TM 5-852-4/AFM 88-19, Chap. 4
proportionately greater pile load capacity simply because
1/AFM88-3, Chapter 7. Full-scale load tests performed
of this increased surface. Although extraction tests to
on piles installed by the planned construction procedures
failure of H-piles show that such piles come out clean,
best integrate the variables involved. Figure 4-80 shows
i.e., without soil included between the flanges, the
an example of such a test, though the interval between
included perimeter should be used in design rather than
load increments is less than the 10 kips per three days
the actual surface, since yield in creep will tend to occur
which is recommended (see below).
However, if
on that basis.
sufficient time is unavailable in the construction
(1) Available data indicate that when steel
schedule, such deviation may sometimes have to be
piles are driven into permafrost by conventional
accepted and corrected for as indicated in the
methods, adfreeze bond over the pile surface area is
"Computation of Allowable Design Load" in figure 4-80.
less complete than is possible with a properly placed
If these tests cannot be performed at the design ground
slurry backfill. This conclusion is based both upon load
temperature (as is frequently the case for field tests),
tests and inspection, for evidences of contact, of piles
they may be adjusted to the design temperature by using
which have been completely extracted. Therefore, the
the applicable curve in figure 4-82 for guidance and
allowable load-bearing capacity of a conventionally
assuming that the strength for the test case varies as a
driven steel pipe pile should be reduced to 75 percent of
fixed percentage of this curve with temperature.
(m) Since the effective unit adfreeze
that for a slurried pile in which the slurry is made from
the same foundation soil mixed with fresh water. For H-
bond strengths are directly related to permafrost
type and other irregular section piles, the reduction
temperatures, reasonably accurate assessment of the
should be less since the pile surface area allowable is
permafrost temperatures with depth, for the life of the
partly direct pile soil contact surface and partly surface
structure, is required. The warmest temperatures with
through frozen soil as computed for the included
depth to be experienced in the life of the structure should
perimeter. For example, the allowable load bearing
be used for design. For ventilated pile foundations this
capacity for a driven 10 inches by 10 inches steel H-pile
normally will occur in the early part of each winter. If a
would be 87.5 percent of that for a slurried pile, if the
residual thaw zone should exist or develop, there will be
sustainable creep strength of the soil is approximately
no seasonal variation in permafrost temperatures and
the same as the unreduced adfreeze strength between
the permafrost will tend to eventually reach a thawing
soil and steel. However, if a pile is driven by a method
condition"'; in such a case the recommended procedure
which generates enough heat to produce a slurry film on
is to design for thawed-condition skin friction values. If
the entire pile surface in permafrost, no reduction is
the permafrost or slurry contains excess moisture in the
needed.
form of ice, these values will tend to be on the low side
(k) The
possibility
must
be
for the type of soil involved, and negative friction forces
considered that the natural foundation soil may have
may need to be considered. However, since thaw at
sufficiently low shear strength, as compared with the
depth in the ground is usually slow, consolidation will
adfreeze bond strength at the slurry/pile interface, to be
normally occur in small annual increments.
(n) If permafrost temperatures in the
the controlling factor in determining the load-carrying
capacity. However, since the perimeter of the drilled or
seasonal period selected for design are essentially
augered pile hole is ordinarily a minimum of 30 to 50
uniform with depth, the permafrost supporting capacity
percent greater than the perimeter of the pile itself, the
may be estimated (using fig. 4-82) by simply multiplying
natural frozen ground would have to be much weaker
the total surface area of the part of the pile in permafrost
than the strength at the slurry/pile interface for the
by the average sustainable tangential adfreeze bond
strength of the natural soil to be controlling. This is very
strength at that temperature, applying a correction factor
unlikely if the slurry is made from the natural foundation
for the type of pile surface and slurry if necessary.
soil. However, it may occur if select slurry backfill is
Should the ground temperatures vary appreciably with
used. It is possible to intentionally make the augered
depth, a more refined computation of the permafrost
hole larger than needed for pile placement purposes
supporting capacity may be made by plotting, first the
alone, in order to decrease stress at the outside
variation of average sustainable bond stress with
perimeter of the slurry cylinder when this condition may
temperature, then temperature with depth, then the
apply. However, the resulting increase in slurry volume
average sustainable bond stress for the applicable
would significantly retard freezeback time.
temperature with depth, and finally the sustainable
(1) For preliminary design purposes, the
adfreeze bond load capacity per foot with depth. By
average sustainable adfreeze strength values in figure 4-
determining the area under the latter curve as shown in
82, adjusted by the appropriate factor if necessary,
the right hand diagram of figure 4-83, the potential pile
should be used. Normal negative frictional resistance
load capacity is obtained.
values (combined friction and cohesion) for unfrozen soil
may be determined from guidance given in TM 5-818-
4-135
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