TM 5-852-4/AFM 88-19, Chap. 4
(b) As with footings, allowable
ward frost heave thrust occurs exceeding the combined
weight of the pile, load on the pile, negative skin friction
loadings of piles in frozen ground are determined by
in thawed zone(s) and the adhesion in the permafrost.
creep deformation which occurs under steady loadings at
(1) Friction piles.
stress levels well below the rupture levels measured in
(a) Analytical considerations and
ordinary relatively rapid tests to failure.
A creep
methods of making preliminary estimates of bearing
deformation rate of only 0.01 in./day will result in 3.65
capacity of piles which develop their support in skin
inches of settlement per year or about 3 feet in 10 years,
friction along their surfaces are discussed in this
which is wholly unacceptable for permanent type
paragraph. Bearing capacity of such piles should be
structures. Creep occurs in the adfreeze bond zone at
calculated for the ground conditions which exist in the
the contact surface between the pile and the frozen
most critical period of the year. In permafrost areas this
ground and is attended by punching at the pile tip if the
will usually be late summer through early winter when
pile is overloaded. The stress-strain behavior of frozen
permafrost temperatures at the depths of primary load
soil in unconfined compression tests may be used to
support are at their warmest. Allowable downward load
illustrate the deformation phenomena associated with
on piles supported in adfreeze bond in permafrost should
support of loads on piles in frozen ground. Response of
be computed in accordance with equation 14 (see right-
frozen silt to various conditions of loading is shown
hand side of fig. 4-77):
diagrammatically in figure 4-78 (the same relationships
Qa = 1 (Qp Qnf)
are represented in another form in fig 4-47). Elastic
(Equation 14)
behavior is limited to a negligibly small portion of the
FS
stress-strain curve. Nonelastic deformation begins only
where
a short distance from the origin and increases with
Qa = Allowable design load on pile
increase in stress. At rapid rates of loading or at low
FS = Factor of safety
temperatures, the stress-strain curves are relatively
Qp = Maximum load which may be developed
steep, relatively high stress levels are reached, and the
in tangential bond between pile and
deformation ends in brittle-type rupture, as shown by the
permafrost,
two left-hand curves in figure 4-78. At slow rates of
loading and warmer permafrost temperatures the curves
are flatter, lower stress levels are reached, and
deformation continues plastically to large strain values. If
rate of applied strain is reduced at a point such as A, the
stress intensity will tend to relax as indicated by curve AB
to a stress level compatible with the new, lower rate of
strain. In saturated, fine-grained frozen soils the peak
and ultimate strengths tend to be virtually identical when
where
loading rate is at a level producing extended
Ap = surface area of pile in permafrost
deformation, as shown by the two right-hand curves in
fa = maximum tangential adfreeze bond stress
figure 4-78. Peak stresses higher than ultimate strength
which may be developed between frozen
values are observed in saturated, granular frozen soils,
soil and pile, a function primarily of
even for slow rates of loading, but such soils less
temperature, everything else being equal.
frequently require pile-type foundations.
Qnf = Maximum skin friction force from thawed
(c) Figure
4-79
illustrates
soil on pile.
Under normal summer
diagrammatically the manner in which load applied at the
conditions this will be a negative force,
top of a pile is transmitted with time into relatively warm
acting downward:
permafrost. For simplicity, load is assumed carried
solely in skin friction in permafrost with zero load on the
tip of the pile. Conditions are shown in figure 4-79 for
three separate times after instantaneous application of
t1 represents a time immediately after load
load:
application, t2 represents a time intermediate between t1
and t3 and t3 represents time when complete stress-strain
adjustment has occurred under the applied load.
Immediately after load application, load transfer to
where
permafrost is concentrated in upper sections of the pile.
At = surface area of pile in thawed soil
Load transfer to lower parts of the pile is at that time
fs = sum of unit friction and adhesion between
restricted because shortening of the pile in compression
thawed soil and pile
is restrain-
4-129
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