TM 5-852-4/AFM 88-19, Chap. 4
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.
4-100
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