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A word about soils

All rental operators should be able to understand soil compaction and be able to relay this information to their customers. No contractor today can afford to blindly rent compaction equipment without careful consideration of the nature of the soil they are compacting.


September 15, 2009
By Tom Asbil

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All rental operators should be able to understand soil compaction and be able to relay this information to their customers. No contractor today can afford to blindly rent compaction equipment without careful consideration of the nature of the soil they are compacting.

The compaction of soil in confined areas is a very important, yet not always fully understood, subject. With an understanding of soil mechanics and machines used in compaction, rental operators can increase their comprehension of soil compaction to discover new construction approaches to increase revenue and reduce maintenance costs.

Recommending the proper machine to properly compact soils depends on having a good understanding of the soil that is to be compacted.

Vibratory rammers, plates and rollers are designed to compact a variety of different soil types depending on job size and specifications. While every application has its unique features, there are some basic selection guidelines that can help rental operators remove some of the guesswork in helping a customer match the right machine for the job.

What type of soil will be used?
This is the single most important variable in compaction equipment selection. Soil is comprised of four basic components: clay, silt, sand and gravel. These components are classified into two soil groups: cohesive = clays and silts (fine soils); granular = sands and gravels (course soils).

In granular soils, the particles are held in place by frictional forces that exist at the contact surface. In the dry state, they can be formed into desired shapes but will easily crumble apart.

Granular soils are best compacted by vibration energy. The type of machine would be either a vibratory plate or roller. The vibration action reduces the frictional forces at the contact surface, allowing the particles to fall freely under their own weight. At the same time, as the soil particles are vibrating, they become momentarily separated from each other, allowing them to turn and twist until they can find a position that limits their movement.

Soils, such as sands, will respond best to very high frequencies, in the range of 11,000 to 14,000 vpm (vibrations per minute). Larger gravels will respond best to lower frequencies in the range of 2,000 to 4,000 vpm. Whenever possible, it is best to match the frequency of the vibratory compactor to the largest fraction of particles present in the soil.

In cohesive soils, there is a molecular attraction between the microscopic particles which holds the soil in place. These particles are small and very densely arranged, so the cohesive force in the soil is very high. This makes cohesive soils in the dry state very hard. When moist, the forces and structure break down and become very plastic and easily moulded.

Cohesive soils are best compacted by impact force. The type of machine recommended is a vibratory rammer or pad-foot roller. The impact force of the rammer or pad-foot roller causes a shearing effect that squeezes out the air voids and excess water from between the particles.

What are the compaction specifications for the job?
Most compaction specifications deal with the density required for the job, how thick the lifts are and frequency of the density tests required.

The machine selected should have enough power to compact the soil to the depth required for the necessary density. If the machine does not have enough power, the proper density will not be reached. If it has too much power, over-compaction can occur. This is the result of either excessive passes or too much power on a soil layer that is too thin.

To avoid too much power, look at the machine specifications. To avoid over-compaction, the soil layer should not be less than one-third of its maximum lift. For example, if the machine’s maximum rating is 18 inches, it should not be put on a soil layer less than six inches.

What is the application or job site condition?
Vibratory plates are more efficient than rollers due to the large base plate surface area in contact with the soil. Vibratory plates are also more manoeuvrable so they are ideal for confined areas.

Rollers have the advantage in larger, open areas where there is plenty of room to manoeuvre. Because the roller has a faster travel speed, it will cover the area faster than a plate.

The same logic applies to rammers. The rammer should be used on cohesive soils in confined areas and the trench roller should be used in larger trenches and open areas.

By asking only a few questions about the type of soil, application required and the job site conditions, a rental operator will be in a better position to help a customer select the best machine for the job


Tom Asbil is the technical trainer for Wacker Neuson Limited.