A Lasting Impact

July 1, 2000
Before you help potential customers get the proper compaction equipment, do some digging of your own. Asking your customer about the work site's conditions

Before you help potential customers get the proper compaction equipment, do some digging of your own. Asking your customer about the work site's conditions and the type of soil to be compacted is crucial when determining the best compactor for the job.

Soil is classified into two categories - granular and cohesive. Granular soil consists mainly of sand and gravel. Vibratory plates and rollers are ideal for compacting sand and gravel because vibration reduces the friction that holds the soil particles together.

Cohesive soil is made up of silt and clay. Cohesive soil is hard when dry. When the soil is moist, the cohesive force that holds it in place breaks down and makes the particles plasticlike and easily molded. Unlike granular soil, cohesive soil does not settle under vibration because of the natural binding forces between the soil particles.

The most effective way to compact silt and clay is with impact force, which produces a shearing effect that squeezes the air pockets and excess water to the surface, moving the particles closer together. Compacting by impact force is achieved best with a rammer or a sheep's foot roller. With a rammer, the ramming shoe alternately strikes and leaves the ground at a high speed and ultimately "kneads" the ground to increase soil density.

The R-factor The compaction method also is determined by the job specifications, which typically include lift, or soil depth/layers, and the number of passes needed. Rammers are ideal when the soil depth ranges from 4 to 24 inches. Depending on its size, a rammer can achieve soil lifts to approximately 25 inches. Advise your customers to make three passes when using a rammer under optimal conditions to compact cohesive soil.

Rammers are often used in confined areas where they can deliver high impact power to a space not easily reached by larger plates and rollers. Common rammer applications include trenches, backfill around foundations and forms, around utility towers and poles, and base preparation for concrete slabs, curbs and gutters, pipelines and cables, and under asphalt patches.

A rammer should provide high impact power, with the ramming shoe coming off the ground 2 to 3 inches. It should have good balance, be easy to guide and have good shock isolation to reduce operator fatigue. Rammer manufacturers also are creating more ergonomically and environmentally friendly units.

Rammers are categorized by their power source - electric, diesel, two-cycle gas/oil mix or four-cycle gasoline engine. Diesel-powered rammers eliminate the need for a fuel/oil mix.

Most contractors prefer rammers powered by two-cycle engines because they use a fuel/oil mix in contrast to the four-cycle engines, in which there are no fuel and oil to mix. Four-cycle engines are not as durable as two-cycle engines when used in rugged, high-impact applications. The risk of a fuel leak is also very high when a rammer with a four-cycle engine is laid on its side.

Compaction 101 * The unit should be checked to ensure that the fuel/oil mix is correct, the oil is leveled and all fasteners are tightened.

* The bottom surface of the ramming shoe must be parallel to the soil while the rammer moves.

* Advise users to experiment with the hand throttle once the engine is warm to find the right rpm; this is generally at three-quarters to full-open throttle. The ramming shoe should make a steady, rhythmic sound.

* On level surfaces, the rammer must be guided, not constantly pushed. However, when moving up or down a grade, the rammer must be pushed or held back.

* Advise users to stay close behind the machine and be ready to take control if the rammer becomes unbalanced.