TM 5-852-4/AFM 88-19, Chap. 4
ed by the surrounding frozen ground. As yield occurs in
content, the chemical composition of the pore water, the
the permafrost and the adfreeze bond, more and more
temperature, and the surface condition, shape and
compressive strain develops in the pile with depth,
length of the pile. With augered and slurried piles some
progressively readjusting the pattern of load transfer
control can be effected over the adfreeze bond strength
from pile to soil, the strain at the pile-permafrost
that can be developed, by controlling the type of pile
interface, and the tangential adfreeze bond stress. As
material and surface, the soil type and moisture content
shown in diagrams b. and c. in figure 4-79, the final
of the slurry, the mode of freezing, and the
distributions of load and strain along the embedded
characteristics of the water used to make the slurry.
length of pile in permafrost at time t, are approximately
With driven steel piles such control is not possible except
triangular. An assumed pattern of adfreeze bond stress
for removal of oil, paint, rust or scale from the pile
is shown in diagram d. of figure 4-79. As also shown in
surface before driving; however, there is no freezeback
this diagram, the adfreeze bond may be ruptured as a
delay or uncertainty, a common problem with slurried
result of excessive stress or excessive rate or magnitude
piles.
(g) Experimentally determined values
of strain, beginning at the top of permafrost. The stress
strain-time relationship and possibility for bond rupture
of average sustained and average peak adfreeze bond
are affected not only by the behavior characteristics of
strengths for frozen slurries made with silt of low organic
the frozen soil and the adfreeze bond zone, but also by
content in contact with steel pipe piles of 18 to 21-feet
the deformation characteristics of the pile. Piles which
lengths (in frozen soil) are shown in figure 4-82. Factors
exhibit high deformation per unit length under load or
for adjusting the curves for different types of piling and
which are especially long are more susceptible to such
slurry backfill, based on field and laboratory testing, are
bond rupture.
also shown in the figure.
(The curve "Average
(d) That a triangular distribution of
Sustainable Adfreeze Strength" with the appropriate
load and strain over the depth of embedment, reducing
correction factor for variation in pile and/or slurry type
to zero load at the tip, may be reasonably assumed as a
and with a procedure to be illustrated later may be used
basis for design in relatively warm permafrost is
for preliminary design and planning of pile load tests.)
demonstrated in figure 4-80, which shows load-deflection
Because shear strain along the surface of a loaded pile
data from a compression load test to failure on an 8-inch
varies along the length, decreasing downward from a
pipe pile compared with deflection computed on the
maximum at the ground surface, as has been illustrated
basis of the triangular load distribution assumption. The
in figure 4-79, such values measured on full scale piles
degree
of
correspondence
appears
especially
represent averages over wide ranges of development of
satisfactory when account is taken of the fact that
the stress-strain curve.
The average values are
application of factor of safety in the design will place
therefore always less than the potential maximum
working loads in the area of best agreement. It must be
adfreeze bond stress. Average adfreeze bond strengths
recognized that even though the load was added slowly
at ultimate pile bearing capacity are about 40 percent
over more than two weeks, in the test shown in figure 4-
greater than average sustainable adfreeze bond
80, the stress-strain adjustment under each 10-kip
strengths used in design (before application of any factor
increment was not 100 percent complete. In the test
of safety).
(h) Adfreeze bond strengths and
illustrated in figure 4-81, measured distributions of strain
in an 8-inch. I-beam pile with 10-feet embedment in
creep properties of slurry may range from those
permafrost under various levels of imposed loading also
characteristic of freshwater ice, through those of frozen
83
show the general triangular pattern .
sands, silts, clays and organic soils at various moisture
(e) For friction type piles of average
contents (depending on the type of material selected, the
lengths, bearing on the tip is small enough so that it can
water content at freezeback, and the manner of freezing)
be ignored if the tip diameter is relatively small (of the
to those of the same soils unfrozen, if freezeback is
order of 6 inches) or if the pile is placed in a dry-augered
incomplete or if permafrost degradation should occur. In
hole which is not flat-bottomed and/or if loose auger
temperature ranges a few degrees below 32 F, slurries
cuttings are unavoidably left at the bottom of the hole,
in which the ice fraction predominates may show better
thus requiring appreciable strain at the tip before full end
structural performance than slurries of some soils in
bearing can be achieved. If the tip diameter is relatively
which the soil fraction is more predominant, if the solute
large and if full positive end contact is assured, results
content of the added water is relatively low, depending
obtained by ignoring the load on the tip may be too
on the soil type. As shown in figures 2-12 and 2-13, ice
conservative. In such case, the pile tip load may be
has relatively high ultimate strength compared to most
computed as the sum of the first two factors in the
frozen soils at temperatures immediately
applicable equation of figure 4-61b.
(f) Effective unit values of the
strength of adfreeze bond between frozen soils and
foundation piles under long term loading depend
primarily on such factors as the type of soil, the moisture
4-131
Previous Page
