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-