TM 5-852-4/AFM 88-19, Chap. 4
Q = L w γd (Equation 8)
assume allowable tangential adfreeze bond stress
corresponding to whichever is the weaker, under the
critical permafrost design temperature, of pure ice or
where
consolidated slurry. (For further discussion of strength of
L = latent heat, 144 Btu/lb of water
ice vs. strength of frozen soil, see f(1) below.) This
w = water content, expressed as decimal
λd = dry unit weight, lb/ft .
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problem will not arise when piles are installed in
permafrost by driving.
Examples are as follows:
(f) While it may be possible to avoid
For w = 80 %, λd = 53, Q = 144 (0.80)(53) =
formation of an ice layer on the pile during installation at
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6100 Btu/ft
low temperatures by use of non-frost-susceptible slurry a
For w = 40%, λd = 80, Q = 144 (0.40)(80) = 460
number of problems may cause difficulty in achieving this
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Btu/ft
result. In the first place, a slurry conforming to the
For w = 19%7, λd = 109, Q = 144 (0.19)(109) =
common definition of non-frost-susceptible material
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3140 Btu/ft
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presented in TM 5-818-2 is not necessarily completely
(c) Thus, a silt slurry or one with an
non-frost-susceptible. That criterion assumes that a
excess of water may introduce considerably more heat
certain low level of frost susceptibility is tolerable in
than a sand slurry or one in which the amount of water is
pavement applications and is based upon freezing rates
carefully controlled.
experienced under pavements. Validity of the criterion
(d) When the slurry moisture content
for slurry freezeback conditions has not been
is carefully controlled, the slurry will retain relatively
investigated. Also, under field conditions it may be
uniform characteristics after freezing. However, if the
difficult to insure that some contamination with fines may
slurry has an excess of water, consolidation of the soil
not occur from contact of the above-freezing slurry with
component may result in separation of excess water
the wall of the drilled hole, or from other sources.
from the slurry. Even while freezeback is proceeding
(g) Some test pits made around piles
inward from the wall of the hole, bridging of the soil in the
which have been in place a year or more have shown a
relatively narrow annular space may result in formation of
layer of ice on the pile extending from the ground surface
essentially soil-free slugs of water between masses of
through the annual thaw zone, attributed to segregated
consolidated slurry. Because of their high heat content,
freezing of seasonally thawed moisture. Such an ice
the water inclusions will freeze back more slowly than the
layer limited to the annual thaw zone is not significant in
consolidated slurry. If the water inclusions occur within
terms of the pile bearing capacity.
the annual thaw zone, they may thaw and escape to the
(h) Measurements on 8-inch steel
surface in subsequent seasonal thawing, even though
pipe piles exposed to the atmosphere and sunlight above
frozen initially, and settlement of the overlying slurry may
the ground in a region of borderline permafrost have
then occur, requiring backfilling of the resulting
shown that thawing may typically reach several inches
depression around the pile.
below the top of permafrost immediately adjacent to the
(e) Freezeback of slurry proceeds
pile surface at a location where the permafrost table is
primarily from the wall of the hole inward toward the pile.
3.8 feet below the ground surface. An additional few
If the pile itself is below freezing temperature, freezing
inches may have especially low tangential adfreeze bond
may also occur from the pile surface outward, particularly
strengths. Therefore, the assumed effective length of
if the pile is a pipe type open at the surface to admit air at
embedment in permafrost of all properly installed dark-
low temperatures. When the slurry is composed of frost
surfaced piles exposed to sunlight should be reduced by
susceptible fine-grained soil, multiple small ice lenses
a nominal 15 inches. No reduction is required for piles
will form during freezeback; these are oriented vertically,
completely shaded, shielded, or painted a highly
parallel to the wall of the hole. An annular layer of ice
reflective white, regardless of type of pile. For piles
normally forms at the contact between the slurry and the
improperly installed, as by uncontrolled steam thawing,
wall of the hole, sometimes as much as an inch thick;
no valid guidance can be given.
this has no significant effect upon the pile bearing
(i) Knowledge of ground temperature
capacity. A similar layer of ice may also form on the
with depth is essential to estimate the freezeback time
surface of the pile; because the ability of the material at
and overall effect of the installation on the permafrost.
this contact surface to endure tangential shear stresses
Plots showing seasonal variation of depths of isotherms
is controlling in determining allowable pile bearing
in the ground or plots of temperature with depth may be
capacity, the occurrence of such an ice layer may be
used to select the optimum installation period for rapid
significant. If piles of any type are placed during below
freezeback. Available methods of computing natural
freezing air temperatures by the slurry method it should
freezeback of piles in permafrost assume the slurried
be assumed, unless evidence can be presented to the
pile to be a finite cylindrical heat source inside a semi-
contrary, that an ice layer, however thin, will form on the
surface of the pile. In such cases it will be necessary to
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