Getting Down to Earth

Sept. 1, 2007

What would happen if we didn’t do the right compaction? If we were building a house, the walls would crack and the whole structure could fall down. If we were making a sidewalk or driveway, the surface would crack quickly to become a hazard for users, pedestrian and driving. If we were building a road, its surface would deteriorate quickly and vehicle operators would cuss its dangers. Good compaction of the soil beneath any construction project is essential. If you mention compaction to most contractors they start to think about asphalt and pavements, but every day there is compaction completed that does not include big machines (or even convoys of machines). We are talking about soil compaction-ground compaction, if you will.

At the outset we must recommend a publication copyrighted by Multiquip Inc. It’s called Soil Compaction: a Basic Handbook. Thorough, easy to read, and most helpful, it starts by posing the question: “What is soil?”

Soil is the ground, what we stand on, what we dig, what we build on. It’s everywhere there is dry land, but it seldom looks or acts the same. Consider the different soils faced by contractors and gardeners in Iowa, Tennessee, California, Florida, and Wyoming. Each soil varies according to its source-which could be a river, lake, glacier, or even the wind-and each soil may require different compaction treatment. Whatever soil you have on which to build, the three facets that are most important are the type of soil, its moisture content, and the compaction effort necessary for transforming the soil into a stable base. “I walk across soil, drive over it, and I dig it, so why is compaction so important?” asks a neighbor. “Haven’t millions of years made the soil firm and strong enough to hold anything?” What happens when it rains? Or the snow melts? The very shape of the soil seems to change, and not in any controlled configuration. Ruts, puddles, and potholes can result from insufficient compaction.

There are many examples of poor compaction before construction, some of them more obvious than others. I’ve heard a worker wonder why he had to be so careful when compacting a trench in which some pipe was going to be laid. It was just a ditch. You put some pipe in it and filled it up again. But if the ground beneath a pipe joint settles or sags, then that stretch of pipe sags with it and a joint can be fractured, leading to enormous (and, sometimes, apparently mysterious) loss of water or sewage. The dangers are more obvious for foundations. Foundations built on poorly compacted soil will shift as the soil shifts, so that the very structure of the foundation is weakened. Cracks will allow moisture to invade, causing problems inside the structure apart from weakening the overall strength of the structure itself. If the structure is a house or a commercial building, the financial penalties for poor compaction could be enormous.

Track drive helps a Sakai compactor negotiate a steep slope.

Vibratory and Static
The soil in our gardens or on our farms and countryside is, basically, loose soil. The little pieces that make up its body are not packed closely together; there’s plenty of air between the little pieces. That’s great for gardening and agriculture, but it’s not good enough for construction. Compaction forces out the air and brings the little bits tightly together so that they can carry more load, suffer less seepage of water, keep their shape better, and prevent settling of the soil. There are two fundamental methods of mechanical compaction: vibratory and static. When you use static force, the weight of the machine pushing downward on the surface of the soil compresses the soil particles. You can change the static force by increasing or decreasing the weight of the machine. You should remember, too, that static force works well only to a limited depth.

You’ll get deeper compaction with a vibratory force. You would usually use an engine-driven vibrating force with an action consisting of something like a rotating eccentric weight or a combination of spring and piston. These compactors (which can be quite small for projects like paths or narrow trenches) give a fast sequence of blows to the surface of the ground, but they also affect lower levels of the soil. The vibration goes through the soil and, by shuffling the particles, as it were, and moving them closer together. Thus, it develops a high density. And, yes, you will need some force to get the process going. Some soils stick together naturally but not necessarily in a strong, supportive way, and you may have to encourage them with force and vibration to stick in the way you want. Those are cohesive soils.

How do know which soils you are compacting? There are three general types: granular, cohesive, and organic. Organic soils are, by their natural makeup, unsuited for compaction, and you are unlikely to be asked to make solid construction on any organic base. Most of the soils you meet will be granular or cohesive. This is a generalization. The American Association of State Highway and Transportation Officials (AASHTO) has named 15 groups in soil classification, all of them mixtures in which density and moisture vary; but for most of us it will help greatly if we consider whether the soil to be compacted is cohesive or granular. Cohesive soils have the smallest particles. Clay is cohesive (with particles ranging from 4 hundred-thousandths of an inch to 2 thousandths of an inch) and it has been successful for retaining pond beds, embankment fills, and similar projects. Cohesive soils are dense, bound together tightly by molecular attraction. When wet, they are plastic and can be molded. When they are dry, they can become very hard. For these soils it is essential, for good compaction, to have the correct water content evenly distributed. The cohesive soils are the ones that demand force such as impact and pressure. Granular soils have larger particles. Sand is considered to be anything from 3 thousandths of an inch to 8 hundredths of an inch, while gravel (fine to medium) can range from 8 hundredths of an inch to 1 inch.

How the soil accepts moisture is important for construction. Some soils are turned into a kind of plastic state by rain, while others (like those in the alley behind my office) can become downright liquid. In those conditions, the soil can support very little-hence the ruts and channels. Moisture, however, is essential for proper compaction. Think of moisture as the lubricant that helps the particles glide together. When there’s too much, some of the voids between particles become little ponds and there is no load-bearing capability. When there’s too little moisture, the particles find it hard to move to form the desired density.

Testing the soil both before and after compaction has clear merits. Before compaction, you can tell if the soil is of the right moisture content and density to accept compaction. After the compaction, testing will show how well you achieved the specified density. That information can be most helpful if there are arguments about the efficiency of your compaction at a later date. We suggest you ask local experts about testing (and recording) for moisture and density; it is not something adequately explained in a brief article. For those thousands of small projects that occur, a hand test may be adequate. This is how the Multiquip handbook describes it: “Pick up a handful of soil. Squeeze it in your hand. Open your hand. If the soil is powdery and will not retain the shape made by your hand, it is too dry. If it shatters when dropped, it is too dry. If the soil is moldable and breaks into a couple of pieces when dropped, it has the right amount of moisture for proper compaction. If the soil is plastic in your hand, leaves traces of moisture on your fingers, and stays in one piece when dropped, it has too much moisture for compaction.” For larger projects, it is strongly recommended that you use professional help for testing the soil. You’ll hear names like The Proctor Test and the Modified Proctor Test, and you’ll hear phrases like “sand cone,” “nuclear gauge,” “Shelby tube,” “Balloon Dens meter,” “nuclear density,” and “soil modulus.” Each method has its advantages and disadvantages.

This is everyday work for small soil compactors.

Equipment That Suits the Soil
Let’s stay with the smaller (and much more frequent) projects. If the soil you must compact before finishing construction is cohesive, you’ll need a compactor that impacts the soil strongly and pushes out the air. Many parts of our country have mostly clay soils. If that’s the local ground condition, you’ll need a rammer for the smallest jobs and a padfoot roller for those that are slightly bigger, such as common trenches, and need more production from the machine.

Rammers (sometimes called tampers) can be powered by gasoline, diesel, or even air. Their good production is given by the impact and speed of the blows they aim at the cohesive or semi-cohesive soils. Multiquip has five models of four-cycle gas rammers, with impact forces from 1,215 pounds to 4,550 pounds, “shoe” sizes ranging from 5.9 inches by 10.6 inches to 13 inches by 15 inches, weights from 101 pounds to 156 pounds, and areas of coverage ranging from 1,453 square feet for the smallest model to 3,900 square feet for the largest. The same company offers two heavier diesel rammers with impact forces of 3,550 pounds and 4,400 pounds, each of which is capable of covering more than 2,000 square feet per hour.

There are some residential projects where the homeowner could tamp or ram the soil on which he hopes to lay a garden path, and the impact of a manual tamper may be enough for a path that is lightly walked. But compare the force, speed, and impact of such a tool with even the smaller, engine-powered rammers. Could you do, let alone maintain, several hundred impacts per minute? With force of more than 1,000 pounds? Bomag offers a couple of vibratory tampers (the BT60/4 and the BT65/4) with frequencies of 540 to 708 blows per minute and an impact force of more than 3,000 pounds, capable of compactions deeper than 20 inches. Interestingly, Bomag says these two tampers can work with gravel-sand, mixed soils, or silt-clay. The gravel-sand will be tamped adequately at about twice the speed of the clay. Among other features that caught our eye on these two models were the anti-vibration handle, engine protection guards, infinitely variable frequency, and the polyethylene shoe with a wear plate.

Rammers, tampers, and vibratory plates work hard and fast, so it is not surprising that one of the chief concerns for users is that of durability. It couldn’t hurt to ask other contractors about their experiences with specific equipment: how long a particular brand has lasted or how easy it might be to maintain.
Wacker Construction Equipment, another leading manufacturer of this type of equipment, last year offered an innovation for its four-cycle rammer: a low-oil shutoff. An electronic capacitive switch is mounted in the base of the engine crankcase. When the operator starts the rammer, a red flashing light will alert the operator if the sensor detects low oil (or no oil) and the engine will shut off within about 10 seconds. As well as four-cycle vibratory rammers, Wacker offers oil-injected two-cycle and diesel models. We saw seven models of Wacker reversible plates (which have been especially successful for compacting semicohesive soils) as large as 21.7 inches by 35.4 inches, offering centrifugal force up to 13,600 pounds.

Many soil compaction tasks are easily completed by
the efforts of one man and one machine.

How quickly can you work? The P-Series vibratory plate compactors from Arrow-Master Inc. can travel up to 135 feet per minute with centrifugal forces from 2,500 to 5,300 pounds. Do you have a favorite engine? Arrow-Master uses Honda, Briggs, Kohler, or Kawasaki. These vibratory plates have their own unique vibration system that incorporates a full-width weight shaft and heavy-duty tapered bearings directly into the base to give them most powerful, hard impacts. Each unit also offers a reversible, swing-over guide handle and double-formed leading edge (for improved climbing). There are Arrow-Master vibratory plate compactors for both asphalt and soil.

MBW Inc. describes its rammer compaction equipment with performance and maintenance in mind: “We attack high-maintenance issues usually associated with this product type. The MBW delivery system is the lowest-friction, -heat, and -maintenance percussion unit in the industry. Less friction, heat, and wear in the delivery system translates into lower continuous horsepower demands to keep the rammer running. That means fewer engine problems and longer life.”

The R420, R421, and R440 series of rammers from MBW can compact granular, mixed, and cohesive soils in confined areas. Each weighing less than 140 pounds but with up to 3,700 pounds of compaction force, they can compact to 18 inches. The Smart Rammer series from MBW offers four sizes. They can compact up to 25 inches in depth and give 4,550 pounds of compaction force. These four rammers have integral tachometers and hour meters, so the operator knows when maximum operational performance is reached and when maintenance intervals are required.

Production Rates
Rammers, vibratory plates, and tampers are excellent for small projects, of which there are thousands every day, many of them in restricted spaces where larger equipment could not even go. When the area to be compacted is larger, something more than those one-person tools is required. Rollers are still one-person machines, but they are bigger and heavier, producing results more quickly than their little brothers and sisters. Many of the bigger compaction machines are made by the same group of manufacturers: Multiquip, Wacker, MBW, and Bomag. Add Ampac Machinery Co., JCB Vibromax, Rammax, and Mikasa. JCB Vibromax produces a vibratory compactor (JCB VM 1500 R) that shows us the productive advantage of the bigger machines. Its operating weight is more than 3,000 pounds, and its centrifugal force is 18,880 pounds. The frequency of vibrations is 1,860 per minute. JCB Vibromax points out that this model has dual scrapers on each drum, a safety control bar for the operator, and radio remote control. The working width can be 24.8 inches or 33.5 inches for its padfoot drums. It offers the feature of automatic engine shutdown in the event of a tipover.

Hamm’s new 3307 VIO rumbles along at a brisk 7.7 miles per hour.

Walk-behind or ride-on rollers with smooth (not padfoot) drums are often suitable for soil or asphalt. They are much heavier than the tampers and vibratory plates, and they do more work. You should look at the configuration of these small rollers to see how close they can get to the edge of a trench or any other obstacle. The MRH800 walk-behind tandem roller from Mikasa (distributed by Multiquip) asks us to consider these features: deadman safety controls, a noncorrosive water tank, a light on the front, front and rear drum scrapers, convenient tie-downs and lifting eye, and hydrostatic drive with no drive chain involved. That machine has a centrifugal force of 5,300 pounds and gives 3,300 vibrations per minute. By looking at the specs of competitive machines, you can soon see the questions you should ask before purchase. Is the vibrator assembly readily accessible? Is it hydrostatic drive? How easy is transportation? What is the visibility like for the operator of a ride-on roller? How close can you get to obstructions?

Frequency is not the only asset of these compacting machines. Of equal importance is the amplitude. Let’s quote Multiquip’s great handbook again: “Frequency is the speed at which an eccentric shaft rotates or the machine jumps. Each compaction frequency machine is designed to operate at an optimum frequency to supply the maximum force. Frequency is usually given in terms of vibrations per minute. Amplitude (or normal amplitude) is the maximum movement of a vibrating body from its axis in one direction. Double amplitude is the maximum movement a vibrating body moves in both directions from its axis. The apparent amplitude varies for each machine under different job-site conditions, and the apparent amplitude increases as the material becomes more dense and compacted.” How fast and how far your compaction machine vibrates are both important.

Compaction on a Large Scale Although most compaction jobs are probably small (and often forgotten as part of the compaction sector in our industry) there will always be the large-scale projects. Most people who talk about compactors are thinking of those rollers that handle whole streets or highways of asphalt. They must be big in order to finish. Their makers produce similar rollers for soil compaction, and the areas covered are of like dimensions, too. The compactors are rollers, with flat drums for most soils and padfoot versions for when the situation requires higher compression.

Sakai America Inc., a leading maker of rollers for both asphalt and soil, offers a helpful explanation of its vibratory machines: “Unlike static rollers that depend on the weight of the machine to generate the forces required to compress materials, Sakai vibratories introduce a dynamic force that helps to generate a high-compaction effect with far less effort at far less cost.” That dynamic force is made by rotating an off-balanced weight within the steel compaction drum. The rotation develops a centrifugal force, enough to lift and drop the heavy steel drum as it moves through its cycle-a cycle that is repeated between 28 and 67 times per second as the machine travels across the surface of the material. Such dynamic force will increase the compacting force of the drum (typically, six times the accrual static weight) while the vibration helps to rearrange the particles for the most dense results.
In Sakai’s list of soil compactors the two heaviest (over 30,000 pounds of centrifugal force each) have track drive. Excavation contractors of large projects can see the usefulness of such machines. The other 14 in the Sakai list offer centrifugal forces from 16,100 to 29,875 pounds; drum widths of 54 inches, 67 inches, or 87 inches; and the variety of single drums you’d expect: smooth, padfoot, pad-blade, or combination.

Caterpillar is another leader in soil compaction. Two of its many models are described as, simply, “soil compactors”-the 815F series 2 and the 825H. With respective operating weights of 45,765 pounds and 72,164 pounds, these are heavy compactors. The three models mentioned as “landfill machines” weigh 52,364 pounds, 81,498 pounds, and 118,348 pounds. Again, these are not designed for small jobs. There are eight models of Caterpillar vibratory soil compactors. Toward the top of the list, among the heaviest, are the CS-533E and CP-533E. (CS models have smooth drums, usually for granular and semi-cohesive soils; CP models have padfoot drums, for cohesive soils.) These two models offer an interesting feature found on all Cat 550E models. Two pumps give the required pressure and flow to the drum drive motor and wheel drive motor independently, so the user gets maximum torque and full-time tractive power to the drive motors regardless of footing or rolling resistance. Such a dual pump arrangement will be instantly appreciated by those of us who know that soil to be compacted is rarely provided in flat tracts. The dual pumps will also give good machine control on a grade in forward or reverse.

Other considerations for these larger compaction machines are the visibility and comfort of the operator (similar to excavators and loaders, because they are of similar size and weight). Sloped hoods help visibility. Low sound levels at the operator’s station would be desirable. On Caterpillar’s machines there are large isolation mounts under the operator’s platform that isolate the platform from the machine frame to reduce vibration levels for the operator. Can you imagine what it would be like (from the comfort and productivity aspects) trying to run a full shift of these powerful, vibrating compactors without good vibration control for the operator?

From Hamm, the compaction division of Wirtgen America, you can choose either oscillation or conventional vibration for your soil and base compaction. Oscillation? Hamm was the originator of this technique, which imparts a horizontal force into the base material, side by side, rather than a downward force by an up-and-down movement. The drum never leaves the material, whether it’s soil or asphalt. On the new 3307 VIO soil compactor, you would have this choice with a 66-inch drum. The oscillation or conventional vibration frequency is 2,160 vibrations per minute, with a centrifugal force of 27,675 pounds (oscillating or vibrating). Travel speed for this Hamm model is 7.7 miles per hour, with a Deutz diesel engine generating 87 horsepower.

“With our oscillation technology, the horizontal forces are transmitted from the drum into the material,” notes Bruce Monical for Hamm. “The result is better compaction in fewer passes, with less vibration-related wear and tear on operators and surroundings.”

What is obvious from research into sources and techniques for soil compaction is that there is ideal equipment available, whatever the size of your project. It’s an area of construction equipment in which the little jobs are certainly not overlooked and in which sophisticated, well-engineered machinery is available to complete the largest compaction projects in a profitable and efficient fashion. Not too many years ago, the success of compaction work depended very much on the skill of the operator. Today, manufacturers have addressed problems reported from the field, and the equipment not only works well but is easily maintained. Perhaps our greatest advance has been that semi-skilled workers can run the machines effectively today, a significant benefit in today’s labor marketplace.