What Your Rental Staff Needs to Understand About Compaction to Best Serve Your Customers

Soil compaction is defined as the method of mechanically increasing the density of soil. In construction, this is a significant part of the building process. If performed improperly, settlement of soil could occur and result in structure failure.

Editor's Note: When planning editorial for Rental, we inevitably revisit many topics, and after years of doing this, it's easy to adopt a tendency toward thinking everyone already knows this stuff. But while attending the Northwest Regional Rental Conference in Seattle last fall, we were reminded that going over the basics never gets old. Evidence of this fact was seen in the seminar program, when dozens of rental professionals packed a conference room to learn about "Basic Compaction Principles." It turns out soil compaction can be pretty scientific, and being knowledgeable about it can help you to help your customers. So with that in mind, we bring you a synopsis of that seminar. 

Soil compaction is defined as the method of mechanically increasing the density of soil. In construction, this is a significant part of the building process. If performed improperly, settlement of the soil could occur and result in unnecessary maintenance costs or structure failure. Almost all types of building sites and construction projects utilize mechanical compaction techniques. These different types of effort are found in the two principle types of compaction force: static and vibratory.

Static force is simply the deadweight of the machine, applying downward force on the soil surface, compressing the soil particles. The only way to change the effective compaction force is by adding or subtracting the weight of the machine. Static compaction is confined to upper soil layers and is limited to any appreciable depth. Kneading and pressure are two examples of static compaction.

Vibratory force uses a mechanism, usually engine-driven, to create a downward force in addition to the machine’s static weight. The vibrating mechanism is usually a rotating eccentric weight or piston/spring combination (in rammers). The compactors deliver a rapid sequence of blows (impacts) to the surface, thereby affecting the top layers as well as deeper layers. Vibration moves through the material, setting particles in motion and moving them closer together for the highest density possible. Based on the materials being compacted, a certain amount of force must be used to overcome the cohesive nature of particular particles.

Soil types and conditions

Every soil type behaves differently with respect to maximum density and optimum moisture. Therefore, each soil type has its own unique requirements and controls both in the field and for testing purposes. Soil types are commonly classified by grain size, determined by passing the soil through a series of sieves to screen or separate the different grain sizes. Soil classification is categorized into 15 groups, a system set up by AASHTO (American Association of State Highway and Transportation Officials). Soils found in nature are almost always a combination of soil types. A well-graded soil consists of a wide range of particle sizes with the smaller particles filling voids between larger particles. The result is a dense structure that lends itself well to compaction. A soil’s makeup determines the best compaction method to use. There are three basic soil groups:

  • Cohesive
  • Granular
  • Organic (this soil is not suitable for compaction and will not be discussed here)

Cohesive soils have the smallest particles. Clay has a particle size range of .00004" to .002". Silt ranges from .0002" to .003". Clay is used in embankment fills and retaining pond beds. Cohesive soils are dense and tightly bound together by molecular attraction. They are plastic when wet and can be molded, but become very hard when dry. Proper water content, evenly distributed, is critical for proper compaction. Cohesive soils usually require a force such as impact or pressure. Silt has a noticeably lower cohesion than clay. However, silt is still heavily reliant on water content.

Granular soils range in particle size from .003" to .08" (sand) and .08" to 1.0" (fine to medium gravel). Granular soils are known for their water-draining properties. Sand and gravel obtain maximum density in either a fully dry or saturated state. Testing curves are relatively flat so density can be obtained regardless of water content. The tables on the following pages give a basic indication of soils used in particular construction applications.

The response of soil to moisture is very important, as the soil must carry the load year-round. Rain, for example, may transform soil into a plastic state or even into a liquid. Moisture content of the soil is vital to proper compaction. Moisture acts as a lubricant within soil, sliding the particles together. Too little moisture means inadequate compaction—the particles cannot move past each other to achieve density. Too much moisture leaves water-filled voids and subsequently weakens the load-bearing ability. The highest density for most soils is at a certain water content for a given compaction effort. The drier the soil, the more resistant it is to compaction. In a water-saturated state the voids between particles are partially filled with water, creating an apparent cohesion that binds them together. This cohesion increases as the particle size decreases (as in clay-type soils). 

Achieving the right level of compaction

The desired level of compaction is best achieved by matching the soil type with its proper compaction method. Other factors must be considered as well, such as compaction specs and jobsite conditions.

■ Cohesive soils—clay is cohesive; its particles stick together. Therefore, a machine with a high impact force is required to ram the soil and force the air out, arranging the particles. A rammer is the best choice, or a pad-foot vibratory roller if higher production is needed.

■ Granular soils—since granular soils are not cohesive and the particles require a shaking or vibratory action to move them, vibratory plates (forward travel) are the best choice. Reversible plates and smooth drum vibratory rollers are appropriate for production work. Granular soil particles respond to different frequencies (vibrations) depending on particle size. The smaller the particle, the higher the frequency necessary to move it. As you compact soils with larger particles, move up to larger equipment to obtain lower frequencies and higher compaction forces.

Normally, soils are mixtures of clay and granular materials, making the selection of compaction equipment more difficult. It is a good idea to choose the machine appropriate for the larger percentage of the mixture. Equipment testing may be required to match the best machine to the job.

Asphalt is considered granular due to its base of mixed aggregate sizes (crushed stone, gravel, sand and fines) mixed with bitumen binder (asphalt cement). Consequently, asphalt must be compacted with pressure (static) or vibration.

Two factors are important in determining the type of force a compaction machine produces: frequency and amplitude.

Frequency is the speed at which an eccentric shaft rotates or the machine jumps. Each compaction machine is designed to operate at an optimum frequency to supply the maximum force. Frequency is usually given in terms of vibrations per minute (vpm).

Amplitude (or nominal amplitude) is the maximum movement of a vibrating body from its axis in one direction. Double amplitude is the maximum distance a vibrating body moves in both directions from its axis. The apparent amplitude varies for each machine under different job site conditions. The apparent amplitude increases as the material becomes more dense and compacted.

Lift height (depth of the soil layer) is an important factor that affects machine performance and compaction cost. Vibratory and rammer-type equipment compact soil in the same direction: from top to bottom and bottom to top. As the machine hits the soil, the impact travels to the hard surface below and then returns upward. This sets all particles in motion and compaction takes place.

As the soil becomes compacted, the impact has a shorter distance to travel. More force returns to the machine, making it lift off the ground higher in its stroke cycle. If the lift is too deep, the machine will take longer to compact the soil and a layer within the lift will not be compacted.

Soil can also be over-compacted if the compactor makes too many passes (a pass is the machine going across a lift in one direction). Over-compaction is like constantly hitting concrete with a sledgehammer. Cracks will eventually appear, reducing density. This is a waste of man-hours and adds unnecessary wear to the machine.

What type of equipment is used for which type of soil?

Rammers deliver a high impact force (high amplitude) making them an excellent choice for cohesive and semi-cohesive soils. Frequency range is 500 to 750 blows per minute. Rammers get compaction force from a small gasoline or diesel engine powering a large piston set with two sets of springs. The rammer is inclined at a forward angle to allow forward travel as the machine jumps. Rammers cover three types of compaction: impact, vibration and kneading.

Vibratory plates are low amplitude and high frequency, designed to compact granular soils and asphalt. Gasoline or diesel engines drive one or two eccentric weights at a high speed to develop compaction force. The resulting vibrations cause forward motion. The engine and handle are vibration-isolated from the vibrating plate. The heavier the plate, the more compaction force it generates. Frequency range is usually 2500 vpm to 6000 vpm. Plates used for asphalt have a water tank and sprinkler system to prevent asphalt from sticking to the bottom of the baseplate. Vibration is the one principal compaction effect.

In addition to some of the standard vibratory plate features, reversible plates have two eccentric weights that allow smooth transition for forward or reverse travel, plus increased compaction force as the result of dual weights. Due to their weight and force, reversible plates are ideal for semi-cohesive soils. A reversible is possibly the best compaction buy dollar for dollar. Unlike standard plates, the reversible’s forward travel may be stopped and the machine will maintain its force for “spot” compaction. 

Rollers are available in several categories: walk-behind and ride-on, which are available as smooth drum, padded drum and rubber-tired models; and are further divided into static and vibratory sub-categories. 

A popular design for many years, smooth-drum machines are ideal for both soil and asphalt. Dual steel drums are mounted on a rigid frame and powered by gasoline or diesel engines. Steering is done by manually turning the machine handle. Frequency is around 4,000 vpm and amplitudes range from .018 to .020. Vibration is provided by eccentric shafts placed in the drums or mounted on the frame.

Padded rollers are also known as trench rollers due to their effective use in trenches and excavations. These machines feature hydraulic or hydrostatic steering and operation. Powered by diesel engines, trench rollers are built to withstand the rigors of confined compaction. Trench rollers are either skid-steer or equipped with articulated steering. Operation can be by manual or remote control. Large eccentric units provide high impact force and high amplitude (for rollers) that are appropriate for cohesive soils. The drum pads provide a kneading action on soil. Use these machines for high productivity.

Configured as static steel-wheel rollers, ride-ons are used primarily for asphalt surface sealing and finishing work in the larger (8 to 15 ton) range. Small ride-on units are used for patch jobs with thin lifts. The trend is toward vibratory rollers. Tandem vibratory rollers are usually found with drum widths of 30” up to 110”, with the most common being 48."

Suitable for soil, sub-base and asphalt compaction, tandem rollers use the dynamic force of eccentric vibrator assemblies for high production work. Single-drum machines feature a single vibrating drum with pneumatic drive wheels. The drum is available as smooth for sub-base or rock fill, or padded for soil compaction. Additionally, a ride-on version of the pad foot trench roller is available for very high productivity in confined areas, with either manual or remote control operation.

Rubber-tired rollers are equipped with 7 to 11 pneumatic tires with the front and rear tires overlapping. A static roller by nature, compaction force is altered by the addition or removal of weight added as ballast in the form of water or sand. Weight ranges vary from 10 to 35 tons. The compaction effort is pressure and kneading, primarily with asphalt finish rolling. Tire pressures on some machines can be decreased while rolling to adjust ground contact pressure for different job conditions.

Information for this article was provided by Stevan Garcia, compaction product manager at Multiquip.