The equation for Tx predicts the maximum

temperatures occurring at particular depths. These

maximum temperatures do not occur simultaneously but

the assumption that they do is conservative for this

situation.

The diffusivity of the sand is greater than that of

The cohesion, c, of the frozen silt must be

the silt, which will induce higher temperatures in the sand

determined from creep test data at about 30F (average

layer than would result if all the soil were silt. An

temperature in the top 2 feet of silt as shown in fig 4-65.)

adjustment can be made for the layered soil condition by

Take failure cohesion at 25 years.

using the procedure outlined below. To determine

Unconfined compressive strength determined by

temperature distribution with depth in the sand, it is

conventional laboratory test is

necessary to convert the sand layer to an equivalent silt

layer. From page 38, TM 5-852-6/AFM 88-19, Chapter

14

6 , the thicknesses are proportional to the square roots

of the thermal diffusivities.

Results of laboratory unconfined compression creep

Diffusivity for silt,

tests are:

i.e. I foot of sand is equivalent to 0.77 foot of silt, as

regards temperature penetration under transient

conditions.

Values of Tx are computed for the entire depth of

possible interest assuming silt, and depth adjustments

are then applied in the sand layers. Computations for

selected depths are shown in table 4-7 and the

temperature distribution with depth is illustrated in figure

4-65. For use in settlement computations, a simplified

temperature distribution is also shown in the figure.

(d) Checking bearing capacity in critical period

of year. Use equation:

Since the gravel layer beneath the footing is a

high bearing value material, using D = 7 feet for depth of

footing might be too conservative. However, using D = 9

feet at the base of the gravel layer might not be

from figure 4-61, using terminology thereon, or from TM

conservative enough. Therefore, both values will be

5

5-818-1/AFM 88-3, Chapter 7 , for square footings.

checked below to determine the effect on indicated

capacity. For D = 7 feet, Ultimate Bearing Capacity, qu =

qu = 1.3c Nc + γDNq

1.3 c Nc + yDNq

From figures 4-61 and 4-62.