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