TM 5-852-4/AFM 88-19, Chap. 4
sand. Clays are difficult to mix and place and do not
hole depth and permitting the lower portion to freeze
achieve good adfreeze strength values.
Gravel,
back before completing the backfill. Care must be taken
unsaturated soils or only water (ice) should not be used
not to coat the pile or hole wall with the slurry above the
for backfill. Concrete should not be used to backfill
level of the backfill since it will freeze in place, and tend
around piles in drilled or augered holes in permanently
to prevent complete filling of the voids when the final lift
frozen ground. Organic matter (peat) should not be
is placed later. To avoid flotation no additional slurry
permitted in the material used for preparation of the
backfill should be placed around the pile until the pile has
slurry. Strength and creep properties of slurries are
frozen solidly in place.
(f) Backfilling may also be accomplished
discussed in more detail in f(l) below.
(c) Slurries are normally prepared in
by filling the hole with silt-water slurry, just prior to
portable concrete mixers by adding sufficient water to
placing the pile, sufficiently to bring the slurry to the
bring the slurry within a prescribed range of water
surface when the pile is inserted to proper depth. Piles
content. The mix water temperature may be warm or
thus placed are more difficult to plumb and position but
even heated when using frozen cuttings. When thawed
when such factors are not critical this method is much
borrow material is used water is chilled by the addition of
faster. H-piles are easily placed by this method; timber
snow and/or ice. When mixed, slurry temperature
and closed-end pipe piles have a strong tendency to
should under no circumstances exceed 40 F. The
float. Such prior backfilling of holes should not be
slurry should have the consistency of 6 inches slump
attempted when using sand backfills unless the piles are
concrete, the consistency being specified and monitored
to be driven into the unfrozen slurry.
(g) Normally no attempt is made to
by field inspectors using a calibrated container from
which the acceptable range of wet unit weights can be
completely clean or to bell the bottom of augered holes,
quickly determined. Normally the mixing crew can
some loose cuttings always remaining. Bottom portions
reproduce the desired consistency quite easily after
of holes drilled too deep can be backfilled as necessary
establishing the appearance and viscosity during the first
with sand or gravel slurry while the pile is suspended in
or second batch. Methods for control of components
the hole. The pile may be dropped a short distance to
may be similar to those used in concrete batching.
compact the cuttings or backfill. Piles (except H-piles)
Based on past experience, continuous inspection of the
should not be placed closer than 1 inch to the walls of
mixing operation should be made to insure that no more
augered or drilled holes.
(h) To minimize the amount of heat to
water than necessary to saturate the soil and to produce
the desired consistency of the slurry is used and that
be introduced into the ground by the slurry, the annular
temperature of the mix is kept low.
space is made only just large enough to allow the slurry
(d) The slurry is normally placed by
to be efficiently placed and vibrated with a small diameter
direct backfilling by wheelbarrow on small jobs or by the
concrete vibrator. Vibration is needed to insure that the
use of concrete buckets with cranes on large jobs.
space will be completely filled without air entrapment.
Tremie pipes or direct pumping may be advantageously
Except for H-piles, a minimum of about 3 inches oversize
used on some jobs but during cold weather, operations
in diameter is required to permit use of a 1-inch-diameter
are often a source of major trouble because of freezing.
vibrator in the annular space. Additional allowance is
Normally the pile after being centered and plumbed in
usually made because of the difficulty of achieving exact
the hole is backfilled with the slurry in one continuous
centering of the pile in the hole and because of pile
operation. It is very important that the material be
irregularities such as lack of straightness. A hole size 4
vibrated with a small diameter concrete spud vibrator
to 6 inches larger than the pile diameter commonly has
and rodded to ensure that there are no bridging and no
been used, but it should be kept as small as practical for
voids left along the pile, which can often happen when
the particular situation.
(2) Installation in bored holes. Holes for
backfilling around cylindrical piles, especially if tremie
pipe is not used. Care should be taken during placement
the piles may also be made by rotary or churn drilling, or
of the slurry to avoid moving or bending the pile by
even by drive coring under some conditions, using
placing the slurry too fast from only one side. Small form
various bits, drive barrels, etc., and removing frozen
vibrators may also be attached directly to the pile to aid
materials
with
air,
water
and/or
mechanical
55,74,153
in uniform flow around the pile during backfilling and to
. Procedures are otherwise the same as
systems
hasten consolidation of the backfill. Rapping the sides of
for dry augered holes. By proper selection of equipment,
the pile with a sledge hammer also aids the backfilling
any type of frozen ground may be handled. Use of water
and consolidation.
or warm air for removal of cuttings may introduce
(e) Timber piles quite often float or
undesirable quantities of heat into the permafrost and
rise up when backfilled with silt-water slurries and must
must be carefully controlled.
(3) Installation by driving.
be restrained or weighted. Anchoring timber piles may
also be accomplished by backfilling only a portion of the
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