Do your counter employees know the difference between cohesive and granular soils? Do they understand what impact force is? Or vibratory force? If the answer is no, they need to read this article.

Basic knowledge of soil types and what machines are best suited for compacting them is important to effectively serve customers looking to rent light compaction equipment. To ensure your customers are getting the service they need, start by educating your employees on the science of compaction.

Particle physics
Different types of soils are made up of different types of particles. These particles each possess their own properties and behave in their own unique ways.

Cohesive soils, for example, include clays and silts. The particles in these soils are smaller and are held together by magnetic attraction. They can be recognized by the fact that they will form a ball when squeezed in the palm of your hand.

"If you could analyze the particles that make up a ball of clay, you would see that the particles are shaped like a flat pancake," says Peter Price, manager of product & training at Bomag Americas Inc. "They need to be compressed with impact force."

Granular soils, on the other hand, include gravel and sand and are held together only by gravity. If you hold them in the palm of your open hand, the particles will freely slip through your fingers. These particles are larger than cohesive soil particles and they have rough surfaces which can be agitated to minimize the air voids between them, Price explains.

Because cohesive soils require impact force to achieve compaction, the best equipment for the job is a rammer. The rammer's up and down motion, or amplitude, pounds the particles into each other until a stable mass is created. Granular soils, however, require vibratory frequency to excite the particles so they settle into a dense material.

Tools of the trade
The world of light compaction equipment is generally thought to include rammers, plate compactors, trench compactors and walk-behind rollers.

"Vibratory rammers are the most universal type of compactor," says Mark Conrardy, sales engineering manager at Wacker Corp. "They work in any soil type with the exception of sand. They are ideal for very confined spaces such as narrow trenches and along footings, foundations or walls. Rammers work the best in cohesive soils where the shearing action of the shoe can work the soil and break the bonds of the clay where vibration alone will not."

For their part, vibratory plates are ideal in granular-type soils such as sand and gravel. "They're the most efficient type of compactor due to the large contact area of the plate and the high number of vibrations that they send into the soil," says Conrardy. "The limitation of a plate is when the soil contains approximately 30 percent cohesive material. At this percentage, the clay and silt will act like glue and make the sand and gravel stick together, not allowing the vibration impulses to move them into a denser configuration."

Reversible plates work the same as forward vibratory plates but are larger. "They use the same concept as forward plates, but the weight goes up and so does the cost," says Fabian Salinas, product manager for concrete and light compaction products at Dynapac. "If you need more output and higher compaction rates, you need a reversible plate."

Reversible plates also offer increased versatility because they can switch directions easily, making them a snap to get in and out of tight spaces. "It's an issue of ROI," says Salinas. "In some cases, a reversible would be overkill."

Trench compactors are typically used in cohesive soils. They are used in deep trenches and are effective at covering areas larger than what a rammer can handle but yet aren't large enough to warrant the use of a ride-on roller. With a padfoot drum, a trench compactor concentrates its static weight on the pads, while the vibratory frequency works to compact just about any material, except asphalt.