TM 5-852-4/AFM 88-19, Chap. 4
Figure 4-83d. Example of computation of sustainable load capacity of pile in permafrost.
(Adfreeze bond load capacity vs. depth below top of permafrost
(o) Within reasonable limits
a
which has been described above because the stress-
strain behavior typified by the two left-hand curves in
deflection of the pile relative to the surrounding
figure 4-78 will apply rather than the extended-strain type
permafrost, which exceeds the minimum strain required
behavior shown in the two right-hand curves of that
to develop peak adfreeze bond stress at the top of
figure.
Even though higher stress levels can be
permafrost, is normal and acceptable in permafrost at a
accepted under rapid loading, reduced capacity for
temperature of about 20 to 25 F or warmer, provided
readjustment by creep may nullify this.
opportunity for gradual development of this displacement
(q) The computation of the allowable
by creep is available. The curves for slow loading shown
load on the pile should be completed using equation 14
in figure 4-78 typify this condition.
However, the
above and factor of safety from h below. TM 5-818-
possibility of complete rupture of the adfreeze bond in
1/AFM 88-3, Chapter 7 may be referred to for guidance
upper permafrost strata must be considered and special
concerning skin friction of thawed soil.
analysis should be made when considering piles of
(2) End bearing piles. As described in the
significantly more than 30 foot embedment in permafrost,
preceding paragraph, the point bearing (Qe in fig. 4-77)
or if the temperature of the permafrost when loads are
may often be assumed negligible. However, if a firm,
applied is colder than about 20 F, or full design load is
reliable bearing stratum such as ice-free bedrock is
suddenly applied on the pile. The criteria in this manual
within economical depth, the bearing capacity can be
are based on experience with piles of conventional
augmented by or solely derived from end bearing.
lengths of permafrost embedment, that is 15 to 30 feet in
Design procedures for end bearing piles should be the
permafrost at temperatures of 20 to 25F or warmer,
same as in temperate zone practice (TM 5-818-1/AFM
under gradual application of load.
20
(p) In permafrost of low and very low
88-3, Chap. 7' and/or EM 1110-2-2906 ) except that
safety against frost heave must be assured in
temperatures (colder than about 20F) unit adfreeze
accordance with the following paragraph. Drilling and
bond strengths are higher, allowable deflections are
anchoring of
lower, optimum pile lengths are less, possibility for
rupture of the adfreeze bond is increased, and the
patterns of distribution of load, strain, and stress along
the embedded length of pile may differ from the pattern
4-138
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