Contractors use a number of different types of soil and aggregate compactors these days. Common types include: sheepsfoot rollers, which run static and are typically towed; pneumatic-tired rollers, which use rubber tires to provide the familiar kneading action of soil or subgrade; vibratory rollers (smooth drum), typically used for granular and mixed soil materials; and tamping foot, which combines the advantages of a vibratory roller with a sheepsfoot.
The sheepsfoot roller is most effective for compaction of plastic soils like clay or silt, according to Bomag Americas, a leading roller manufacturer. The sheepsfoot compacts from the bottom of each lift towards the top. High contact pressures cause the feet to penetrate through the loose material and actually compact the material directly with the foot tip.
A number of passes are required with a sheepsfoot roller because of the small area compacted by each foot. Self-propelled embankment compactors, such as the Bomag BC462EB and the BC772EB, are capable of higher productivity than towed sheepsfoot rollers because they can travel at higher speeds. And while the teeth differ slightly in design from a sheepsfoot, the Bomag embankment compactors achieve compaction the same way.
Pneumatic-tired rollers generally compact from the top of the lift downward. The relationship between the tire contact area and the ground contact pressure causes a kneading action, which helps seek out soft spots that may exist. Light- to medium-weight self-propelled units are used primarily for compaction of granular base as well as hot mix asphalt.
Vibratory rollers work on the principle of particle rearrangement resulting from dynamic forces generated by the vibrating drum hitting the ground. As particles in the soil rearrange themselves, voids between particles become smaller, causing an increase in material density. The best vibratory application is the compaction of granular and mixed soils.
A tamping foot roller has feet, or pads, that penetrate the soil, compacting from the bottom to the top for uniform density. The forces of gravity and vibratory impact simultaneously compact from the top down. Due to the foot shape, and in combination with vibration, these rollers achieve a kneading and impact effect while the imprints left contribute to a reduction of water content.
It is often recommended, says Bomag, that a vibratory smooth-drum roller be used on materials having up to 10% cohesive content. A tamping foot, or padfoot roller can compact soils having as much as 50% cohesive content.
Large Versus Small Projects
Compactors required by a contractor will depend on the size of the job, the type of material, lift thickness and job-site congestion. On a smaller project, material is generally spread and compacted until density is achieved, followed by another lift of material until the process is completed. In such a situation, compactor maneuverability is more important than size, as hauling and spreading equipment may be sitting idle on the job site during the compaction process.
On a larger project, such as a highway, dam, levee, or runway, the compaction process may be continuous. In this case, Bomag says it is imperative that the compactor’s size and productivity complement the hauling and spreading equipment on the project. Maneuverability is less of a factor as compactors frequently shuttle back and forth in order to reduce turnaround time. Thicker lifts or rock fill will require the largest rollers available in order to maintain project timelines.
Caterpillar recommends selecting a machine that matches the production of your earthmoving fleet—weight is the key. The machine must be able to keep up with the pace of the project. Caterpillar has charts that show general productivity of its rollers.
The size of your compaction equipment depends on the factors above. For example, if you need to compact thick lifts of limestone rock for a mile-long levee in Florida, you will want the largest smooth-drum roller you can get. If you are filling utility trenches on a congested job site in Georgia, you will want a mid-size tamping-foot (padfoot) roller with more maneuverability. If you are building a large 20-acre pad for a new shopping center, you may want a sheepsfoot roller.
Frequency and Amplitude
Bomag says that large vibratory rollers usually offer a choice of two amplitudes and two frequencies. That allows the contractor to adjust to job-site conditions. You would use high frequency/low amplitude on granular material or thinner lifts—and low frequency/high amplitude on cohesive material or thicker lifts.
There is little advantage to running a vibratory roller in static mode on soils or base material. A sheepsfoot roller, on the other hand, always runs in static mode and uses manipulation and impact to achieve compaction.
Caterpillar Global Paving, a leading compactor manufacturer, says most of its machines are equipped with one standard frequency. Variable frequency is available as an option.
Frequency is a measure of the number of complete cycles or revolutions of the eccentric weights around the axis of rotation in a given length of time. Frequency is usually expressed in units of vibrations per minute (vpm) or hertz (Hz). Amplitude is a measure of the vertical movement of the drum during a vibration.
The relationship between frequency and working speed, says Caterpillar, is sometimes simplified to a simple rule of thumb which states that frequency and working speed should be adjusted to yield approximately one impact per inch or 25 mm. Too high a working speed can cause “washboarding” with impacts spaced too far apart; and too low a working speed negatively impacts machine productivity. There is an optimum working speed and frequency for each compaction application, but they may not yield exactly one impact per inch. That is due to many variables of soil mechanics and composition.
Frequency is not so important by itself, says Jon Sjoblad of Caterpillar Global Paving. Traditionally operators use a process of trial and error to find the balance of frequency, amplitude and ground speed that most efficiently compacts the soil at hand. Operators adjust the speed and amplitude as they work, says Sjoblad.
An operator can sense when over-compaction has occurred. When a soil becomes dense, as it nears its maximum density, it deflects some of the impact energy from the drum and transmits it back into the drum. The drum will rebound high enough for an entire cycle of the eccentric weight to occur without the drum making an impact. In essence, the drum makes a “double-jump.” Such a phenomenon, called “decoupling,” or “double-jumping,” is easily recognized by the operator because it causes violent vibrations throughout the machine. To stop it, the operator will adjust amplitude down.
There are a number of compaction meter devices that can help an operator determine when proper compaction has occurred. Caterpillar’s AccuGrade Compaction is one such device, but it is a far more complex system than simply a compaction meter. Bomag Americas, Caterpillar, Dynapac, Hamm, and Volvo Construction Equipment all offer complete lines of soil compactors.
Volvo Construction Equipment offers a full line of both vibratory smooth drum and padfoot compactors. Volvo acquired these compactor lines in 2007, and their legacy is traced back to 1969, says Bob Marcum, segment manager, Volvo Road Machinery.
The majority of Volvo’s vibratory soil compactors have five frequency settings. “This allows the optimum match of drum frequency to the natural resonances of the type of soil you’re compacting,” says Marcum. Frequency settings range between 1,200 to 2,500 vpm. Volvo vibratory soil compactors, seven tons and larger, have two amplitude settings which can range from 0.015 inch up to 0.080 inch. Higher amplitudes allow the compaction of deeper lifts of materials for more production.
Most Volvo compactors have a traction enhancement system for maximum gradability. This system ties the power of the front drum to the rear axle to enable the machine to climb steeper grades. “We have more centrifugal force in our compactors than most of our competitors,” says Marcum. “Greater compaction energy can provide increased production on difficult materials.”
All of Volvo’s compactors have engines that are certified to meet today’s emission standards. “Volvo currently has several smaller compactors that have Tier 4 (interim) engines and we are moving toward all engines meeting Tier 4 standards as they become applicable.” says Marcum.
Volvo’s emphasis on safety is demonstrated with all compactors featuring a spring-applied, hydraulically released (SAHR) safety brake, says Marcum. Volvo places a test switch on compactors so that an operator can assure himself that the SAHR brake is in working order. “With the test switch on, the hydraulic system can be pressurized, but the brakes will remain engaged, which demonstrates that the brakes are in working condition,” says Marcum.