TM 5-852-4/AFM 88-19, Chap. 4
(a) Conventional or modified pile
may be used in colder heat input into the ground if
driving procedures including diesel and vibratory
alternative methods of installation are not feasible.
d. Freezeback of conventional piles.
hammers may be used to drive open-ended steel pipe
and H-section to depths up to 50 or more feet in frozen
(1) General.
(a) Piles and anchors in permanently
ground composed of silty sand or finer-grained soils, at
48,49,55,74,169
.
ground temperatures above about 25F
frozen ground attain their holding capacities only after
Some experience indicates that, under favorable
they are frozen solidly in place. Pile-supporting capacity
conditions, heavy pipe and H-sections can be driven into
in permafrost is dependent primarily on the strength of
ground at lower temperatures, as described in paragraph
the adfreeze bond between the permafrost and the pile
2-6. Although the pile is heated by the driving action and
surface. The strength of the bond is a function of
a thin zone of soil may be thawed at the soil/pile
temperature and is at its lowest and most critical value in
interface, the amount of heat thus introduced into the
the fall and early winter when permafrost temperatures at
permafrost is usually negligibly small and freezeback is
the levels in which the piles are supported are at their
normally complete within 15 to 30 minutes after
warmest. Therefore, any unnecessary transferal of heat
completion of driving. Because no drilling of pile holes is
from the structure to the foundation will tend to have an
required, because no slurrying is involved, and because
adverse effect on the supporting capacity.
In far
total installation equipment can be minimal, this
northern areas the reserves of supporting capacity and
installation technique can be even simpler than the dry
stability may be so large that small variations in heat
augered hole technique. However, the procedure is
input to the foundation will be of little consequence; in
limited to steel pipe and H-piles, and it is necessary to
marginal permafrost areas, however, the effect of even
make certain that sufficient driving energy is available to
small unanticipated heat inputs may be extremely critical.
reach the depths of penetration needed for bearing and
(b) Freezeback of slurry or otherwise
to resist frost heave. Templates should be used to
thawed soil surrounding piles must be assured before
assure accurate placement of piles.
imposing any load upon the pile. Thus, in addition to the
(h) The smallest H section to be
time required to install the pile, the construction schedule
considered for driving in frozen soil should normally not
depends on the time required for freezeback of the pile.
be smaller than 10BP42 and the rated hammer energy
If foundation piles are installed well in advance of the
should not be less than 25,000 ft-lb. When piles are
structural construction or if the permafrost temperature is
driven with conventional or vibratory hammers, advance
well below freezing, there may be adequate time
should be continuous, since there is a negligible amount
available for natural freezeback by permafrost. If the
of heat evolved, and stops longer than 5-10 minutes can
construction time is short and the work is to be
permit freezeback to the extent that resumption of driving
continuous or if the permafrost temperature is warm, use
will be impossible, or possible only after a prolonged
of artificial refrigeration or thermal piles may be required.
period of heavy driving. Necessity for stops to weld on
Thus, the foundation thermal conditions may determine
additional lengths should be avoided. The use of
both the design and the method of construction to be
chemicals, jetting or steaming should not be permitted
employed.
during driving although the pile may be preheated
(c) In order to measure the rate and
(particularly lower half) as it enters the ground to
effectiveness of freezeback of slurried piles, and to
minimize side friction during pile penetration.
permit
monitoring
of
subsequent
foundation
(4) Installation by steam or water thawing.
performance, thermocouple or thermistor assemblies
Until the early 1950's, piles were traditionally installed in
should be attached to representative piles or thermistor
permafrost by prethawing the ground at the pile locations
assemblies should be attached to representative piles in
by steam points before driving. An alternative was water
the foundation. When artificial freezeback is employed
thawing.
However, these techniques have the
using tubing attached to the pile exterior, the
disadvantage of introducing so much heat into the
temperature sensors should be placed midway between
ground that freezeback may be almost indefinitely
tubes where freezeback will be longest delayed. A
delayed. This involves not only the volume of permafrost
control thermocouple assembly installed at an adjacent
thawed by the steam or water, which is difficult to control,
undisturbed area and in equilibrium with the ground
but also the warming of a large volume of surrounding
temperatures is essential for comparison with
frozen material. The result may be failure to develop
temperatures at the piles.
To avoid conflicts all
adequate bearing capacity and/or progressive working of
monitoring equipment should be provided, installed and
the piles out of the ground by frost heave with
observed by the Government.
consequent damage to supported structures. Many such
(d) Since fine-grained soils tend to
failures have occurred. Therefore, steam or water
freeze or thaw at temperature levels depressed below 32
thawing should not be used in any area where the mean
F,
annual permafrost temperature is greater than 20F and
4-114
Previous Page
