TM 5-852-4/AFM 88-19, Chap. 4
Average thaw-season surface temperature differential
to ensure maintenance of design ground temperature
conditions under the footings. Ground temperatures
vs. = nI = (1.0) (2900) = 19.3 (above 32F)
under individual footings should be as nearly the same
as possible in order to obtain uniform support. By
t
150
successfully achieving an n = 1.0 condition for the actual
Initial temperature differential
o = MAT -32 = 23-32 = 9F (below 32F)
V
foundation support area, for the thaw season, the
permafrost table may be expected to become somewhat
higher under the building than under adjacent non-
α =
9
=
0.47
shaded areas.
19.3
(c) Determining
temperature
Fusion parameter = s
V
distribution with depth below base of footing for critical
Average volumetric heat capacity, C = λd (c +
period of year. Given that the highest temperature at the
0.75 w/100)
top of permafrost is 32F and the permafrost
For silt: Dry unit weight, λd = 85 lb/ft .
3
temperature at a depth below the influence of annual
Moisture content of soil (percent of dry
temperature fluctuations is 27F it is assumed that
weight), w = 33 percent
erection and operation of the structure does not
Specific heat of dry soil, c = 0.17.
significantly affect the mean annual temperature at the
(Average value for near 32F; TM 5-852-6/AFM 88-19,
latter depth.
Studies of field data show that the
14
Chap 6 )
temperature of permafrost, Tx5 at depth X below the
3
C = 85[0.17 + 0.75 (33/100] = 35.5 Btu/ft
permafrost table may be determined from the
L = 144 (λd) (w/100) = 144 (85) (33/100) =
3
TX = 32 - (AO -AX)
4050 Btu/ft
where:
λ = 0.88 (from TM 5-852-6/AFM 88-19, Chap 6,
Ao =
amplitude of temperature wave that the
top of permafrost above the temperature at the depth of
fig. 13)
no annual variation.
Since the annual thaw zone includes both frozen and
In this case,
unfrozen soil except at the start and the end of the
Ao = 32 + 27 = 5F
thawing season, an average value of thermal
and from p. 36, TM 5-852-6/AFM 88-19, Chapter 6
conductivity, K, is the best approximation for this
Ax = Ao exp (-X√π/ap)
condition. Select individual K values from figures 3 and 4
of TM 5-852-6 (they may also be determined by test).
These have been shown in figure 4-62. Then,
where:
K
= [K
+K
ave
unfroz froz] = [0.68 + 1.2] = 0.94 Btu/ft
a = thermal diffusivity = K/C
hr
P = period of sine wave, 365 days
Estimated depth of thaw X = λ√48knI =
The footing size is in this case assumed to be
L
0.88√48(0.94)(1.0)(2900)
small enough so that the foundation temperatures are
not significantly affected by the differing thermal
4050
properties of the footing and underlying gravel.
The footing should be founded a foot or more below the
top of the permafrost, depending on the reliability of the
For frozen silt:
data used in the estimate and the degree of confidence
that the assumed thermal regime will be maintained. In
this case, a depth of 7 feet is used with a footing design
of the general type shown in figure 4-63. Because
stresses are most intense within about one to one and
one-half diameters below the base of the footing, as
shown in figure 4-64, temperatures within this depth are
most critical. By placing high-bearing value material
within the most critical part of this depth, as illustrated by
the gravel in figure 4-63, design certainty can be
increased.
At the perimeter of the building where transition
occurs from the shaded, cooler interior surface under the
building to the unshaded natural ground surface, the
building should cantilever out beyond the footings or
special shading should be provided for sufficient distance
4-98
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