Both reversible plates and trench rollers can compact soil in confined spaces. When considering which of these compactors to use, the type of soil and size of the job need to be taken into account. But there are other things you also need to consider.
Match compaction to the soil
All vibratory plates - not just reversibles - are suited to granular soils, such as sand or gravel. Cohesive clay or silt-type soils, on the other hand, 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, Price points out. In between those pieces is either air or water. Impact force is required in order to squeeze the water or air out.
"With granular material, you have to excite the particles via vibratory frequency so they start moving in all different directions," Price continues. "The frictional force between the particles is then reduced and the particles settle under their own weight."
This means operators working with smaller aggregate sizes (1 in. and smaller) need a higher frequency unit to move the material and achieve compaction. As a rule of thumb, reversible plates tend to provide high-frequency vibration, but low amplitude (height at which the machine "jumps" off the ground), while rollers provide low-frequency vibration and high amplitude.
"With their low frequencies and high amplitudes, rollers 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, trench rollers can be used in granular soils. "But they're not as productive in granular soils as reversible plates," says Price. "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.
Obviously, it's important to consider the type of soil you will be working in when deciding if a reversible plate or a trench roller is the answer. In areas where the soil is predominantly sandy, such as in Florida, reversible plates are more commonly used. However, in an area with more clay-like soil, such as in Georgia, trench rollers are typically more prevalent.
Job size and lift thickness
Along with soil type, the size of the job needs to be taken into account. Mickey Benedict, product manager - soil with Ingersoll-Rand, recommends using common sense when choosing which machine to use. "If you have a small, narrow trench, a rammer or small trench roller would suffice," he notes. "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 up to 34 in., depending on the manufacturer. "The narrower machines can be used for burying utilities. As the machines get bigger, they can be found doing foundation work, working around curbs and walls and 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. In addition, they can 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.
Some of the larger reversible plates can successfully compact lifts of material up to 30 in. thick. "Due to the reversible plates' size and higher centrifugal force, they can compact deeper lifts than conventional forward plates," says Boersch.