The science of landfill compaction

The idea that a steel-wheeled landfill compactor can help pack more trash into the limited space of a landfill is a simple one, largely because it makes logical sense. The step between the tool and the result, however, is where many operators struggle.

Waste compaction might not be rocket science, but it is a science.

When Fred Caron, founder of Caron Compactor Co., unveiled the first landfill compactor, the Pactor, in the 1960s, he understood—long before anyone else—that waste compaction is a vital part of running a landfill.

Since releasing its first compactor, Caron Compactor Co., based in Escalon, California, has stood for innovation when it comes to landfill compaction. Yet more than half a century later, the industry still is trying to apply basic laws of physics to something as obscure as waste compaction. Let’s talk about how to put some science into the compaction process.

Manage the moisture

Much of the organic portion of trash consists of cellulose fibers, which include wood, green waste, paper, cardboard and many other subtypes of materials.

Cellulose fibers are rigid and can hold their shape against strong forces. They also can resist bending, flexing and compressing when dry. When wet, however, cellulose fibers lose much of their strength and soften, becoming more susceptible to the compressive force of a landfill compactor.

That’s why experienced operators always are looking for ways to get moisture into dry loads comprised of cellulose fibers. Residential trash, sludge and food waste are common sources of moisture in the waste stream.

Some landfills even will spray liquid on the active face to minimize dust and litter and to increase compaction. As a rule of thumb, 25-30 gallons of liquid applied per ton of waste is adequate to improve compaction without generating free liquid that could create other issues. Of course, many variables must be considered, and every landfill will have its unique recipe.

The point is, though, that moisture can be beneficial in providing higher rates of compaction in landfills.

Spread waste in thin layers

The downward force of a landfill compactor is reduced with depth, looking like a subsurface pyramid. At some point, the weight of the compactor will not be felt. Even at depths of just 4-6 feet, the weight of the heaviest landfill compactor will not crush pallets or break bottles.

Have you ever seen a contractor dump a load of soil to build a ramp when crossing a concrete curb? That soil ramp acts like a bridge to help dissipate, or spread out, the force exerted by a heavy truck or tractor, thereby protecting the concrete curb from damage. That 4-6 feet of trash in a landfill creates the same type of “bridge” that inhibits the compressive force of the landfill compactor.

To achieve the optimum compaction, the compactor’s teeth should extend fully through each layer of trash. That way, every bit of trash that goes into your landfill is impacted by the teeth.

For that reason, trash should be spread in thin layers to maximize the force applied to all waste. Most operators probably have heard of the 2-foot rule. Even here, there is science.

When uncompacted trash is spread across the active face of a landfill, it has a density of approximately 400 pounds per cubic yard (pcy).

After the first pass with the compactor, the density increases 250 percent to around 1,000 pcy. Accordingly, that initial 24-inch layer is compressed to less than 10 inches, which is close to the tooth height on many compactors.

As compactor teeth wear, waste must be spread in thinner lifts to achieve that same full-length penetration. The solution is to continue reducing the layer thickness or replace the compactor teeth more frequently. Studies indicate that more frequent tooth replacement saves landfill airspace and makes financial sense.

Make long runs

Landfill compactors are at their peak productivity when traveling at full speed, even if it’s only 3 miles per hour. But at the end of every run, that machine must slow down, stop, reverse direction and then accelerate back up to production mode. This is lost, nonproductive time.

From the perspective of maximizing the compactor’s productivity, long runs are better because the machine spends a greater percentage of its time at full speed, and less operating time slowing, stopping, reversing and accelerating.

If you are covering with soil, the concept of making long runs conflicts with the idea of maintaining a tiny active face to minimize cover soil usage.

This was once standard practice, but conditions have changed, and for landfills using alternative daily cover (ADC), the cost of covering more active face is a small price to pay for the increased density achieved from longer runs.

Use aggressive teeth

When it comes to teeth on landfill compactors, size matters.

Compactors with many large teeth consistently will get the best results. You could save money by going with smaller teeth or by spacing them out on the wheel, but these savings only consider your upfront costs. In the long run, the cost of airspace trumps everything else.

Larger compactor teeth also have greater demolition ability. They also can reach further into every layer of waste to ensure thorough compaction.

Tooth spacing is an important consideration, as well. Have you ever seen a landfill compactor that was not achieving full depth penetration with the teeth? In other words, have you ever seen daylight between the drum and the trash? I have not. That indicates that a denser tooth pattern would more effectively get more teeth into the waste.

The one drawback is that a dense tooth pattern can exacerbate wheel packing, where mud or sticky waste gets stuck between the teeth, and the compactor works more like a smooth-drum roller rather than a toothed compactor.

This can be mitigated by installing cleaner bars on the machine. Some compactors are designed with cleaner bars on all wheels. The operator also can help prevent wheel packing by staying on the trash and not traveling across areas of wet soil.

Many factors should be considered when selecting the right tooth design, size and placement pattern for your compactor. Just be aware that your decisions can affect the overall operation.

Prep for cover

Waste compaction ranks high on the list of important considerations for landfill managers. After all, it’s all about airspace. But when it comes to airspace, many don’t think about a hidden cost. It’s the cost associated with excavating, hauling and placing cover soil. The hidden part of that cost is related to the airspace consumed by the cover soil.

The single biggest avoidable cost in the landfill industry is the excessive use of cover soil. So, in talking about compaction and airspace, the conversation isn’t over until it’s addressed the issue of cover soil.

Prepping the waste before cover soil is applied on the landfill face can help the compactor operator reduce the consumption of cover soil.

Whether it’s soil or some form of ADC, cover is placed most efficiently when the waste surface is properly finished.

This starts with placing homogeneous, nonbulky trash on the surface. Sometimes this requires the dozer to warehouse a few loads until the end of the day to ensure the compactor has good material to work with. Then it comes down to the compactor operator doing a good job of trimming and filling to create a surface with a smooth, uniform plane.

The process of compacting trash at a landfill truly is a science. Yes, a component of the process is art, but only where it lines up with the basic laws of physics.

Neal Bolton is president of Blue Ridge Services Montana Inc., Hamilton, Montana. He can be reached at neal@blueridgeservices.com.