Good Vibrations

Both reversible plates and trench rollers can compact soil in confined spaces. When considering which type of compactor to use, the type of soil and the size of the job need to be taken into account. Here are some more things to consider when contemplating selecting these soil machines for your inventory.

Down & dirty

All vibratory plates, not just reversibles, are suited to granular soils, such as sand or gravel, whereas cohesive clay or silt-type soils are best compacted by a trench roller or rammer.

"To compact granular material, you need vibratory force," says Peter Price, manager of product and training for Bomag Americas Inc. "With cohesive material you need impact force. When looked at under a microscope, cohesive material is made up of very small pancake-shaped particles. In between those pieces will be either air or water, so what you have to do is use impact force to squeeze out the water and air.

"With granular material, what you have to do is excite the particles via vibratory frequency so they start moving in all different directions," he continues. "The frictional force between the particles is then reduced and the particles settle under their own weight."

This means if operators are working with smaller aggregate sizes (one inch and smaller), they need a higher frequency to move the material and achieve compaction. The rule of thumb tends to be that reversible plates have high frequency vibration, but low amplitude (the height at which the machine "jumps" off the ground); while rollers have low frequency vibration and high amplitude.

"Rollers, with their low frequencies and high amplitudes, are better in cohesive soils because they tend to push the soil particles together — like kneading bread — forcing molecules together and squeezing air out," says Frank Wenzel, vice president of engineering at Stone Construction Equipment. "Rollers won't work on granular materials as well, but plates, with their high frequencies and low amplitudes, vibrate the granular material better to compact it."

When equipped with polygonal drums with small pads, like Bomag Americas offers, trench rollers can be used in granular soils. "But," says Price, "they're not as productive in granular soils as reversible plates. Ninety-nine percent of the time trench rollers are used, it will be in cohesive soils."

On the other hand, some larger reversible plates, with triple shafts, can be used on cohesive soils. This will be discussed later.

Obviously, it's important to consider the type of soil your customers will be working in when deciding if a reversible plate or a trench roller is the answer. In areas of the country where the soil is predominantly sandy, such as Florida, reversible plates are more commonly used. However, in an area with more clay-like soil, such as Georgia, trench rollers will be seen on more jobsites.

If the job fits

Along with soil type, the size of the job needs to be taken into account when considering reversible plates and trench rollers.

Mickey Benedict, product manager - soil with Ingersoll-Rand, notes that it's common sense when choosing which machine to use. "It depends on the size of the job," he says. "If you have a small narrow trench, a rammer or small trench roller would suffice. As the job gets bigger, the machine gets bigger."

The name "trench roller" tends to pigeon hole these units. Trench rollers vary in width, from 24 inches up to 34 inches, depending on the manufacturer. "The narrower machines can be used for burying utilities. As the machines get bigger, the can be found doing foundation work, working around curbs and walls and for doing site prep," says Benedict.

Reversible plate compactors are also easy to maneuver in tight, confined areas such as trenches, along walls and around pipes. They can also compact thick lifts, says Udo Boersch, president of Ammann America Inc.

"If you have a pipeline trench with a really thick lift, then you'll want a reversible plate to do the job," he explains.