Soil Science Simplified: Roller Manufacturers Offer Monitoring Devices to Take the Guesswork Out of Compaction

Sept. 1, 2003

Few people think of soil compaction as a science. Though most understand the important role compaction plays in the eventual success, or failure, of a roadway, a building, or other structure, many hold the erroneous belief that more is always better.

In reality, proper soil compaction is more complicated than that. It involves chemistry with the different mixtures of soil types and how moisture or additives can affect their compactive properties. And it involves physics with the effects of weight and vibration on certain materials and how air voids can influence results. The fact is that soil compaction is a science in itself. But there’s one problem: Most excavating contractors are not scientists. Therefore, they don’t completely understand every soil intricacy, nor can they be expected to.

So how can the industry encourage better soil compaction practices? It can advocate that contractors think more about the science behind compaction – or it can take the thought out of the process entirely. The latter is exactly what many manufacturers are doing with the introduction and development of roller-mounted density-measuring devices.

Smart Rollers

Top: Overall soil density system represented on the printout. Bottom: Showing how density can vary over one pass.

First introduced in the early 1990s, onboard density-measuring devices are now offered by several roller manufacturers. Though each differs in its specific method, most are designed to measure the changing stiffness of the material being compacted, which directly equates to achieved density.

The Bomag measuring device, known as the Terrameter, utilizes a set of accelerometers that measure the speed and reaction of the drum while it’s compacting. At first, when the material is soft, most of the drum’s energy is directed into the ground and little reaction is read by the accelerometers. But as the material stiffens, essentially meaning it’s becoming more dense or compacted, less energy is transmitted into the ground and more is reflected back into the equipment. The result is a more violent, or reactive, response by the drum, which is read by the accelerometers and immediately translated by the cab-mounted microprocessor as a higher-density reading, or “omega value.”

With the Terrameter system, the operator receives this information via an analog omega meter, a computer printout, and an LED console. Each method provides an easy-to-read indication when required compaction has been achieved, signaling the operator to cease rolling or to move on to another area.

Though seemingly a simple technology, the Terrameter system provides results that are as accurate and dependable as popular onsite testing devices while additionally providing results on the fly. Therefore, this gives the operator more confidence in the job that he or she is doing and helps optimize compaction results.

Compaction Confidence

No matter what the project, the primary goal of compaction is to reach the optimum density for the particular material type, ensuring the most effective base for the structure it will eventually support. Likewise, the primary purpose of a density-measuring device is to make achieving this goal as easy as possible.

The main problem operators face with standard soil compactor technologies is knowing when enough density has been achieved. The lack of confidence and insufficient information many times result in either undercompaction or overcompaction – both offering equally disastrous results.

Undercompaction simply involves compacting a material to a less-than-adequate density. When structures are built on undercompacted subgrades, significant settling can result. With roadways, this can translate into sinking or collapsing driving surfaces, whereas with buildings this can mean foundation and wall cracking, doors that won’t open properly, and perpetually stuck windows.

On the other end of the spectrum, there are those who simply err on the side of rolling more than is necessary. This is an operating philosophy that will surely result in overcompaction at some point in time. Providing almost the same outcome as undercompaction, overcompaction occurs when a material is driven beyond its maximum possible density. Because the material has nowhere to go after reaching 100% density, the roller’s continued centrifugal force begins to break down the material particles themselves, quickly diminishing the structural integrity of the soil.

Although the eventual results are similar, overcompaction is not as easy to fix compared with undercompaction. In many cases, overcompaction requires adding moisture or a strengthening agent to the material; in severe situations, it means physically digging up the subgrade and starting over.

Fortunately, density-measuring devices provide the necessary information to eliminate these concerns. First of all, there’s no need to guess what progress has been made on the material because compaction results easily can be tracked using the system’s visual indicators. With each pass over the material, the operator sees the rise in stiffness via printout or omega-meter reading. As the material becomes stiffer and energy is reflected back into the drum, compaction performance levels off. At this point, the operator is confident that optimum compaction has been achieved and it’s time to move to another portion of the job. Additionally, most systems include an LED indicator that changes color when optimum density has been reached. This makes the information easy to understand for both veteran and rookie operators. Therefore, using the system properly, an operator – with or without experience – can easily avoid any undercompaction or overcompaction situations.

Another goal of density-measuring technology is to help streamline the overall compaction process. On many jobs, onsite compaction meters are used after every roller pass to indicate how much density has been achieved. Though relatively effective, this step results in a major slowdown in production. With every test, the roller operator must stop and wait for the results before completing another pass or moving to another area of the job.

Conversely, using a density-measuring device allows the operator to keep working while simultaneously receiving compaction results. The elimination of the time-consuming onsite soil testing keeps the operation on task and productive.

Proof Rolling and Surgical Compaction

In addition to simplifying the operator’s role and ensuring that optimum compaction has been achieved on every job, density-measuring technologies have revealed some new operational roller practices. One of these is proof rolling.

Before beginning actual production, an operator can use a roller with a density-measuring device and initially run over the entire job site. This “proof roll” provides a printout for the area to be compacted. On the printout, lines that are lower on the chart (low omega values) indicate areas of weakness, whereas higher lines (high omega values) show stronger, more compacted areas. Offering a comprehensive compaction snapshot of the job site, the proof roll lets contractors develop a more logical plan of attack for the material.

As lifts of material are deposited and production rolling begins, operators watch the density improve while still keeping an eye on the weak spots. At the point that optimum density has been achieved over most of the job site, operators can go back to those specific areas of weakness and surgically compact them to achieve density. This allows for consistent compaction values to be accomplished across the full range of material without risking overcompaction on the stronger areas; it maximizes productivity by addressing only those portions that need extra attention.

Paper Trail

How a soft spot shows on a Terrameter printout.

In addition to providing a map of the job-site surface, the printout from the density-measuring device can offer the contractor substantial value from a documentation standpoint.

All contractors must agree to some type of guarantee on their work. This is typically specified in the bid and locks the contractor into a particular quality and performance standard. Because of this, contractors should document the results achieved on every job in order to offer some sort of recourse in the event that a problem arises within the warranty period. If the documentation isn’t available or sufficient enough to prove that the job was done right the first time, the contractor may be required to come back and fix the problem at his own expense.

Onboard density-measuring devices that offer printouts provide the ultimate documentation for compaction jobs. The printouts provide a readout of the entire area compacted with the roller. In comparison with engineering core samples or onsite measuring devices that prove definitive compaction results only where the test was administered, density-measuring-device printouts indicate compaction achieved for the entire length of the lane traveled and the entire width of the drum. Therefore, they are able to show what was achieved for 100% of the compacted area. Additionally, the printouts exhibit a progression of compaction, from one pass to the next, and include a time/date stamp to indicate the job’s pace.

So if a project goes awry and the county, state, or federal authority comes back to the sub-base contractor for answers, a density measurement printout not only allows the contractor to show compaction results for every square inch of the job but also indicates how much was completed before and after lunch.

More Bang for the Buck

With the obvious benefits of comprehensive documentation for every compaction job and added efficiency and productivity, it’s puzzling that most contractors are not currently utilizing an onboard density-measuring device. For the most part, this is due to cost.

Many contractors do not feel they can justify the expense for such a device. Since they might be getting by with their current rolling technology and onsite testing devices, contractors see any extra expense as directly cutting into potential profits. But the opposite is actually the case.

Exploded illustration of the drum and how accelerometers are positioned.

What most contractors fail to see is the amount of resources wasted with some standard practices. With every pass a roller makes over a certain area, time, operator dollars, and maintenance costs are invested. If the roller makes eight passes when only five were required for optimum density, that’s a waste of time, money, and equipment longevity, which ultimately translates into lost profits – not to mention the potential costs associated with overcompaction.

Additionally, factoring in the time and money it takes for an onsite measuring device also favors new roller-mounted technologies. Again, onsite testing utilizes one operator to administer the test while the roller operator waits. Repeating this same practice after each pass, throughout the entire duration of the job, results in a substantial loss of operator productivity.

Using a roller-mounted density-measuring device basically eliminates these wasteful actions. It minimizes the amount of time a roller must be utilized to compact a surface and requires only one operator for both the testing and equipment operation. Because the production rate of the machine is increased, the job is completed sooner – resulting in an immediately noticeable increase in profits.

Some contractors hesitate to employ this technology because they don’t want to incur the expense of buying a new roller. But manufacturers are countering this argument by offering field-installable kits that will add density-measuring devices to an established roller fleet. By simply attaching the accelerometers to the drum and running communications cables back to the cab-mounted microprocessor, contractors can enjoy the benefits of density-measurement technology as though it were pre-installed at the factory.

The science of compaction remains complicated, but with the use of roller-mounted density-measuring devices, understanding basic soil properties might no longer be required to achieve compaction goals. Manufacturers continue to refine the technology, gaining more acceptance of the process and overcoming objections to the price. But the key to the furthering of this technology lies with the contractors. As they begin to experiment with it and realize the benefits of increased production, reduced job costs, and better, more provable results, the question will no longer be how contractors can afford it but, rather, how they can afford to work without it.