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Compaction is a method of increasing the density of soil, which is an important part of the building and construction process. Almost all types of building sites and construction projects utilize mechanical compaction techniques. Soil compaction increases load-bearing capacity, prevents soil settlement and frost damage, provides stability, reduces water seepage, swelling and contraction. If performed improperly, settlement of the soil could occur and result in unnecessary maintenance costs or possibly structure failure, which is why it is necessary to follow proper soil compaction techniques. Three factors: soil type, soil moisture content, and compaction effort are important in ensuring proper compaction.
Cohesive soils are dense and tightly bound together by molecular attraction. These soils are plastic when wet and can be molded, but become very hard when dry. Proper water content, evenly distributed, is critical for proper compaction. On the other hand, granular soils are known for their water-draining properties. Sand and gravel obtain maximum density in either a fully dry or saturated state.
Effect of Moisture The response of soil to moisture is also very important, as the soil must carry the load year-round. Too little moisture means inadequate compaction?EUR??,,????'????EUR??,,????'???the particles cannot move past each other to achieve density. Too much moisture leaves water-filled voids and subsequently weakens the load-bearing ability.
Soil Density Tests To determine if proper soil compaction is achieved for any specific construction application, there are many tests available such as soil density tests and proctor tests.
Rammers Rammers deliver a high impact force (high amplitude) making them an excellent choice for cohesive and semi-cohesive soils. Rammers cover three types of compaction: impact, vibration and kneading.
Vibratory Plates 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. Frequency range is usually 2,500 vpm to 6,000 vpm. Plates used for asphalt have a water tank and sprinkler system to prevent asphalt from sticking to the bottom of the base plate. Vibration is the one principal compaction effect.
Rollers 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.
Walk-Behind Smooth-drum machines are ideal for both soil and asphalt. 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.
Ride-on Configured as static-wheel rollers, ride-ons are used primarily for asphalt surface sealing and finishing work in the larger (8 to 15 ton) range.
Rubber-tire 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.
Applications 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 job site conditions.
Cohesive soils call for equipment with a high impact force 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. 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 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.
Compaction Machine Characteristics Frequency and amplitude are important factors in determining the type of force a compaction machine produces. Frequency is the speed at which an eccentric shaft rotates or the machine jumps. Amplitude (or normal amplitude) is the maximum movement of a vibrating body from its axis in one direction.
Lift Height and Machine Performance Lift height is also an important factor in ensuring good compaction machine performance and compaction cost. As vibratory machines hit the soil, the impact travels to the hard surface below and then returns upward. This sets all particles in motion and compaction takes place. 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. On the other hand, 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.
Compaction Specifications Generally, compaction performance parameters are given on a construction project in one of two ways: Method Specification ?EUR??,,????'??? detailed instructions specify machine type, lift depths, number of passes, machine speed and moisture content. This method is outdated, as machine technology has far outpaced common method specification requirements. The second way is End-Result Specification, where engineers indicate final compaction requirements, thus giving the contractor much more flexibility in determining the best, most economical method of meeting the required specs. Fortunately, this is the trend, allowing the contractor to take advantage of the latest technology available.
Francisco Uviña, University of New Mexico
Hardscape Oasis in Litchfield Park
Ash Nochian, Ph.D. Landscape Architect
November 12th, 2025
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