TM 5-852-4/AFM 88-19, Chap. 4
vide a large surface area and holding capacity within the
tensile stresses sufficient to crack the piles and expose
permafrost but with a small circumference and area
the reinforcing steel to corrosion.
Steel in lightly
exposed to heaving forces within the annual frost zone.
reinforced and lightly loaded concrete piles may be
Composite piles employing wood as either the upper or
stretched and substantially by frost heave forces,
lower member are not recommended unless there is no
causing multiple cracking The upward forces may be
possibility that frost heave forces will act on the pile,
double the design loadings, causing complete stress
because of the difficulty of providing a joint capable of
reversal. Therefore, if the piles will be subject to heave
resisting high tension forces under the effects of heaving.
forces, careful analysis should be made to insure that the
The type of pile selected will depend on initial costs,
amount of reinforcing steel in combination with structural
shipping costs, installation method, load levels,
loading is sufficient to prevent cracking. Pretensioned
resistance to corrosion, difficulty in splicing or cost and
precast piles may be advantageous.
difficulty of providing installation equipment, labor
(b) Cast-in-place
concrete
piles
availability and other factors. Displacement piles, which
should not in general be used in permafrost because of
densify or force aside a relatively large volume of soil as
the hazards of either thawing the permafrost or freezing
they are driven, can be used only in thawed soils.
of the concrete. When such piles may be required in
(1) Timberpiles. Timber piles are normally
special cases, approval of HQDA (DAEN-ECE-G),
less expensive than other types, easy to handle and
WASH DC 20314 or HQUSAF/PREE, WASH, DC should
normally readily available in lengths from 30 to 70 feet.
be obtained based on field test evidence.
Timber piles frozen into saturated permafrost soils are
(c) The average compressive stress
durable for centuries. Their structural characteristics are
on any cross section of a pile should be in accordance
5
with requirements in TM 5-818-1/AFM 88-3, Chapter 7 .
discussed in paragraph 2-6. The maximum allowable
(d) Precast piles may be round,
average compressive stresses on the cross section of
square, multisided or double T's. Handling, transporting
round or square timber piles are given in TM 5-818-
5
1/AFM 88-3, Chapter 7 .
and cutoff of concrete piles may be relatively expensive.
(4) Special types of piles.
(2) Steel piles. H-piles and pipe piles are
the most useful types of steel piles, although box
(a) To assist freezeback at time of
sections and angles have been used. Pipe or shell piles
installation and to increase the degree of thermal stability
filled with concrete or sand may be used in some
under service conditions during period of below-freezing
designs to provide high load capacity. Pipe piles that are
weather, several special types of metal pipe piles may be
capped or closedended at the bottom may be installed in
considered. Such piles, called as a group thermal piles,
premade holes but cannot be driven in permafrost as
including self-refrigerated piles, serve not only to carry
displacement piles. Open-ended steel pipe and H-piles
structural loads, but also to remove heat from the ground
can be driven in great lengths, can be readily cut off or
surrounding the embedded portion of the pile, and move
made longer and can carry high loads. The average
it upward to the surface where it is dissipated to the
compressive stress on steel pipe and H-piles under the
atmosphere. In some cases only the heat removal
120
design load should not exceed 9000 psi . For steel
function may be needed.
shells less than 1/10 in. in thickness, no contribution to
(b) In the two-phase system which
bearing capacity from the shell should be credited. For
operates on an evaporation-condensation cycle,
steel shells 1/10 inch thick or greater, stress should not
analogous to a steam heating system with gravity
120
exceed 9000 psi . Laboratory investigations should be
condensate return, the pile is charged with propane,
made of the soils and water to which steel piles will be
carbon dioxide or other suitable evaporative material"'.
183,184
.
exposed to determine if corrosion will be a problem
Evaporation of this material by heat flow from the ground
If corrosion protection is required, steel piles should be
and its condensation in the portion of the pile exposed to
protected by a coal-tar preservative over a lead based
cold winter temperatures above the ground surface
primer from the pile cap to not more than 5 feet below
provides the heat transfer mechanism. Finned radiation
the long term permafrost table. No corrosion protection
surfaces are commonly employed above ground.
is required, nor should it be used, below the latter depth
Condensate returns by gravity to the liquid reservoir at
in frozen soils. Reduction of the potential adfreeze bond
the lower end of the pile.
may be expected as a result of shear failure in the
(c) In the single-phase system or
23
coatings applied to the pile surface . Any portion so
liquid convection cell, analogous to a gravity hot water
coated extending below the permafrost table should be
heating system, the pile is completely filled with a
discounted in computing bearing capacity or frost heave
suitable nonfreezing fluid, and heat is moved upward
from the ground by a natural circulation induced by a
(3) Concrete piles.
density gradient of fluid resulting from the temperature dif
(a) Concrete piles should not be used
under conditions where frost heave forces may produce
4-111
Previous Page
