TM 5-818-1 / AFM 88-3, Chap. 7
conditions are necessary. Underlying soft clay layers
percent are obtained. Bearing capacity increases of 200
may dampen vibrations.
to 400 percent are usual for sands and marls, with a
c. Vibratory rollers.
Where cohesionless
corresponding increase in deformation modulus. The
cost is reported as low as one-fourth to one-third that of
deposits are of limited thickness, e.g., less than 6 feet, or
vibroflotation.
where cohesionless fills are being placed, vibratory
(3) Because of the high-amplitude, low-
rollers are likely to be the best and most economical
frequency vibrations (2-12 Hz), minimum distances
means for achieving high density and strength. Use with
should be maintained from adjacent facilities as follows:
flooding where a source of water is available. The
Piles or bridge abutment
15-20 feet
effective depth of densification may be 6 feet or more for
Liquid storage tanks
30 feet
the heaviest vibratory rollers (fig 16-3a). For a fill placed
Reinforced concrete buildings
50 feet
in successive lifts, a density-depth distribution similar to
Dwellings
100 feet
that in figure 16-3b results. It is essential that the lift
Computers (not isolated)
300 feet
thickness, soil type, and roller type be matched. Properly
c. Vibroflotation.
matched systems can yield compacted layers at a
(1) A cylindrical penetrator about 15
relative density of 85 to 90 percent or more.
inches in diameter and 6 feet long, called a vibroflot, is
attached to an adapter section containing lead wires and
16-3. Vibrodisplacement compaction. The methods
hoses. The whole assembly is handled by a crane. A
in this group are similar to those described in the
rotating eccentric weight inside the vibroflot develops a
preceding section except that the vibrations are
horizontal centrifugal force of about 10 tons at 1800
supplemented by active displacement of the soil and, in
revolutions per minute. Total weight is about 2 tons.
the case of vibroflotation and compaction piles, by
(2) To sink the vibroflot to the desired
backfilling the zones from which the soil has been
treatment depth, a water jet at the tip is opened and acts
displaced.
in conjunction with the vibrations so that a hole can be
a. Compaction piles. Partly saturated or freely
advanced at a rate of about 3.6 feet per minute; then the
draining soils can be effectively densified and
bottom jet is closed, and the vibroflot is withdrawn at a
strengthened by this method, which involves driving
rate of about 0.1 foot per minute. Newer, heavier
displacement piles at close spacings, usually 3 to 6 feet
vibroflots operating at 100 horsepower can be withdrawn
on centers.
One effective procedure is to cap
at twice this rate and have a greater effective penetration
temporarily the end of a pipe pile, e.g., by a detachable
depth. Concurrently, a cohesionless sand or gravel
plate, and drive it to the desired depth, which may be up
backfill is dumped in from the ground surface and
to 60 feet. Either an impact hammer or a vibratory driver
densified. Backfill consumption is at a rate of about 0.7
can be used.
Sand or other backfill material is
to 2 cubic yards per square yard of surface. In partly
introduced in lifts with each lift compacted concurrently
saturated sands, water jets at the top of the vibroflot can
with withdrawal of the pipe pile. In this way, not only is
be opened to facilitate liquefaction and densification of
the backfill compacted, but the compacted column has
the surrounding ground. Liquefaction occurs to a radial
also expanded laterally below the pipe tip forming a
distance of 1 to 2 feet from the surface of the vibroflot.
caisson pile.
Most vibroflotation applications have been to depths less
b. Heavy tamping (dynamic consolidation).
than 60 feet, although depths of 90 feet have been
(1) Repeated impacts of a very heavy
attained successfully.
weight (up to 80 kips) dropped from a height of 50 to 130
(3) A relationship between probable
feet are applied to points spaced 15 to 30 feet apart over
relative density and vibroflot hold spacings is given in
the area to be densified. In the case of cohesionless
figure 16-4. Newer vibroflots result in greater relative
soils, the impact energy causes liquefaction followed by
densities. Figure 16-5 shows relationships between
settlement as water drains. Radial fissures that form
allowable bearing pressure to limit settlements to 1 inch
around the impact points, in some soils, facilitate
and vibroflot spacing.
Allowable pressures for
drainage. The method has been used successfully to
"essentially cohesionless fills" are less than for clean
treat soils both above and below the water table.
sand deposits, because such fills invariably contain
(2) The product of tamper mass and
some fines and are harder to densify.
height of fall should exceed the square of the thickness
(4) Continuous square or triangular
of layer to be densified. A total tamping energy of 2 to 3
patterns are often used over a building site.
blows per square yard is used. Increased efficiency is
Alternatively, it may be desired to improve the soil only at
obtained if the impact velocity exceeds the wave velocity
the locations of individual spread footings. Patterns and
in the liquefying soil. One crane and tamper can treat
spacings required for an allowable pressure of 3 tons per
from 350 to 750 square yards per day. Economical use
square foot and square footings are given in table 16-3.
of the method in sands requires a minimum treatment
area of 7500 square yards. Relative densities of 70 to 90
16-10
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