The Mechanics of Waste Compaction

Photo Credit: Caterpillar
Compression is a vital part of the waste compaction equation.

Many landfill managers believe there is a simple formula for achieving maximum landfill compaction density: A + B = C

And it sounds pretty straightforward.


A-Achieving optimum waste compaction is a cornerstone of proper landfill operation. True

B-Buying a steel-wheeled landfill compactor is the industry-accepted means of achieving optimum density. True


A + B = C

…Where C equals Optimum Compaction. Not necessarily true.

Unfortunately, A + B does not always result in C. Despite the fact that compaction is an operational cornerstone, buying a compactor does not guarantee you’ll get great compaction density…there are a few more variables in this equation.

The concept of using a landfill compactor to pack more waste into your landfill’s limited airspace is a given. Nothing new here: We’ve been doing it for decades. It’s expected that most landfills will have a compactor, and you’d have some explaining to do if you didn’t have one.

But having a compactor and using it to its fullest extent is not the same thing, any more than having a pencil will make you another Norman Rockwell. You’ve got to have both the tool and the technique.

So to help you get more out of your compactor, we’re going to dig deeper into the mechanics of waste compaction and explore how compaction equipment and methods can be applied to various types of waste to get optimum results.

To be clear, when we talk about using landfill compactors to compact trash, we’re talking about a short-term cause-and-effect relationship. This is something that you could measure immediately: It’s real time.

But as important as these results are to your operation, most landfills do not measure the actual compaction performance. Rather, they measure the periodic average waste density at their facilities. Or, more often, they measure the cumulative (effective) density, which is simply a measure of tons of waste disposed divided by the overall (gross) airspace consumed during the same period.

Commonly referred to as the airspace utilization factor (AUF), the denominator of this equation (the volume) includes waste, cover soil, construction materials, gas system materials…and anything else that consumes landfill airspace.

AUF is typically expressed in tons (of waste) disposed per (gross) cubic yard of airspace consumed. Thus, if during the last measurement period your landfill disposed 70,500 tons of waste and consumed a total of 151,200 cubic yards of airspace, the AUF was .466 tons per cubic yard. Some folks prefer to replace tons with pounds, in which case the AUF would be 933 pounds per cubic yard. Either way, it’s the same number, just different units.

Chances are, your landfill tracks annual AUF, and so you’re probably wondering, “What’s wrong with using the AUF as a metric of landfill performance?” Answer: There is nothing wrong with it per se, it just doesn’t provide the full characterization of your compactor’s performance.

Think about this: The average monthly mean temperature in Bozeman, MT, is 45°F, which sounds a bit cool, but by no means arctic. In fact, compared with San Francisco’s monthly average of 60°F, we can wonder if all those folks from Montana are just fooling when they talk about those chilly Montana winters.

Nope, they aren’t fooling. The unreported difference between an annual average of 45° and 60° is in the variation. It’s that day-to-day variation that really tells the story. A very cold day in San Francisco is 30°…in Bozeman it’s -30°. That same type of variation happens with your landfill compactor.

There are times when your compactor achieves very high density and other times when it’s embarrassingly low. These variations in compaction rates become dampened to the point of invisibility when rolled into an annual AUF number. That’s because it doesn’t allow you to measure the daily changes in compactor performance and because it (your annual AUF) also includes other contributors to waste density, such as biological and chemical decomposition, surcharging with stockpiled soil, greenwaste, or simply more layers of trash. It also includes the ongoing force of gravity. All of these factors combine to create your annual AUF, but dissecting it can be nearly as impossible as reverse engineering Colonel Sander’s 11 herbs and spices.

So instead of suggesting that you conduct weekly compaction density surveys to track that density variation, we’ll focus on using the right techniques to minimize it. In other words, even though you can’t dissect the colonel’s secret recipe, you can still take a few cooking classes to produce consistent cooking results on your own.

Two Mechanical Processes
The right techniques boil down to two mechanical processes: compression and size reduction. These two things, when applied at the right time and to the appropriate degree of intensity, will consistently result in good compaction.

Compression refers to the process of pushing waste into less space, mainly through the weight of the machine and the pressure-typically measured in pounds per square inch (psi)-exerted by the compactor wheels (drums).

Size reduction refers to the process whereby waste materials are broken into smaller pieces, mainly through the action of the compactor’s teeth.

Both mechanical processes are important, and their correct application depends almost entirely on the type of waste being disposed.

Machine Weight
Compression is a vital part of the waste compaction equation. You can think of compression in terms of what happens when trash is placed in a packer truck and compacted. The force of hydraulic pressure uses a large plate (or push blade) to compress the waste. Wet, homogeneous waste in this situation can be very well compacted by pressure alone, with the key word here being wet.

Wad up a piece of paper in your hand and note the result. If you have average strength you’ll end up with a wad smaller than a golf ball. But now do something else. Closely examine the crushed paper. What you’ll see is a random matrix of bends and creases. These perform as tiny trusses. In engineer speak, they are structural members, a mixture of geometric shapes that work together to strengthen that ball of wadded-up paper. And the more you crush it, the more trusses you create…and the stronger the wad becomes. At some point, the ability becomes limited for compacting trash with compression only, because the overall waste mass becomes a broad, self-supporting matrix of trusses, beams, and bridges.

Of course, you know what would happen if the paper was wet: you could compress it into a much denser shape …more like the size of a large marble. Why? Because the cellulose fibers that provided rigid support for all of those tiny trusses would be softened.

If you are still not convinced about this structural member idea, consider the simplicity of corrugated cardboard. Using tiny rows of corrugated paper sandwiched between two flat sheets, the humble sheet of cardboard can provide incredible strength. I recently saw an advertisement showing a 7,500-pound load stacked on a single pallet-a pallet made entirely of cardboard.

This exact same phenomenon occurs in every landfill and can be found on a very small scale (like the wad of paper) on up to a large scale (with branches, wood, and other bulky items).

So, to a degree, a more compressive force (say, in the form of a heavier compactor or narrower wheels, both of which would increase ground pressure) could increase density by breaking down those structural members. This doesn’t just apply to paper and cardboard, but also to large, bulky items that must be smashed and compacted.

But even with largest compactors in the industry, some of which weigh more than 60 tons, there is a limit to what pure compressive force will do.

The other mechanical action is that of size reduction. As noted above, moisture plays a significant role, but we’ll address that topic after we’ve talked about size reduction.

Size Reduction
When we talk about size reduction, we’re talking about breaking up bulky items within the wastestream that might tend to bridge and, as a result, limit uniform waste compaction. Such items might include furniture, wood, branches, and various types of construction-and-demolition (C&D) debris.

In many cases, the weight of the compactor may not be adequate to effectively break these materials into small enough pieces to fill-and thereby eliminate-voids. For example, a 7-inch by 9-inch wooden beam (i.e., a railroad tie) bridging across a 4-5 foot span, may not be broken simply by the weight of a compactor because as a structural member, it’s just too strong. But after a few passes with the compactor’s teeth hacking out chunks of wood, the beam will be reduced in size.

Same concept applies to branches, -couches, C&D wastes, and other bulky items.

The degree to which bulky items within the wastestream can be reduced in size is dictated by the point load imposed by the teeth. While the steel wheel drums may impose something over 20 psi, the load imposed by the tips of the teeth may be 100 times greater.

The point-load of the individual teeth and the work they do to crush, chip, and shatter bulky items is all a result of the much higher psi imposed by the compactor’s teeth.

So for waste that is not homogeneous (which is what you would commonly find in self-haul, roll-off, and contractor loads), the teeth become infinitely more vital to the compaction process. This is especially true when the wastestream includes a higher percentage of C&D material.

So we have some waste types that form small supporting structures-a matrix that can be crushed by heavy compressive loads, a process further enhanced by the addition of moisture.

And we have another category of waste that forms a much more massive structure within the landfill. These are those bulky items that require super-heavy point loads (from the teeth) to effectively demolish and compact them.

Our operational approach must address both of them.

Different Waste Types, Different Forces
Tricky part is, your landfill likely receives both of these types of waste, in combinations that are changing throughout the day, the week, and the year. This means you need a compactor that’s heavy enough to give good compaction by compression…and is also equipped with aggressive teeth to provide the chewing and grinding required for breaking down those larger, bulky items.

And here we are back with the selection of the tool…and the need for the correct technique. At this point, the compactor operator must be able to read the wastestream, apply various levels of effort to different types of waste, and then blend them together to achieve the ultimate goal of minimizing airspace consumption.

Blending Waste Types
Here’s an example of a demonstration I’ve used to explain blending of various waste types. Inside this transparent container are some tennis balls and large pieces of foam. This represents bulky items after they’ve been crushed and downsized by the compactor’s teeth. At this point the pieces have been reduced in size, and there is less bridging, but as you can see, there are still plenty of voids.

The crushing of this material also creates a quantity of fine material. These fines will typically be mixed with the larger items. Keep in mind this doesn’t necessarily mean that you have to crush all of the waste in the matrix into fines. Instead, you have to place the waste in thin enough layers and blend it with materials that have the ability to create fines or already contain enough fine material so that it naturally fills in those voids with the least amount of machine effort.

If a load of bulky material, such as a 40-yard rolloff container of C&D, is not properly spread across the face and compacted/mixed with some finer material (residential waste), there will still be too many voids remaining.

However, if the bulky materials and fines are well mixed, the voids in the bulky chunks will be filled with fines, and you’ll be placing more waste in less airspace.

Again, this doesn’t happen automatically. The lesson here is for compactor operators to look at individual loads and identify the type of waste each load contains. Then mix loads with fine material and bulky material together to accomplish this same thing in a landfill. While processing these various waste types, the compactor operator must layer.

Layer Thickness: Thinner Is Better
When processing any type of waste, but especially when blending bulky material and packer-truck waste, layer thickness is vitally important-and in that regard thinner layers are better.

Filling the voids in bulky material must be done one thin layer at a time because fines, in the context of a waste type, don’t sift into the underlying voids like dry sand. Instead, compactor operators must avoid creating pockets of voids in the first place. Again, this is accomplished by layering.

Proper layering also comes back to blending: the intermixing of various types of trash so that each is compacted to its maximum density. And as you might expect in an integrated system, blending is an integral part of distributing moisture throughout the waste mass.

Moisture Content
Recall our wad of paper example-specifically, that adding moisture makes a difference? With homogeneous waste, typical office/restaurant /residential waste that contains lots of paper, cardboard, pressed board and other similar materials, moisture will weaken the fibers and allow for considerable compression.

Some landfills add moisture to the inbound wastestream to control dust, reduce wind-blown litter or to increase landfill gas production. But along with the other reasons for adding moisture, it also results in better compaction.

At many landfills, moisture enters in the form of wet waste. This may be as obvious as sludge, biosolids, cannery waste, or some other high-moisture content material. Or it may be in the form of residential waste or commercial waste such as that which comes from hotels, restaurants, or schools. The thing to remember is that moisture is moisture in any form. The key is to transfer that moisture to the paper, cardboard, and pressed board, where it can help soften those structural fibers.

Wet loads should always be spread across loads containing lots of paper and cardboard-and visa versa. This includes loads of restaurant waste, sludge or dead chickens. Regardless of its origin, the goal is to distribute any available moisture to where it will do the most good.

All of this discussion on compression, size reduction, thin lifts, and moisture is fine and dandy, but in the end it really comes down to the person on the compactor understanding his or her job…and then doing it consistently every day. And to be most effective, those decisions must result in the compactor putting more teeth into the trash. Here are some steps to help get you there.

Before you place waste for the next daily cell, remove the previously placed cover soil, right down to where trash is showing, and then recompact the underlying surface of waste. Remember the importance of moisture? Well, that trash that has been covered with soil for several weeks or months has been in a warm and humid environment (i.e., inside your landfill). There’s a good chance it has taken on a relatively uniform moisture content, so a few good passes with the compactor before placing that first lift of new waste can produce some fast and easy settlement. Remember: Wet trash packs better, and moisture is moisture wherever you find it.

Work the compactor on a flat surface. I know, I know…I’ve heard every reason there is for working the compactor on a slope. None of them hold air. Over the years, we’ve conducted operational reviews of one type or another on hundreds of landfills…and have reviewed video data from scores of them. A compactor on a slope will typically travel at a velocity of 1.5 miles per hour (about 2.2 feet per second). At that rate, depending on the machine and wheel/tooth configuration, it will impose a certain number of tooth penetrations into the trash. Typically, this will be somewhere around 500,000 tooth penetrations per day. But when working on a horizontal surface, the compactor’s average speed increases to approximately 3-4 miles per hour. Simple arithmetic shows that this would increase the number of tooth penetrations by 100% to as much as 167% per day.

Make longer runs. Historically the landfill industry has constructed daily cells with a more or less square geometry. Typical cells were 100 feet by 100 feet…or 75 feet by 75 feet…or whatever. This was neither right nor wrong; it just was the way it was done. But when thought of in terms of maximizing tooth penetrations, long rectangular cells make good sense. A longer cell allows for a longer compactor run, which means more time traveling and less time spent accelerating or decelerating. It means a lot fewer starts and stops during the day.

Provide good training for the compactor operators. They are the ones who see every load of waste. They see every pass the compactor makes. They are the ones who will make or break the compaction operation. Give them the best possible tools and training, and then let them do their job.

Okay, after all this discussion about how to run over trash with a steel-wheeled compactor, somebody somewhere is going ask, “Hey, is all this stuff really that important?” Well, let’s see. We’ve seen landfills increase their AUF by 15% to 30% by making these types of changes-and combining better compaction technique with reduced soil use. What would a 30% reduction in your annual liner capitalization cost be worth? Or how would you like to push your closure/post-closure costs out 30% further into the future, and at the same time reduce your annual contribution to the financial assurance fund by a like amount. Finally, how would you like to present 30% more landfill capacity (i.e., longer site life) at your next board meeting?

We do lots of planning and calculating in regard to waste compaction, but in the end, it all boils down to the ability of your compactor operator to process waste efficiently…one pass at a time.  MSW_bug_web


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