TM 5-852-4/AFM 88-19, Chap. 4
(2) Natural freezeback.
theoretical computations of freezeback and pile spacing
(a) Soil-water slurries placed in drilled
involving such materials should use the actual freezing or
thawing temperatures, rather than 32 F.
or augered holes introduce heat, which in natural
This is
freezeback is conducted into the surrounding permafrost.
especially
important
when
normal
permafrost
temperatures exceed about 29 F to 300F. If either the
The heat content of the water, soil, and pile can be
computed, if the water content and dry unit weight of the
slurry material or the permafrost is other than silt, sand
slurry are known or determined by experiment. Slurry
or gravel in which practically all the moisture freezes at
placed at temperatures from 32 F to 40 F, under
the nucleation temperature, the volumetric latent heat of
normal conditions, has a total heat content above 32F of
the slurry and/or the volumetric heat capacity of the
3
less than 200 Btu/ft ; this is small enough to be merely
permafrost within the range of the placement,
approximated and added to the latent heat in the
freezeback, and thermal adjustment temperatures will
computation procedures outlined below.
have to be determined by test.
The freezing
(b) The latent heat per foot of pile
characteristics of the soil can be ascertained in the
length is computed by the equations shown in figure 4-
160
laboratory by generating a cooling curve with time or
70. Note that the latent heat is governed only by the
by calorimetry; they can also be inferred from study of
volume of slurry, the water content (w) and dry unit
natural in-ground temperatures during seasonal freeze-
weight (d). Thus, the heat input into the permafrost can
thaw flux. In important projects, test piles instrumented
be minimized by control of the dimensions of the annulus
with thermocouples or other temperature indicating
(which is also a function of the type of pile) and by
devices should be used to verify the freezeback potential
selection and control of the slurry. Simple comparisons
of the permafrost prior to actual construction. Since test
of the amount of latent heat per unit volume of slurry can
conditions can seldom be identical with the actual
be made using the following equation, assuming all the
construction ground temperatures, the results must
water freezes:
usually be analytically transformed to the construction
conditions.
U. S. Army Corps of Engineers
4-115
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