Selecting a Soil Compactor

Editor’s note: This article first appeared in the March/April 2016 issue of Grading and Excavation Contractor.

Soil compaction is the removal of air voids from soil by applied force in such a way that it conditions or compacts soil beneath or next to the construction, resulting in a denser soil with the ability to hold greater weight, points out Fred Paul, Wacker Neuson sales engineer and application manager.

Air voids are undesirable because they provide a place for water to accumulate, and also allow material to settle. Individual particles can over time re-orient and occupy the voids, says Jon Sjoblad, Caterpillar Paving Products spokesperson.

“Using the proper materials, having the proper moisture content, following the recommended lift heights, and making the required number of passes are the most important factors in achieving good compaction,” points out Vince Hunt, Wacker Neuson product application and training specialist.

“The natural ability of soil to support a structure becomes altered during excavation,” says Paul. “Its density changes and some soils become exposed. Mechanical soil compaction accelerates the natural process of the soil settling, and in some cases makes construction possible on marginal building sites.”

Soil compaction serves as preventive maintenance against settling, cracking, or heaving.

“The effects of inadequate soil preparation may take weeks or even years to become apparent,” says Paul. “However, once the results begin to show, the costs of repairs far outweigh the time and rental costs of proper soil compaction the first time around.”

Choosing the right size compactor is critical to every compaction project, points out Tim Hoffman, rollers product manager for the Atlas Copco road construction equipment division.

“Using one that’s too heavy or too light can have a huge impact on productivity and costs,” he adds. “If it doesn’t have enough force or weight to compact the material efficiently, you’ll need to make more passes to achieve the desired compaction.

“On the other hand, a roller with too much force and weight will crush the material rather than compact it, which requires replacing the material. And if a roller is too narrow for a job, it will require more passes to achieve the desired compaction. This all results in more fuel costs and more time needed to finish the job.”

There are three types of compaction equipment: static, vibratory, and impact, points out Hoffman.

Static rollers come in three-wheeled, tandem, or pneumatic-tired designs and rely on the weight of the drum and machine to compact material. They work best for compacting thin layers of soil or asphalt.

The company’s walk-behind and hand-guided equipment is targeted to small- to medium-sized jobs.

“If it’s native soil, you have to look at the conditions,” he says. “If it’s very cohesive—a very water-logged, clay-like substance—certain types of equipment are more suitable than other types of machines.

“If it’s soil being brought in—for example, at a highway on and off ramp with steep grades where you may be removing the native soil and replacing it with something being brought in from the outside—you have much more control over the type of soil and the type of equipment you can use and you’re much more effective.”

Soil is primarily classified into two main categories, says Paul: granular and cohesive.

Granular soils consist mainly of sand and gravel. The particles are coarse and large enough to see with the naked eye. When granular soil is wet, it may be molded but will crumble easily.

Granular soils are best compacted with vibration energy either with a vibratory plate or vibratory roller. The vibration action reduces the frictional forces at the contact surfaces, allowing particles to fall freely under their own weight.

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.

For non-cohesive soils, a large reversible compactor is usually more cost-effective than a small roller might be, says Witt.

“For cohesive soil, you’d have an issue with reversible plate compactors as they tend to dig themselves in,” he adds. “This has to do with the physics of the reversible plates technology and you either use a large trench roller or impact compaction rather than vibratory compaction.”

An example of impact compaction in road building would be a small jumping jack—an upright machine that pounds the ground “so it’s brutal physical pushing rather than agitating the soil and removing water and moisture that way,” says Witt.

Cohesive soils are made up of silts and clays. The grains are very small and feel smooth when rubbed between one’s fingers. When wet, cohesive soils are sticky and can be molded or rolled into almost any shape. When dry, this soil type tends to be very hard and difficult to crumble, notes Paul.

Cohesive soils are best compacted by impact force, says Paul. “The type of machine recommended is a vibratory rammer or sheepsfoot roller,” he adds. “The impact force of the rammer or sheepsfoot roller causes a shearing effect that squeezes out air voids and excess water from between the particles.”

Mark Eckert, compaction product manager for Volvo Construction Equipment, says that for cohesive material, “you would want to choose a pad foot drum compactor or possibly a smooth drum compactor with a pad foot shell kit.”

Other factors considered in choosing compaction equipment include “layer thickness, size of the project, water content, stiffness of the underlying layer, compaction specifications, capacity requirements, and climatic conditions,” says Hoffman.

“It’s nearly impossible to achieve a high degree of compaction of soil over a low-bearing capacity soil, including soil with fine grains or high water content. For these types of soils, a vibratory roller will be the most effective,” he adds.

Dave Dennison, BOMAG product manager, adds other factors that influence equipment choice. They include:

• Compaction depth. “Contractors must remember that vibratory rollers compact the lift from the bottom up,” notes Dennison. “Therefore, the contractor must know what the lift thicknesses and fill material type will be in order to select a compactor that delivers enough vibration force to penetrate to the bottom of the lift. Otherwise, bridging can occur and the material that is not densified can lead to road failure.”
• Terrain. When building a road, compacting base and subbase on steep terrain will require a single drum roller with both drum and rear wheel drive, such as BOMAG’s smooth drum BW 177 DH-5 and padfoot drum BW 177 PDH-5 rollers, says Dennison. These rollers feature a dual-travel pump drive system with one travel pump dedicated to drum drive and the other to the rear axle. This enables the BW177DH-5/PDH-5 Series rollers to climb up to 24% higher grades than rollers with a single travel pump and parallel flow hydraulic system.
• Project area size. The larger the project size and the deeper the lift thicknesses, the larger the roller and wider the drum required on the job, says Dennison. BOMAG’s single drum rollers have drum sizes from 47.4–84 inches wide. Its double drum rollers range from 35.4–84 inches wide.
• Production goals. As with project area size, the larger the compactor, the deeper the lift thickness and the more area covered in a shorter period of time, says Dennison.

Along with the type of soil being compacted, the method of compaction used is largely determined by the job specifications that need to be met.

“Machine selection is often dictated by compaction specifications,” says Paul. “The most common specifications include lift, or soil depth/layers and the number of passes made with the machine to achieve optimum compaction.”

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

The number of passes depends on the situation, but to attain 95–100% Standard Proctor density, use three passes for a rammer, four for a vibratory plate, and five for a vibratory roller, notes Paul.

The actual job site is an important factor when selecting equipment, says Paul.

“In general, vibratory plates are more efficient than rollers due to the large baseplate surface area in contact with the soil,” he adds. “Vibratory plates also are more maneuverable, so they are ideal for confined areas. Rollers have the advantage in larger, open areas where there is plenty of room to maneuver.

“Because the roller has a fast travel speed, it will cover the area faster than a plate. The same logic applies to rammers, which should be used on cohesive soils in confined areas. The trench roller should be used in larger trenches and open areas.”

Smaller equipment is generally used on smaller-scale applications, such as commercial work (driveways, parking lots, small paths) and utility work (trenches and asphalt patching). Smaller equipment also is used on larger sites where there are many small tasks or the need to frequently trailer the equipment from site to site, such as compacting highway approaches, points out Sjoblad.

Typically, contractors can use walk-behind light compaction equipment, such as double drum rollers or plate compactors for small projects, including driveways or asphalt patching, notes Hoffman.

“Some are static rollers for thin layers of granular soil, while others are vibratory rollers or plate compactors that can be used on soil or asphalt,” he says. “Walk-behind rollers have narrow drums of about 25 inches, which are ideal for narrow areas such as bike paths.

“For similar projects that are longer in length, contractors also can drive compact, gas-powered asphalt rollers, such as Atlas Copco’s CC900G. The roller’s drum is 35 inches wide and has 18-inch curb clearances on the left and right sides of the machine so operators can compact close to curbs and landscaping.”

“Often you will see single direction and reversible plate compactors, like the BOMAG BVP 18/45 and BPR35/60, compacting granular and mixed soils,” says Dennison of the use of compaction equipment on small jobs. “If upgrading utilities, the articulated BOMAG BMP8500 is used for backfilling of trenches. BOMAG’s small single and double drum vibrator rollers will be used to compact base materials prior to paving the driveway.”

Rollers used on smaller jobs typically fall in the 3- to 5-ton weight class, and range from 39–54 inches in compaction width, such as rollers found in Sakai America’s SW/TW 300 Series and the SW502-1 Series, says Josh Steele, marketing manager with Sakai America.

“On some of the smallest jobs, you may even see only a small 1.5-ton roller, such as Sakai’s 36-inch CR271, or even only a walk-behind roller such as Sakai’s steerable front drum HS67ST,” he adds.

Sakai’s 58-inch, 10-ton class roller, the SW652-1, is used for small- to medium-sized jobs, notes Steele.

“Additionally, on these thin lift pavements, you also could find oscillating rollers, which make use of horizontal vibration patterns in order to effectively compact without damaging the aggregate,” says Steele. “Oscillatory rollers are also more effective on colder asphalts than typical vertical vibratory rollers.”

Sakai’s oscillation system is designed for versatility, making use of a gear-driven system with no belts to change or drums to disassemble for routine maintenance to switch back and forth from oscillation to vertical vibration on the fly. It enables the contractor to use the roller in a wide variety of applications without investing additional capital on more equipment, notes Steele.

Larger equipment is used on jobs in which scale or size counts. “This can include large site development, roads, large parking lots, and airport runways,” says Sjoblad. “The larger equipment is more efficient and can process more material than the smaller equipment in the same amount of time.”

For medium to large jobs, most contractors will use a combination of small tandem and/or small to large single drum ride-on vibratory rollers to compact the base and subbase material, says Dennison, adding the roller size and type utilized depends on such factors as project size and the contractor’s production goals.

A static tamping roller can be used for compacting cohesive soils on large projects, says Hoffman.

“Atlas Copco’s CT3000 moves as fast as 12 miles per hour, which is much faster than a vibratory soil roller,” he says. “Like other tamping rollers, the CT3000 features four padfoot drums, but the drums are mounted on rubber tires to absorb impact, designed to minimize wear on the drum and vibrations to the operator platform.”

For large soil compaction projects, a ride-on soil roller should be considered, says Hoffman.

“These weigh four to 27 tons and feature a single vibratory drum that ranges from 54 to 84 inches wide. Most soil rollers are suitable for compacting soil in relatively thick layers, but when compacting rock fill, the roller selection would be determined on depth of material and size of project,” he says. “This ensures there’s enough force for effective compaction. Soil rollers typically feature a smooth drum, but contractors also can equip them with a bolt-on padfoot shell kit for compacting cohesive soils such as silt and clay.”

Large asphalt projects can utilize a ride-on asphalt roller, says Hoffman.

“The rollers, which range from one to 18 tons, feature two vibratory drums that are 32 to 84 inches wide. Operators can use asphalt rollers for compacting thin to medium layers of base course, sand, and gravel.”

Some asphalt rollers also come with pneumatic tires, such as Atlas Copco’s CP1200, he adds.

“The roller features pneumatic tires and a flexible steel ballast that evenly distributes weight between the tires for consistent ground pressure,” says Hoffman. “Pneumatic-tired rollers compact areas as wide as 7.55 feet and are ideal for finishing and sealing thin layers of asphalt.”

Sakai’s rollers are in the 8- to 12-ton range with a compacting width between 67 and 84 inches and are used for larger base work, allowing for more efficient use of labor on larger jobs, and speed through the work, says Steele.

Sakai’s models in these classes include the SV412, SV540, and SV640 Series rollers. The 5-ton SV201 Series with a 54-inch width is used for smaller roads and base work.

“The drum choice for each of these will depend on the type of material being compacted and the current task in the base construction,” says Steele. “For initial backfill and placement, a padfoot drum cuts through thicker lifts to build quicker roads and maintain compaction throughout. On top layers of the base work, switching to a smooth drum roller gives the smooth finish you need in order to begin placing asphalt.”

Sakai mounts a smooth shell on top of a padfoot drum to add to the operating weight of the roller, improve the compaction ability for the smooth application, and enable easier installation in contrast to trying to clamp pads together, says Steele.

Smaller rollers are used on laying asphalt for smaller jobs, such as driveways, small parking lots, and bike paths, he says.

“These applications will typically see a smaller lift of asphalt pavement placed,” points out Steele. “The thinner lift means that the roller used needs to be lighter weight to avoid shoving the mat. Additionally, due to the reduced size of the job, a smaller roller can accomplish the same job in the same amount of time as a larger roller with the same dynamic force per inch.”

That enables contractors to save money on fuel burn and freight costs to and from the job site, he adds.

For large areas, wider drums—up to 84 inches in width—may be selected, so fewer side-by-side passes are required to cover the area, and heavier machines with more force may be needed for thicker lifts,” says Eckert.

Most single drum compactors of 7 tons and larger also have high and low amplitude selections for adjusting drum performance when lift thickness changes, he adds.

“High amplitude may be selected for thick lifts, while low amplitude may be selected for thinner lifts,” says Eckert. “Some single drum vibratory compactors also have a selection of vibration frequency settings, so a frequency can be selected to more closely match the resonant frequency of the material being compacted, adding efficiency to the compaction process.

“Many types of ride-on compactors are used for the compaction of asphalt pavements,” says Eckert. “There are double drum or tandem static and vibratory compactors, pneumatic tire compactors, combination compactors with a steel vibratory drum on the front of the machine, and rubber tires on the rear of the machine and three steel wheel static compactors.”

There also are special features such as oscillatory drum technology, multiple amplitude, and variable vibration frequency drums to consider when choosing tandem compactors, says Eckert.

“Many times there are method specifications that tell the contractor which compactors to use and where in the ‘roller train’ to utilize them,” he adds.

Choosing the size and weight of the compactor, the width of the drum, or the number of tires for pneumatic rollers depends on the pavement panel width and type and thickness of the pavement material, says Eckert.

“Choosing the appropriate drum width for the pavement panel can optimize the number of side-by-side passes of the compactor required to cover the pavement panel uniformly,” he adds.

Wacker Neuson makes the following equipment recommendations:

• For cohesive/clay and mixed soil in a confined area, use a rammer for excavations 4–14 inches wide or adjacent to structures. In larger areas, use a sheepsfoot roller for excavations 24 inches wide or adjacent to structures.
• For sand/gravel in a confined area, use single direction plates for smaller areas and shallow lifts less than 12 inches. In larger areas, use reversible plates for areas with lifts deeper than 12 inches. Smooth drum rollers are used in parking lots and sidewalks.
• For asphalt, patches, and edging along curbs and light poles, use single direction plates. Ride-on rollers are used in larger areas for compaction of asphalt, gravel sub base, and driveways, among other uses.Sjoblad explains the uses of the five types of compactors offered by Caterpillar:
• Single drum vibratory soil compactors, large and small, which can be equipped with smooth or padfoot drums (also known as sheepsfoot or tamper foot). “Soil machines are used on soil and aggregate materials,” he says.
• Static tamper foot soil compactors for large-scale thin lift applications. “A smooth drum machine is used with granular materials; padfoot drums—also known as sheepsfoot or tamper foot—are used in cohesive and semi-cohesive materials. Vibratory rollers are the most productive, as the vibration reorients soil particles and significantly enhances the static pressure of the machine,” says Sjoblad.
• Tandem drum vibratory asphalt rollers, large and small. “Asphalt rollers have two smooth drums and are usually vibratory in both drums. They are equipped with water spray systems to keep the asphalt from sticking to the drums,” he says. “The vibratory systems often can provide multiple amplitude and frequency settings to help optimize the performance on varying asphalt mixes and thicknesses.”
• Pneumatic tire rollers, used on both soil and asphalt. “Pneumatic rollers are static with eight to 11 wheels—depending on model and configuration—that are arranged in an overlapping pattern,” says Sjoblad. “This provides a kneading effect on the material, helping to tighten the surface. They are used on both asphalt and soil applications.”
• “Combi” rollers—smaller compactors equipped with a vibratory drum on the front and pneumatic tires on the back. They are used primarily on asphalt but also are used on soil applications. They offer the kneading effect of a pneumatic roller combined with the smoothing effect of a tandem drum roller, says Sjoblad.

Advances in compaction technology, such as high frequency vibration, multiple amplitude settings, and oscillatory drums are a few innovations that have increased compaction performance and productivity over the years, notes Eckert.

“Being able to adjust the amplitude based on material type and lift thickness can optimize the amount of force to achieve the specified density efficiently. High frequency vibration allows for faster rolling speeds to keep up with faster paving speeds, while maintaining proper impact spacing to ensure smoothness on the finished pavement,” he adds.

Oscillatory drum technology provides an alternative for traditional vibratory drums, points out Eckert.

“Oscillatory drums direct the drum forces into the material tangentially compared to the standard vertical amplitude provided by traditional vibratory drums,” he says. “This can be particularly beneficial on extremely thin paving lifts or while compacting over utilities, on bridge decks, or on tender asphalt mixes.”

In general, technology makes work more efficient, and also improves its quality. For the contractor, this means less cost and less risk, points out Sjoblad.

“Traditionally, operators use experience and intuition to guide compaction—they work until they feel the work is up to specification,” he says, adding this process is vulnerable to human error.

In some areas, “method specifications” govern compaction, notes Sjoblad.

“Engineers determine that a certain size roller would need to make a certain number of passes over the material and the compaction should then meet specification,” he says. “This process also is vulnerable to error, as variability in the materials or human mistakes on the rolling pattern can affect the outcome.

“This compaction work is followed up by testing: a crew equipped with portable testing devices such as a nuclear gauge, geogage, lightweight falling deflectometer, or plate load makes a spot check every so many square yards or meters.”

If the test passes, the section passes. If the test fails, compaction work on the section continues until testing passes, points out Sjoblad.

“Often, this manner of spot testing means that less than one square foot per million is assessed for compaction quality,” he says. “That leaves a lot of room for problems, because the test spot is the assumed value for the entire section.”

Intelligent compaction (IC) technology—which may be utilized by virtually any compactor—is most effective when installed on tandem drum vibratory compactors being used for breakdown rollers, the compactor in the first position behind the paver, notes Eckert.

“IC systems typically include high-precision GPS, material temperature sensors, accelerometers, and a computer display screen to display and record information during the compaction process,” he says. “These systems provide the compactor operator with real-time pass mapping, temperature mapping, and intelligent compaction measurement values [ICMVs]. Led by the Federal Highway Authority [FHWA], many state DOTs [departments of transportation] are implementing IC specifications. Intelligent compaction has been extensively tested by FHWA and many state DOTs, and has proven that compaction results are more uniform and consistent when IC is utilized.”

Intelligent compaction is still in its infancy in terms of adoption, says Sjoblad.

But the potential is there to significantly improve the quality and uniformity of compaction while lowering production costs and increasing the life of roads in providing operators with real-time information that helps them determine the state of the compaction work without relying on guesswork, adds Sjoblad.

“It tests wherever it rolls, providing assessments on each square foot. Thus it provides much more data and verification than spot testing can,” he says. “This can reduce the need for the number of tests, reducing costs and delays that occur when testing commences.”

Depending on the intelligent compaction system used, contractors can view many job facets, including the area they have covered and how many times they have been over an area—the pass count, says Sjoblad.

“It also will show areas that have been missed,” he adds. “On soil, various systems can provide an indication of the soil stiffness, which is one way to determine the load-bearing strength. On asphalt, some systems show the mat temperature, a critical factor to proper compaction.”

As it grows in popularity, intelligent compaction is giving contractors a competitive advantage by allowing them to ensure the uniformity of the jobs they perform, notes Steele.

“Documenting the entire compaction process from start to finish allows them greater security while also providing the contractor with additional confidence in the work being done,” he says. “Beyond this, it also allows the contractor a real-time measurement of the effectiveness of their compaction efforts.”

For simple applications that do not require a full-on mapping solution, Sakai offers its compaction control value (CCV) system on both dirt and asphalt rollers of all sizes. The CCV system involves an accelerometer-based stiffness value read on an operator display.

For applications where pass count mapping and full documentation are desired, Sakai has teamed up with Topcon to offer its CCV stiffness values while using Topcon GPS and display technology with Compaction Information System Version 2 (CIS2), which is fully Veta- and FHWA-compliant.

Hoffman points out that the ability of intelligent compaction systems to optimize efficiency and ensure high-quality results through real-time data such as pass counts and relative stiffness. For asphalt applications, temperature eliminates the need to take core samples during the compaction process.

Atlas Copco’s Dyn@lyzer intelligent compaction system is used on both asphalt and soil.

“This is possible because the accelerometers measure the drums’ responses to ground conditions about 3 to 5 feet below the surface—not just the asphalt mat,” says Hoffman. “As the surface density increases, the frequency response of the drum changes. That frequency change is measured by the accelerometers and instantly displayed on a tablet inside the cab, showing the operator if additional passes are required or when optimal compaction has been achieved. Operators also can set project parameters and be alerted at optimal compaction levels.”

For quality assurance, contractors also can use the compaction data to prove to regulating agencies such as a state department of transportation that compaction standards were met, says Hoffman.

“This minimizes the risk of costly litigation or having to restart the project,” he says.

BOMAG’s Economizer is intelligent compaction for its line of plate compactors and single and double drum vibratory rollers. It is designed to offer intuitive operation with real-time compaction results read from the instrument panel’s LED display.

Once the machine’s optimum working frequency has been reached, yellow LED lights indicate the level material stiffness. As compaction increases, more LED lights illuminate.

When the number of yellow lights stops increasing, the maximum level of compaction has been achieved.

If all 10 yellow lights are illuminated, a red LED light illuminates as an indicator to the operator to stop compacting that section. “This avoids over-compaction and fracturing of the aggregate and costly removal and replacement of material,” says Dennison.

Additionally, the Economizer allows the operator to do proof rolling of the area to look for and address weak areas.

Wacker Neuson offers Compatec, a compaction display that gives the operator live feedback regarding the relative compaction in an effort to avoid unnecessary passes. Compatec also protects the machine by alerting the operator when the machine is operating in an over-compacted situation with flashing LED lights.

Intelligent compaction on Weber MT’s walk-behind machines allows users to produce uniformly-compacted soil at the same level throughout the entire job field, notes Witt.

Weber MT’s intelligent compaction is designed to improve time efficiencies by up to 25%, which also saves on costs, points out Witt.

“It’s first and foremost a quality assurance tool,” he says. “But it’s developed into an efficiency tool because this device will let you know when you’ve achieved maximum compaction—so there’s no way you can waste time. Prior to that, it was a bit of a guessing game. Operators would tend on the side of conservative behavior and stay on the soil longer than was necessary and at best would just waste time.

“Worse, soil can also be over-compacted—break up below the surface when staying on the area too long. Any compaction tests would indicate failure. This possibility is eliminated with intelligent compaction.”

Regarding intelligent compaction, Multerer says “most of these technologies are surface testers. They’re good on asphalt because the reaction you’re getting in the machine to the surface conditions of the asphalt means it’s not a huge leap to assume that you’re getting a similar result two or three or four inches below the surface and that happens to be the typical lift or layer of asphalt that’s compacted at one time.”

In the last decade, there’s been a trend to adapt the technology to soils, he says, adding that it’s effective on large machines with a high mass dealing with a specified lift of 12–18 inches—12 being the typical DOT spec.

“Even for heavy machinery on highway applications, the technology is quite good,” he adds. “The problem occurs when you try to apply a surface testing technology to light equipment. In this country, the number one cause of compaction or report failure is the fact that the field crews are out of compliance with good practice in terms of lift deck.

“You have to be very careful when you apply it to light equipment and variable lift depths or inadequate moisture contents. The conditions of compaction have a great deal to do with whether you’re getting good readings or false positives.”

MBW’s intelligent compaction technology is used primarily in gas utility applications. It uses a piezoelectric sensor which is placed at the bottom of a typical utility trench. Lifts are placed over the sensor.

“As you place each lift to compact that, every time the shoe of the rammer or the plate hits the surface of the lift, it imparts a pressure wave which travels through the lift until it hits the sensor,” says Multerer. “We’re watching the decreasing rate of increase in the amplitude of a pressure wave traveling through soil. It is in fact a direct index of soil stiffness taken from the bottom up rather than at the surface, presuming you’re getting a good result at the bottom.