drums on vibratory rollers vary from 3 to 5 feet in
diameter and 4 to 8 feet in width.
The engine, providing power for propulsion, also
powers the hydraulically driven vibrating unit.
Vibrations are generated by a rotating eccentric weight
inside the drum, the speed of which determines the
frequency, or vibrations per minute, of the drum. The
weight and distance from the shaft of the eccentric
determine the amplitude (amount) of the impact force.
Both the frequency and amplitude of vibrations are
controlled independently of roller travel and engine
The vibration frequency of rollers used for com-
paction is generally between 2,000 to 3,000 vibrations
per minute (vpm), depending on the model and
manufacturer. Some models provide only one or two
specific frequency settings; while others may provide a
full range of frequencies within certain limits; for
instance, 1,800 to 2,400 vpm.
Vibratory rollers achieve compaction through a
combination of three factors: (1) weight, (2) impact
forces (roller vibration), and (3) vibration response in
Weight is the natural force in compaction of soils.
Vibrating rollers amplify their static weight through
vibration to increase the overall dynamic weight.
The impact forces are those generated by vibration
of the compaction drum. They are regulated by
controlling the frequency and amplitude of the
vibration. The amount of impact force required to obtain
optimum density depends on the type of material being
compacted. The impact forces also vary with the
diameter of the drum and the width and the ratio of the
roller static weight and dynamic (impact) force.
The vibration response in the soil or material is the
result of the way the forces are exerted upon it by the
vibratory roller. As with other types of rollers, the
material will compact easily or with difficulty,
depending on its moisture content, cohesion
characteristic, particle shape and texture, and
confinement; for example, sandy soil requires more
vibration and less impact force (amplitude). However, a
soil with higher clay content requires more amplitude
than vibration because of the kneading action necessary
to compact the clay. Vibratory rollers exert repetitive
dynamic force on the material, rather than the static
force used by other rollers.
The frequency and roller speed should be matched,
so there will be at least 10 downward impacts per foot
of travel of the roller. The speed of the roller increases
for a given frequency of vibration, and the spacing of
the impacts grows farther apart.
When using vibratory equipment, keep in mind that
the energy imparted by the vibratory wheel must be
absorbed in the material being compacted. Controlling
the amplitude permits the operator to vary the force
developed from the wheel and, therefore, the energy
imparted to the material. A change in the lift thickness
and material gradation content may require adjustment
in the amplitudes being used.
NOTE: It is important that the roller vibrates only
when it is moving. If vibration continues while the roller
is standing still or changing direction, the vibrating
drum will leave an indentation in the material at the
Most modern rollers have automatic cutoffs for
vibration when the roller stops moving.
The rollers used in the Naval Construction Force
(NCF) are equipped with two interchangeable drums.
One is known as the sheepsfoot, as shown in figure
11-40; and the other is known as a smooth drum, as
shown in figure 11-41.
The sheepsfoot drum is used for compacting heavy
lifts of 6 to 12 inches thick. As consecutive passes are
made, the drum will start to walk out of the ground as
the penetration of the sheepsfoots decrease. These
rollers should only be used for initial compaction,
because the footprints they leave will not allow excess
water to drain.
These rollers concentrate the static and dynamic
weight on the relatively small contact area of the
sheepsfoots. This force is exerted through the one row
of feet in contact with the ground. With all the roller
weight concentrated on this row of sheepsfoots, they
exert more than 22,000 pounds of force.
In most heavy fills, a smooth drum roller is worked
behind the sheepsfoot drum and grader. With thinner