TM 5-852-4/AFM 88-19, Chap. 4
was employed to control degradation of permafrost
depths. The bearing stratum can competently support
111
under the structure .
the live and dead loads on a pile but cannot provide
(8) Foundations of the type shown in
sufficient anchorage to resist frost heaving forces unless
figure 4-42a are relatively immune to progressive frost
the pile is anchored at considerable additional expense.
action effects as the foundation readily returns each
Such conditions are quite common at bridges, where the
summer to its original position. However, foundations of
thermal regime of the permafrost is influenced by the
the types illustrated in figures 4-42b and c are
stream. In other situations, such as piles or poles which
susceptible to possible progressive jacking by frost
are lightly loaded or carry only transient loads, the depth
action. In these designs, irrecoverable deflections must
of embedment in permafrost required solely to resist the
be positively prevented.
frost heave forces may produce uneconomical design.
(9) Where fixed type foundations are used
Isolation may also be more economical in seasonal frost
as in figure 4-42c, detrimental frost action effects can be
areas.
controlled by placing non-frost-susceptible soils in the
(15) Heave force isolation may be
annual frost zone to avoid frost heave problems;
accomplished by casing the pile or foundation member
providing sufficient embedment or anchorage to resist
or backfilling around the foundation member with treated
movement under the heaving forces (sufficient integral
soil. The annulus between pile and casing is normally
strength must also be provided in foundation members to
filled with an oil-wax mixture which has a thick
ensure such forces); providing sufficient loading on the
consistency, as shown in figure 4-46a. Simple casings
foundation to counterbalance heaving forces; isolating
within the annual frost zone normally will be jacked
foundation members from uplift forces by various means;
progressively out of the ground by frost action.
or in seasonal frost areas by taking advantage of natural
Therefore, plates or flanges should be employed at the
heat losses to minimize adfreeze and/or frost heave
bottom of the casings to resist casing heave.
(here however, the possibility of future standby
(16) To avoid the difficulties and costs
deactivation of the structure without heat must be
involved with casing, a premixed backfill of soil, oil and
considered).
wax may be used to reduce frost heave thrust on the
(10) A non-frost-susceptible foundation mat
upper sections of the pile to acceptably low values, as
or backfill can be of substantial help in achieving desired
shown in figure 4-46b. This method of heave isolation
control. In addition to providing material which in itself
offers a somewhat greater lateral pile support than the
will not impart frost heaving forces to the foundation, the
casing and oil-wax method shown in figure 4-46a.
mat will impose a surcharge on the underlying soils to
(17) Coating the pile length in the annual
reduce frost heaving and provide a useful thermal barrier
frost zone with low friction material, as well as
which will avoid the extremely low temperatures in the
creosoting", may temporarily reduce frost heaving but
potentially adhering frost-susceptible underlying soils.
must not be relied on for this purpose in permanent
Maintaining warmer (though still below freezing)
construction. Chemical additives have also been added
temperatures decreases the adfreeze strength of the
to the active zone immediately around piles but were not
insitu soils.
found to be significant or of sufficient life to reduce
(11) All forces which might distort the
heaving. Various methods of providing additional shear
supported structure must be carefully anticipated. As
strength or anchorage in permafrost have been studied
discussed elsewhere some types of foundations are self-
to combat frost heaving of lightly loaded piles but were
adjusting for non-uniformities of frost heave.
found to be only partially effective. Some of the methods
(12) Thermal piles offer increased anchorage
investigated including notching the pile, driving railroad
against frost heaving through lowering of pile surface
spikes in timber piles, welding angle iron on flanges of
temperatures in permafrost. Self-refrigerating thermal
steel piles and providing plates on the base of various
piles also tend to reduce depth of summer thaw and
pile types.
furnish heat to the annual frost zone during the period of
(18) While a large surface area in
frost heaving, thereby reducing the adfreeze bond stress
permafrost is desired to provide a greater capacity, the
in the zone of frost jacking.
section of pile passing through the active layer should be
(13) Various methods of isolation are available
as small as possible. Changes in pile cross sections or
to reduce the extremely high adfreeze strength and
surface areas available may be accomplished by the use
upward forces imposed on piling, footings and other
of composite piles. Care should be taken in using
foundations by frost heaving.
composite piles to ensure that the pile has adequate
(14) Heave force isolation of piles is required
strength to resist tension. Reducing the surface area of
when there is insufficient length of pile in permafrost,
timber piles in the annual frost zone may be
particularly when warm permafrost is present. This
accomplished by placing timber piles butt down. In
condition is particularly common when bedrock or
addition to a small cross section in
another bearing stratum exists at relatively shallow
4-71
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