TM 5-852-4/AFM 88-19, Chap. 4
(b) Driven piles, which introduce
parameters influencing the rate of freezeback. Method
of computing freezeback time for a given refrigeration
negligible amounts of heat, have no critical spacing other
14
capacity is given in TM 5-852-6/AFM 88-19, Chap 6 .
than that required to facilitate movement and operation
By limiting the time between slurry placement and start of
of the driving equipment or that introduced by possible
refrigeration to less than a day, the heat gain by the
group action effect in the foundation.
(c) Slurried piles, however, produce
surrounding
permafrost
can
be
minimized.
134
Establishment of a proper freezeback criterion is very
an overall rise in permafrost temperature . The effect
important. Very low temperatures can be produced at a
of pile spacing on permafrost temperature rise at
given moment close to the refrigerant tubing but the soil
different slurry heat values is illustrated in figure 4-76.
may still be unfrozen several inches away. Therefore,
The relationship between temperature rise and slurry
the duration of the refrigeration period should be
heat, as influenced by the volumetric heat capacity of the
established as that which when suspended for 24 hours
permafrost and spacing, is given by the equations in the
will produce frozen ground temperature at the critical
figure. If the rise in permafrost temperature (AT)
freezeback location no greater than the normal ground
indicated by figure 4-76 should exceed the difference
temperature at that position. The controlling depth
between the freezing point and the initial permafrost
where the freezeback is slowest is often about 20 feet,
temperature (Tf-Tp) the permafrost can not freeze more
but may be anywhere between the top of permafrost and
than the amount of slurry water which will raise the
the bottom of the pile. Temperatures may also need to
permafrost temperature to its thawing point.
Heat
be monitored simultaneously at two or more different
exchange cannot occur when permafrost and slurry are
depths for control.
When the required period of
at the same temperature. The remaining slurry will not
refrigeration has been established for one or more
freeze until the surrounding permafrost becomes colder.
monitored piles, it is thereafter necessary to monitor
Actual freezing or thawing temperatures of the materials
freezeback on only a limited number of selected
should be used in the analysis where these differ from
production piles for spot check purposes.
Careful
32F.
(d) No factor of safety is incorporated
records should be kept of the freezing plant and ground
temperature observations.
in the pile spacing effect equations and chart presented
(b) Unless refrigeration is to remain
in figure
-I 4-76.
Therefore, it is essential that
permanently in operation, refrigerant tubes on the piles
temperature indicating devices be required as part of the
should be filled with arctic engine oil chilled to below
design to verify freezeback during construction (again
existing ground temperatures and sealed when the
taking into account the freezing characteristics of the
refrigeration period is completed. Should refrigeration be
soil).
required at a later date, oil can be removed and the
(5) Period of installation.
refrigeration system reactivated with minimum effort; in
(a) The natural freezing rate of
the interim, the oil provides protection against corrosion
slurried piles is primarily dependent on initial ground
and ice blockage.
temperatures of the permafrost and the spacing of the
134,137
(c) Internal refrigeration of pipe or
. As illustrated in figures 1-1 and 1-3, the
piles
other hollow piles can be accomplished by use of
coldest ground temperatures are experienced in the
automatic or forced circulation thermal piles as described
spring. In areas of marginal permafrost, permafrost
in e below.
temperatures are so high that there is insufficient
(d) Artificial freezeback can also be
"reserve of cold" in the permafrost to insure natural
accomplished by the evaporation and expansion from
freezeback of slurried piles except in spring
the liquid or solid state of gases vented to the
(approximately February, March, April and May). If
atmosphere.
Such gases include nitrogen, carbon
slurried piles must be installed in marginal permafrost
dioxide, propane and other similar materials. Dry ice
areas at other times of the year, artificial refrigeration
has, for example, been placed in pipe piles to effect rapid
must be employed to insure slurry freezeback. If
freezeback. Such means of rapid freezeback are usually
freezeback is not completed before the refreezing of the
too expensive for use in large installations but can be
annual frost zone starts in the fall, and frost heaving
effectively employed in small installations.
occurs, the adfreeze bonding required for support of the
(4) Relation of pile spacing to freezeback.
design load may never be achieved. On the other hand,
(a) The spacing of piles is normally
steel piles may usually be installed in fine-grained
based on structural requirements of the floor system or
permafrost soils by driving at any time of the year in
on the need to provide a sufficient number of piles to
these areas without freezeback problems.
Where
permafrost temperatures are below about 25F,
support relatively large concentrated vertical loads.
Consideration of pile spacing in early phases of the
installation by driving will usually be impractical, thus -
structural design may make it possible to provide
requiring a slurried type of installation. However, at
sufficient distance between piles so that in permafrost
areas natural freezeback can be utilized to effect
substantial savings.
4-122
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