Soil compaction machines are used to increase the density of native soils or fill. These rollers may feature a single steel drum for compaction or be configured with tandem (front and rear) drums. Rather than a “static” drums, which compacts with its passing of its weight distributed along a horizontal line of contact, some rollers employ “vibratory” and/or “oscillatory” functionality. While vibratory rollers are used over granular materials and asphalt compaction. The amplitude and frequency of their vibration more efficiently consolidates a base course composed of granular materials such as crushed gravel. The vibratory action may be used in similar fashion to knit the aggregate of some stiff asphalt mixes together faster.
Drum surfaces also vary. They range from smooth surfaced steel to pad-foot and tamping rollers or “sheepsfoot rollers.” Sheepsfoot rollers get their name from the appearance of the earliest tamping roller models, whose protruding spokes with widened ends resembled the leg and hoof of a sheep. The imprints they left on the ground could look as though a flock of sheep had passed.
A smooth-surfaced drum distributes compaction force evenly along a single, linear point of contact with the soil, which may work well with dry or sandy soils or to smooth-finish an asphalt surface. Sheepsfoot drums concentrate compaction force to smaller areas of contact. That makes a sheepsfoot drum more effective than other drum styles in clays and silty clay soils, as their feet knead and knit the soil materials together. They can also be used to help dehydrate wet soils, tending to squeeze out water as they consolidate the soils and exposing saturated soil surfaces to evaporation as they knead them.
As sheepsfoot drums evolved for use on self-propelled compaction machinery, the style of their protrusions changed. The new sheepsfoot-style drums share advantages with their spoke-like, pull-behind predecessors but feature blade-shaped, round, oval or rectangular feet.
When the imprints of a sheepsfoot roller are 0.5 inch or less, the ground is generally considered as dense as that style of roller can compact it. Little benefit will be obtained by passing a sheepsfoot drum over the soil any further.
Like the sheepsfoot roller, a pad foot roller concentrates compaction force to limited spaces of the drum. It’s wider, shallower protrusions, which may be oval, square, rectangular or diamond-shaped, gives the roller greater versatility, such as permitting quicker, smoother compaction runs as soil conditions permit.