TM 5-852-4/AFM 88-19, Chap. 4
the piles into the bearing stratum may be required.
soil, local tangential shear stresses on the surface of the
(3) Pile safety against frost heave.
pile may be substantially higher. For other pile and soil
types the 40 psi value may be adjusted proportionately to
(a) Analysis must be made to assure
factors noted in figure 4-82. Experience shows that the
that the pile is safe against frost heaving under the
surface of the pile within the annual thaw zone will often
normal sustained dead load, or when not loaded if this
show a thin coating of ice which may possibly control the
can occur during a freezing period. The latter is most
observed behavior of the system. Although clays are
likely to happen during construction.
capable of producing considerably higher heaving
(b) For heave stability a satisfactory
pressures than silt (see fig. 2-9), values of fh for clays
relationship as expressed in the following equation must
should be less than for silt because of the limited heave
be maintained under the most unfavorable conditions
rates possible with these low permeability soils and
(see lefthand side of fig. 4-77):
because of their weaker adfreeze bond potentials.
(c) The holding force in permafrost,
Qp, should be computed as in (I) above using average
sustainable adfreeze strength. A factor of safety must be
applied as outlined in paragraph h below.
(d) If the capacity of the pile is
insufficient to resist the frost thrust, Qh, the pile must be
redesigned to provide greater loading, increased
embedment or holding power in permafrost, or a
combination of these alternatives, or one of the heave
force isolation methods discussed in paragraph 4-3 may
be restored to. Where frost heave is possible, wood
where
piles should be installed butt down to increase safety
against heave. Piles which heave as the result of frost
An = surface area of pile in seasonally frozen ground
action destroy the adhesive bond of permafrost. Once
fh = adfreeze bond stress mobilized between frozen soil
broken this bond does not readily reheal and as little as
and pile by heave
1/2 or less of the potential adfreeze bond strength may
QL = effective load of structure, P, and pile, W
be available to support the imposed load and to resist
Qp = same as in equation 14 above, acting from l6 to f 7,
further heaving in subsequent years.
but stress mobilized in opposite direction
(4) Tension loading. It may sometimes be
Qt = skin friction of thawed soil on pile
desirable to use piles in frozen ground to resist tension or
uplift loads. Their advantage usually is their low cost.
However, the use of friction piles in frozen ground to
carry permanent tension loads should be approached
with great caution for the following reasons: friction piles
in frozen ground have inherent potential to fail
progressively in creep; piles in tension tend to
If the pile can experience loading in tension from the
experience reduction in friction by transverse contraction
structure the equation must be adjusted accordingly.
under load; frost heave forces act in the same direction
The nature and mechanism of the heave phenomenon
as the applied stress; permafrost degradation or even
and values of frost uplift pressures which can act at the
warming of ground temperatures during the life of the
plane of freezing are discussed in paragraphs 2-2 and 2-
structure may lead to failure; developing failure under a
4. The effects of frost heave on engineering structures
structure, if observed, may be very difficult to correct;
and methods of controlling these effects are discussed in
and failure, if it occurs, may be accelerative and
paragraph 4-3. Because a pile in tension contracts in the
catastrophic. Some of these adverse factors can be
transverse direction, there is a tendency for skin friction
eliminated by careful design. For example by means of
under high tensile load to be less than in compression.
a rod passing through the pile it can be placed in
As shown in figures 4-44 and 4-45, peak frost heave
compression rather than tension; in such case it may be
forces have been measured of 55,800 lb or 2220 lb per
better designated as an anchorage than as a pile.
perimeter-inch for an 8 inch steel pipe pile and 35,60 lb
Ample factors of safety can also be employed. However,
or 809 lb per perimeter-inch for a creosoted timber pile,
in lieu of friction piles positive gravity or mechanical type
both in frozen silt slurry. For steel piles in silt with ample
anchorages which insure mobilization of the required
moisture available, an average value of fh of 40 psi
mass of soil and are less sensitive to design,
should be assumed to act over the full depth of seasonal
construction and
freezing; in the coldest upper strata of seasonally frozen
4-139
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