ForConstructionPros.com

When you look at the walk-behind compactors in your fleet, chances are you don't think of them as precise scientific instruments. Yet, soil compaction is a science and it requires a certain degree of precision. Understanding some of the science behind it can help you improve efficiency on your jobsites.

What's the dirt?

To choose the right equipment for your job, you need to first understand something about soils and how they are compacted. According to Mark Conrardy, sales engineering manager at Wacker Corp., soil can be broken down into four basic groups: clay, silt, sand and gravel. "The most important characteristic is particle size," he states.

Clays consist of the smallest particles, which are usually less than 0.00024 in., followed by silts, sands and gravels, with particles ranging from 0.08 up to 3 in. in diameter. Anything larger than 3 in. is considered a boulder.

"Clays and silts are grouped together as cohesive soils based on the fact [their particles] will tend to laminate due to their small size. The forces that hold them together are molecular in nature," Conrardy explains. "Sand and gravels are grouped together as granular soils and the forces that hold them together are frictional due to their jagged and rough surface texture."

Mixed soils contain a mixture of both cohesive and granular particles.With mixed soils, a soil gradation analysis can determine the proper classification and assist in machine selection, says Frank Wenzel, vice president engineering, Stone Construction Equipment.