Since foundation work rarely
change the thickness of the annual frost zone and the
extends below a depth of about 30 feet, most foundation
depth to the top of permafrost, and ultimately, possible
design is concerned with the environmental effects
even to affect the existence of the permafrost.
encountered in the upper 30 ft.
d. Wind and other factors. Mean annual wind
(3) The
penetration
of
freezing
temperatures into the ground depends upon such factors
speeds for most arctic and subarctic locations are
as weather, radiation, surface conditions, insulating or
usually of the order of 5 to 10 mph except in coastal
other special courses, soil properties and soil moisture.
areas where the mean is usually 10 to 20 mph. In
43, 83, 181, 185
The most important weather conditions are
mountainous regions wind speeds are generally greater
air temperatures and length of freezing season. These
than those in the plains. Local katabatic winds with
may be combined into indices, based upon accumulated
velocities up to 100 mph or more are not uncommon,
degree-days as explained in TM 5-852-1/AFM 88-19,
particularly along sea coasts. Even though velocities of
10
Chapter 1 .
arctic winds as a whole tend to be low, combination of
(4) It is important to note that the indices
very cold temperatures with wind causes extremely large
found from weather records are for the air about 4-1/2
heat losses from buildings, equipment, and personnel in
feet above ground. The value at the ground surface,
winter. Wind chill values representing the combined
which determines frost effects is usually different, being
effects of wind and temperature are given in TM 5-852-
generally higher for thawing and lower for freezing, and is
1/AFM 88-19, Chapter 1 '. Table 1-1 relates wind chill
the composite result of many influencing variables, some
values to human working conditions.
Drifting and
of which have been mentioned in (3) above. The surface
blowing of snow often creates major construction and
index, which is the index determined for temperatures
operational problems, even where the actual precipitation
is very low. It is of fundamental importance to anticipate
immediately below the surface, is n times the air index,
where n is the correction factor. For paved surfaces kept
such problems in planning and design stages. It is often
possible to reduce greatly adverse effects of drifting and
cleared of snow and ice, n may usually be taken as 0.7
blowing snow by proper site selection and layout alone.
for freezing. Other values are give in chapter 2. Turf,
e. Solar radiation. As previously indicated,
moss, other vegetative cover and snow cover with
solar radiation is an important factor in the thermal
reduce the n value for temperatures at the soil surface in
stability of foundations in arctic and subarctic areas. Net
relation to air temperatures, and hence frost penetration
summer radiational input into the ground may range from
will be less for the same air freezing index.
10
almost nothing in very cloudy or shaded locations to a
(5) TM 5-852-1/AFM 88-19, Chapter 1
predominant part of the summer heat flow at others. It is
gives the approximate distribution of mean air-freezing
a function of latitude, cloud cover, time of year, time of
and air-thawing intensities in North America. More
day, atmospheric conditions, wind speed, subsurface
detailed information for northern Canada is given by
199
64
thermal properties, degree of shading, if any, and aspect,
As demonstrated by Gilman, highly
Thompson.
88
albedo and roughness characteristics of the surface .
useful summaries for local areas can be prepared when
sufficient weather data are available.
Calculation
methods for determining the freezing and thawing
1-6
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