TM 5-852-5/AFR 88-19, Volume 5
and thawing cycle on an annual basis. In the spring
the active layer.
-- The active zone portion of the hole is
this zone may go through several freezing cycles
backfilled with non-frost-susceptible materials
due to warm days and cold nights. This freezing
or this portion of the pole is wrapped with a
causes significant expansion, depending on the soil
10-mil polyethylene sleeve to prevent
type and moisture content. The expansion is very
development of a bond.
significant with fine-textured silty soils when a
The "rule-of-thumb" for poles in the 50-foot range
source of unfrozen water is available. Further details
is to place 10 percent of the above-ground height
on this soil "heaving" can be found in TM 5-852-
plus another 4 feet into the permafrost (example-50
4/AFM 88-19, Chap. 4. The practical effect on
feet above ground height would require 9 feet of
towers and poles is to apply an upward force to the
embedment). A "rule-of-thumb" for shorter poles is
unit that may result in overstress and mechanical
to provide an embedment in permafrost for a depth
failure or in differential vertical movement between
equal to about 2-1/2 times the thickness of the
components. It is necessary to design the towers or
active layer. Holes for these poles or support piles
poles to resist these upward forces, or to allow the
are made with a drill or soil auger and are made
units to "float" up and down with the expansion and
slightly larger (3-4 inches) than the diameter of the
contraction caused by heaving of the active layer, or
pole. A slurry of native soil or sand with water is
to replace the frost-susceptible soils with clean
then placed around the pole to the top of
gravel. In many cases the utility pole will move up
permafrost. This construction is often done in the
due to the heaving forces but cannot return to its
winter when the active layer is also frozen. This will
original position because of the flow of soil into the
allow easier access with minimal environmental
void. The net effect is an annual increment of
disturbance. Rock-filled cribs (fig 11-3) are used
upward movement that will eventually "jack" the
where permafrost is very deep, or rock very
pole out of the ground.
shallow, or for temporary or semi-permanent
a. Towers. Typical designs for tower foundations
construction. Tripod or "tepee" pole configurations
on gravel pads and on or in frost-susceptible soils
using local tree saplings or poles in gravel-filled
are shown in figures 11-2 and 11-3. Technical
drums have been used for lightweight wires and
details for these designs are discussed in TM 5-852-
cables in expedient situations.
4/AFM 88-19, Chap. 4. The above-surface gravel
c. Anchors. Anchors for tower guy wires will be
pads provide some surcharge for resistance to heav-
designed in accordance with TM 5-852-4/AFM 88-
ing forces but some vertical movement is likely. At
19, Chap. 4. The major concern is progressive
the end of the thaw period the pad will settle to its
movement or "creep" of the anchor in ice-rich soils
original position. The anchors for guyed towers
with temperatures just below the freezing point.
provide the major resistance to uplift and provide
Manufacturers' ratings for design capacities of
lateral stability. If the footings for towers are placed
commercially available earth anchors will be reduced
in the frost-susceptible material they will be moved
by 75% if placed in thawed soil above the
upward during the heaving phase, but as described
permafrost layer. As described in TM 5-852-4/A
in the previous section, the footing will not then
FM 88-19, Chap. 4, special helical anchors have
settle back to its original position when seasonal
been developed for installation in permafrost.
thawing is complete. A progressive failure will result
because the footing will be moved upward another
11-5. Special considerations.
increment each year until the resistance to
There are other special considerations that relate to
overturning is insufficient.
construction of electrical distribution systems in the
b. Poles. A very strong bond can develop
cold regions due to responses to low temperatures
between the frozen soil and the surface of an
or other environmental factors:
imbedded power pole. This bond, if developed in
a. Nylon-jacketed conductors (type THWN),
the active layer with frost-susceptible soils, will lift
when used at low ambient temperatures, tend to
the pole out of the ground. Wooden poles are
experience separation of the insulation from the
commonly used for both power and communications
jacket.
systems. Two measures are commonly taken for
b. Molded case circuit breakers and stored
permanent construction where permafrost is
potential switches are not always dependable at ex-
relatively close (3-5 feet) to the surface:
tremely low temperatures. The alternative use of
-- Sufficient imbedment in permafrost is pro-
fuses should be considered or supplemental heat
vided so that the bond developed in that zone
provided to raise the ambient temperature of the
can resist the uplift forces due to heaving in
11-2
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