Since the days of Julius Caesar, his Appian Way Road Department and that Leaning Tower down the road, man and woman have been searching for the ultimate tool to compact soil. Today that search has led to a wide range of large, self-propelled vibratory compactors for site work, dams and road bases. But what’s the contractor to do down in the trenches, around foundations and in other tight spots to gain density quickly and cost-effectively? And hey, what does soil density really mean anyway?

In basic terms, soil density is the elimination of air voids in a lift or layer of loose soil, which has been spread to alter an elevation or slope or has been used as backfill material. The less air left between the particles, the higher the density and load-bearing capacity. Meeting specified density parameters ensures the proper support for the structure to be placed on top of it. Creating soil density in confined jobsites can be done with a variety of compact machines — rammers, vibratory plates, walk-behind rollers and ride-on compaction rollers. But which machine is right for your application?

Picking the wrong machine is costly, especially when the problem is in the bearing surface beneath the structure. Since most all soil compaction provides a foundation for one structure or another, it’s key to ensure that density parameters are met — even on smaller jobs. Any callbacks to remedy failed density will be costly in time and money, so it is mandatory to get it right the first time.

Choices in Ride-on Rollers
Static compactors have been around for thousands of years. It is said that the aforementioned Caesar’s Road gang used to herd sheep up and down the path of the Appian Way to gain compaction. Thus the “sheepsfoot” drum configuration of more modern day compactors.

There are a few different static roller types for soil compaction — smooth steel drum, sheepsfoot drum and pneumatic tire. One common feature (and a negative one at that) is that density is achieved only at the upper layer of the lift. So if it is necessary to compact 12 in. of loose fill material at once, the static roller may only meet density specs on the upper half of the lift thickness. Over time, should water be able to penetrate this lower material, damage in the form of erosion could occur, compromising the structure. It is important to note that since it is not possible to see density, construction must be monitored by a nuclear gauge designed to measure voids. Newer, safer non-nuclear gauges are on the horizon.

Since all uncompacted fill material is subject to settlement, it is critical to compact the complete thickness of the lift. Vibratory compactors, first introduced right after World War II, put the soil particles in motion, allowing them to find their natural place in the matrix, quickly eliminating air voids. These machines provide far greater compaction in depth and permit thicker lifts to be spread. Actual thickness depends on the type of soil to be compacted and the force generated by the compactor.

There are two basic drum configurations for vibratory compactors — smooth and padfoot. The padfoot models have been found to provide better compaction than the sheepsfoot models and are more effective on clay and other cohesive materials. The smooth drums work best on granular and semi-cohesive materials. Contrary to some current practices, small vibratory asphalt rollers should not be used for soil compaction for two basic reasons. First off, soil materials resonate at and thus react better to lower vibration frequencies around 1,800 vibrations per minute (vmp). Today’s asphalt rollers are designed to work faster at frequencies around 4,000 vpm. Secondly, compaction of soil with an asphalt roller can ding the drum. These dings may be reproduced in the asphalt when it is used for that application and contract bonuses are paid for smooth finishes.

The earthmoving site contractor should know that there are many different vibratory soil compactor drum widths available up to about 100 in. and down to 40 in. or so. It is this latter group, with 40- to 55-in. drum widths and operating weights in the 5,000- to 10,000-lb range, that will be considered here. These utility-type compactors are self-propelled and use both the static force (dead weight of the machine at the drum) and the vibratory centrifugal force to gain compaction. They will be far more productive than a static-only roller and bring a far better return on investment.

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Small Vibratory Soil Compactor Specifications
Although specifications alone aren’t everything, here is a brief listing of the important ones for smaller, vibratory compators:

1. Drum Width: As with any compaction application, drum width is the first and foremost parameter. It must fit your primary application.
2. Drum Configuration: Along with width, the contractor should know the type of material being compacted. It is not possible to gain sufficient compaction on clay or other cohesives using a smooth drum. For these applications, a padfoot drum is in order. Some manufacturers, like Sakai, offer a smooth shell that fits over the padfoot drum for application flexibility. Beware of models offering a padfoot shell that fits over the smooth drum.
3. Vibration Frequency: Look for a machine with 1,750 to 2,000 vpm. Faster is not better here.
4. Centrifugal Force: The vibratory soil compactor has an unbalanced weight rotating within the drum. This develops the vibration and the force. Generally speaking, the higher the force, the better. This is the main feature you are paying for with a vibratory soil compactor.
5. Nominal Amplitude: This is the result of the force applied to the weight of the drum assembly measured in inches. This spec will generally follow the centrifugal force spec. All else equal, the amplitude should increase as the force increases.

A good rule of thumb is to take the static weight at the drum and add it to the centrifugal force to determine a Total Applied Force. Divide this total by the width of the drum to determine the PLI or force per linear inch across the drum’s surface. While not totally accurate, this is probably the best model-by-model comparison figure you will find.

What Else Is Important?
Specifications aside, there are a number of productivity and safety features that the contractor needs to look for in a small vibratory compactor. OEM Sakai America of Adairsville, Ga., may be the only compactor manufacturer to include both ROPS and seat belts on every machine it markets here in the United States. The machine you buy or rent should have lifting eyes to enable the machine to be picked up and set down in the trench.

For applications where it is possible, it should have the gradability to climb steep slopes and in and out of the trench under its own power.

All vibratory compactor manufacturers suspend the vibrating drum from the frame using special shock isolators. These shock isolators help direct the powerful vibrations down into the ground and keep them from the operator and critical components. They should be covered by their warranty and should be designed to last many years.

A contractor planning to purchase the roller would do well to ask for a history of the service life of these isolators.

The steering hitch of these machines connects the drum assembly to the tractor half of the machine. In addition to handling the steering functions of the machine, it oscillates radially to keep both the drum and the tires on the ground when operating over rough terrain. The steering hitch needs to be both heavy-duty and reliable as it is holding two halves of the machine together — each weighing about 5,000 lbs apiece.

And in today’s world, fuel efficiency of the contractor’s next soil compactor has to be superb. Compact vibratory soil compactors are a versatile and cost-effective way for contractors to get the job done fast to turn a profit. With a good basic knowledge of both the application and the product choices available, the contractor handling residential site prep and other smaller sized projects can make the decision that will bring him or her to a better bottom line.

Dave Brown is vice president of sales and marketing at Sakai America, Adairsville, Ga.

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