TM 5-3895-346-14
CHAPTER II
OPERATIONS
THEORY OF OPERATION
Compaction is the compression of soil by forcing air and water from between its particles. This can be
accomplished in many ways, but in the case of sand and gravel (the two materials most often used as the
subbases in paving), the best way is through vibration. The Tampo roller can produce as much as 30,000
pounds of impact force up to 1500 times a minute over a path 60 inches wide. The amount of compaction
obtained by the machine is not a result of the number of blows it strikes, but of the number at which the roller
and soil vibrate in union.
All types of soil can be moved to a state known as resonance under vibration. In this state, particles in soil,
particularly sand and gravel, will compact 10 to 50 times more efficiently than when under random impacts or
pressure.
Where time and conditions allow it, some research should be done to determine the resonance of the material
being compacted. However, even when this research cannot be done, vibration in any form will shake sand
and gravel into a more compact base than without vibration.
In addition, compaction can be enhanced by the moisture content of the soil. Particles float into a compact
condition when moisture reaches an optimum condition best determined by engineering studies. But if the
moisture content goes beyond the optimum point for compaction, the force of impact is spread over too great
an area, destroying the compactive force.
A vibratory roller can produce compaction up to a depth equal to three times the width of the roller if soil and
moisture conditions are ideal and resonance can be obtained. Even under this condition, the top inch of soil
will remain loose. Final compaction of this material can be attained by rolling the material without the vibrator
engaged.
The vibratory roller is best used on sand and gravel and is least efficient on loam or clay soils. Loam and clay
not only bear weight through friction among particles, but also through electrical attraction among the particles
and suction from moisture in the soil. Vibration cannot efficiently break down those forces. When clay or loam
is encountered while making a subbase, consideration should be given to changing equipment (a determination
best made by the field engineer).
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