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Compacting Soil

Soil compaction is a necessary step on almost every job site in North America. It improves load support and reduces the chances the soil will settle or erode, which can cause structural failure to the building, road, or parking lot. Several types of walk-behind compactors are available, and while they all have unique characteristics, the soil you’ll be compacting is the biggest factor in determining which type of compactor you will need.

Basically, there are two types of soil: granular and cohesive. Granular soil- sand and gravel- doesn’t stick together, making it ideal for water drainage. Granular soil particles are coarse to the touch, visible, and should crumble easily. Granular soil must be vibrated so the individual particles settle and remove voids, in effect, locking the particles together.

Cohesive soil particles (clays and silts) are attracted to each other molecularly and stick together easily. Because the particles are so small and tightly bound together, they require a shearing or impact force to compact rather than a vibratory force. Cohesive soil is plastic; that is, it can be rolled into thin threads when wet. When dry, cohesive soil is very hard and not easily compacted.

Compacting cohesive soil depends largely on moisture content. To determine the amount of moisture, squeeze a handful of soil. If it holds its shape, it has adequate moisture content. If it does not retain its shape, it’s too dry to compact. If it is wet to the touch, it is too wet to compact and you should allow it to dry before compacting.

Selecting a Compactor

Rammers or Jumping Jacks are commonly used to compact the soil around footings, foundations, and narrow trenches, and are ideal for compacting cohesive soil. The vertical “ramming” action slams the soil, squeezing out the air between the particles. Rammers have limited application in granular soil because the foot can bury itself in the sand. They generate lifts up to two feet.

Vibratory Plate Compactors are used for compacting walkways, trenches, and backfills, and are ideal for granular soils. Unlike rammers, these machines use vibratory force to compact. Vibratory plates come in single-direction and reversible models. Single-direction models only move forward while reversible plates can move in two directions allowing greater maneuverability in tight places.

Safety Considerations

Compactors are designed with vibration-dampening handles to reduce operator fatigue; however, prolonged use of walk-behind compactors can lead to carpal tunnel syndrome and arthritis. If you feel your fingers, hands, or forearms become numb, stop compacting and take a break.

Wear proper clothing when using any type of equipment to protect yourself, including gloves to insulate handle vibration, safety glasses, and ear protection. When working in dry conditions wear a dust mask.


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