UFC 3-220-01N
15 AUGUST 2005
heavy weight (up to356 kN (80 kips)) dropped from a height of 15 to 40 m (50 to 130 ft)
are applied to points spaced 4.5 to 9.0 m (15 to 30 ft) apart over the area to be
densified. In the case of cohesionless soils, the impact energy causes liquefaction
followed by settlement as water drains. Radial fissures that form around the impact
points, in some soils, facilitate drainage. The method has been used successfully to
treat soils both above and below the water table.
The product of tamper mass and height of fall should exceed the square of the
thickness of layer to be densified. A total tamping energy of 2 to 3 blows per square
meter or square yard is used. Increased efficiency is obtained if the impact velocity
exceeds the wave velocity in the liquefying soil. One crane and tamper can treat from
293 to 627 m2 (350 to 750 sq yd) per day. Economical use of the method in sands
requires a minimum treatment area of 2671 m2 (7500 sq yd). Relative densities of 70 to
90 percent are obtained. Bearing capacity increases of 200 to 400 percent are usual for
sands and marls, with a corresponding increase in deformation modulus. The cost is
reported as low as one-fourth to one-third that of vibroflotation.
Because of the high-amplitude, low-frequency vibrations (2-12 Rz), minimum distances
should be maintained from adjacent facilities as follows:
Piles or bridge abutment
4.6 6.1 m
(15 20 ft)
Liquid storage tanks
9.1 m
(30 ft)
Reinforced concrete buildings
15.2 m
(50 ft)
Dwellings
30.5 m
(100 ft)
Computers (not isolated)
91.4 m
(300 ft)
8-7.3.3
Vibroflotation. A cylindrical penetrator about 381 mm (15 in) in diameter
and 1.8 m (6 ft) long, called a vibroflot, is attached to an adapter section containing lead
wires and hoses. A crane handles the whole assembly. A rotating eccentric weight
inside the vibroflot develops a horizontal centrifugal force of about 89 kN (10 tons) at
1800 revolutions per minute. Total weight is about 18 kN (2 tons).
To sink the vibroflot to the desired treatment depth, a water jet at the tip is
opened and sets in conjunction with the vibrations so that a hole can be advanced at a
rate of about 1.2 m (3.6 ft) per minute; then the bottom jet is closed, and the vibroflot is
withdrawn at a rate of about 30 mm (0.1 ft) per minute. Newer, heavier vibroflots
operating at 100 horsepower can be withdrawn at twice this rate and have a greater
effective penetration depth. Concurrently, a cohesionless sand or gravel backfill is
dumped in from the ground surface and densified. Backfill consumption is at a rate of
about .62 to 1.77 m3 per m2 (0.7 to 2 cubic yards per square yard) of surface. In partly
saturated sands, water jets at the top of the vibroflot can be opened to facilitate
liquefaction and densification of the surrounding ground. Liquefaction occurs to a radial
distance of 305 mm to 610 mm (1 to 2 ft) from the surface of the vibroflot. Most
vibroflotation applications have been to depths less than 18 m (60 ft), although depths
of 27 m (90 ft) have been attained successfully.
A relationship between probable relative density and vibroflot hold
spacings is given in Figure 8-7.4. Newer vibroflots result in greater relative densities.
Figure 8-7.5 shows relationships between allowable bearing pressure to limit
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