Far from running out of airspace, changes in landfill operating practices ensured that America had more than enough space to dispose of its waste. Not the least of these changes was the use of in-place compaction to minimize the volume of waste placed in a landfill.
Landfill operators did this on their own. It made good economic sense to reduce the volume of incoming waste by half by doubling its in-place density with compaction equipment. Nobody told them to do it. No regulator required it, and no legislator mandated it. To this day (as far as I know), there is not a single state regulation that specifically directs landfill operators to compact their waste.
Not that it would matter either way; a landfill operator would be foolish not to practice compaction. By reducing the volume of his waste, the operator gains several important financial advantages at minimal cost. Most importantly, it increases the operational lifetime of the landfill in general and its current disposal cell in particular. This pushes back the capital costs of new cell construction and landfill expansion, thus reducing the real capital costs of the site. The only real questions are what are the most effective means of achieving compaction and what equipment design lends itself to optimizing this task?
The How and Why of Waste Compaction
MSW arrives at the landfill with a density of approximately 15 to 25 pounds per cubic foot (equivalent to roughly 0.20 to 0.33 tons per cubic yard). Site compaction efforts occur after the waste has been deposited at the working face in the current disposal cell. The compaction operation is designed to reduce the volume of waste deposited on the working face to half its deposited volume and achieving a doubling of its in-place density to 0.40 to 0.67 tons per cubic yard.
What factors determine the effectiveness of a waste compaction effort? The first and most important is the thickness of the initial layer of deposited waste. When waste trucks arrive at the working face, they are directed by a spotter to the dumping point. After depositing their loads, the now empty trucks exit the current disposal cell by means of an operational access road so they can continue on to their next collection route. The piles of waste they leave at the working face are spread into thin layers by dozers specially modified to operate in the harsh environment of a landfill. The waste should be loosely spread to a thin layer no more than 2 feet to 3 feet thick over the working face.
This is difficult to achieve with consistency, since waste is very heterogeneous both in types of materials and the sizes of objects contained in the wastestream. Everything from thin newspapers to bulky furniture and mushy foodwaste to solid building debris can be deposited in a landfill. Since the mechanics of waste compaction depend on the application of the pressure delivered by the waste compactor and the shredding action of the compactor wheel teeth, a waste layer that is too thick will not receive the full effect of compaction. Like all pressure distributions (active or static) applied to an underlying soil, the pressure of a waste compactor gets dispersed with depth. Ideally, the initial layer of loose waste should be less than 2 feet thick, but this is very difficult to consistently achieve in practice.
The number of times the compactor passes over the waste also determines the effectiveness of the compaction effort. There is a golden mean and a point of diminishing returns where more and more passes do not result in significant additional compaction. In most cases, the optimum number of passes is three to five, with a “pass” defined and movement back and forth across the across the loose lift of waste. More passes than this are usually a wasted effort that will not result in further increases in in-place density of the waste.
The slope of the current working face also plays an important role. Even without a force diagram, it is intuitively obvious that a compaction movement into the waste will be more effective than simple movement over the waste. So geometry plays a significant role, but what are the optimum working face slopes for compaction, and are these practical to achieve in the field? In general, the slope of the working face should be no steeper than 3 horizontal to 1 vertical (a 33% gradient or a slope angle of about 18 degrees to the horizontal) to resolve the effective forces acting on the underlying waste spread over this slope. At this slope angle, about 95% of the equipment’s dead weight is actually being transmitted directly down into the waste (cosine of 18.43 degrees = 0.948).
However, almost a third of the equipment’s horizontal acceleration force is being delivered into the waste slope (sine of 18.43 degrees = 0.316). Best results occur when a pair of compactors are working in tandem, assuming that the current working face is large enough to allow the choreography of two compactors without interfering with other operations at the working face. As a result of pressure distribution of the compaction equipment through the waste, its movement tends to create bulges of waste to its front and sides. The use of a second compactor can eliminate most of these low-density bulges.
The final factor affecting the efficiency of waste compaction efforts is the moisture content of the waste. Waste with higher initial moisture content tends to compact more easily and to a greater extent than drier waste. Weather in general, as opposed to moisture in particular has less effect. Waste arrives at the landfill largely in plastic bags and is only briefly exposed to the elements prior to compaction. It would seem that direct application of water by hose or spray truck would be advantageous. However, most states would ban the direct manual application of water to the water mass for fear of greatly increasing the quantities of leachate generated by the site. Also, there is a point of diminishing returns with moisture as there is with compactor passes. If municipal solid waste were a soil, it would have a field capacity of almost 30%. Waste with a moisture content exceeding its field capacity and saturated waste have a tendency to simply spread out instead of increasing its density during compaction, and the wheels tend to spin, wasting movement and wasting any increased compaction potential.
In addition to these four site characteristics, there are the obvious compaction equipment characteristics such as size, weight, operator skill, and operating speed. What are often overlooked are the characteristics of the equipment that is in direct contact with the waste, the compaction wheels themselves.
The importance of the weight of the compactor to the compaction effort is obvious. What is not so obvious is the effect of the compactor wheels on the efficiency of the operation. The operational characteristics of the compactor wheels that operate “where the rubber meets the road” are crucial to compaction performance. The compactor wheels are usually rated in terms of operating wear life (the duration required between replacement of the teeth or cleats) and their ability to compact the waste (applied pressure at the tip or length of the cleat).
But what is equally important is the traction provided by these cleats. It is this traction that allows the compactor to efficiently move through the waste in the first place. Without proper cleats, time and effort are wasted with the compactor literally “spinning its wheels.” It is not just initial traction but long-term traction for the operational lifetime of the wheel that matters. Cleats that round off after only a few months of operation will greatly reduce the effectiveness of the compaction effort. With rounding comes a shortening of the cleat length, and mud and debris can get caked onto the wheel drum to the thickness of these now shortened cleats. Traction can be significantly compromised. Furthermore, instead of shredding and compacting waste as they should, they merely twist and bend the waste material.
The amount of wasted effort becomes apparent when you realize that the typical new landfill waste compactor has an operational lifetime of only five years before it’s fully depreciated and ready for the resale market. During that relatively short operational lifetime, every moment counts. If a compaction wheel is spinning in place only 10% of the time or reducing the average operational speed by the same percentage due to the inability to achieve traction quickly, a half a year (one-tenth of five years of operations) of compaction operations is being wasted. This is important, as most landfills have 55-hour workweeks (10 hours a day during the week with a half day on Saturday being typical) with compactors operating during these times. At an average annual operating time of over 2,800 hours, a 10% reduction in operating efficiency is equivalent to the machine being idle for a full work week.
The business end of the compactor is the wheel directly in contact with the waste. These parts of the machine actually perform the compaction process through a combination of applied pressure and shredding of the waste material. All the heavy weight of the machine is concentrated on a few cleats that generate significant pressure at their tips. Typical landfill compactors weigh between 35,000 pounds and 60,000 pounds and exert pressures of 105 psi to 1,700 psi. Not only are the cleats narrow, reducing their contact area and increasing applied pressures, but they also have to be long enough to thoroughly penetrate the surface of the waste and prevent “fluffing,” or the rebounding of waste under the compaction wheel as it passes over the waste.
Avoiding wheel wrap is another concern of compactor wheel design. Waste is filled with long, stringy objects such as wires, which could wrap themselves around the axle of the wheel, reducing torque or even causing the axle rotation to jam. So, many compactor wheels come with vertical barriers attached to the inside and/or outside of the wheel. Self-cleaning compactor-wheel designs are also required to prevent clogging of waste as it adheres to the surface of the wheel drum and fills the gaps between the cleats. As a result of clogging, the effective contact area of the wheel is reduced and the penetration depth of the cleat is shortened. Both greatly reduce the wheel’s ability to compact.
Solid waste is also a very difficult material to work with, often with a large percentage of solid or metal objects. These can accelerate the wear and tear on the cleats, shortening their lifetime. Special anti-abrasion materials and coatings can be used to reduce the frequency of cleat replacement.
The number of cleats and their spacing varies with the size of the wheel drum and the anticipated working environment. The better-designed drums can utilize fewer cleats per area of drum surface. Cleats that impart a torque to the underlying waste are more effective than simple, straight cleats. Using fewer, but more efficient cleats results in two important benefits: greater space between the cleats, reducing the potential for clogging; and applying the weight of the machine to fewer tips effectively increases the applied compaction pressures. The arrangement of the cleats should be designed to minimize the potential for clogging while maximizing applied pressures. A staggered chevron pattern of cleats is typical for many compactor wheels. Some machines use cleats with opposite orientation on the front and back wheels to allow for similar compaction results, whether moving backward or forward over the waste.
The spacing and arrangement of the wheels can also affect the efficiency of the compactor. The most common wheel configuration sets the wheels in what most would be considered their normal configuration, at the four exterior corners of the machine (like the tires on almost all vehicles). This results in a wider overall wheelbase, improving operational stability and control. Wheels in alignment (one directly behind the other) subject the same area of waste to two compaction applications with each movement. This also leaves a gap between the left and right wheels that requires repeated passes over essentially the same area to achieve complete coverage. To cover this gap, some models have the front wheels inside the spacing of the back wheels. This presents a complete width of compaction effort. However, the machine may not handle as well on some slopes as machines with a wider wheelbase. So these second types tend to further modify the design with larger-diameter wheels in the rear for greater stability and control.
Terra Compactor Wheel
The Terra Compactor Wheel Corp. is a manufacturer of compactor wheels for all the major industry players. A leader in innovative design, Terra has developed the left- and right-hand Twist-Torque teeth to try to keep more of the compactor’s weight and trash under each wheel, then at the same time change the direction of the flow of weight and trash to use this wheel spin and rolling force as a positive effect. Their design is an answer to the traditional chevron V-tooth alignment. Terra’s tooth patterns are designed to pull trash from under the center of the machine in a blade-down, fully loaded, forward motion on the initial pass through loose trash. When the compactor backs up in the second pass over the waste, they would always hit it again with no load to push. This reduces the chance of wheel spin, improving compaction effort and requiring fewer passes to achieve the same level of compaction.
Twist-Torque “coercion” teeth are a twist cleat with a special trash puncture point offered in 7-inch to 8-inch cleat heights. This is a six-sided rhomboid coercion cleat base paired with opposing mirror images. The right- and left-hand twists are designed for significant size reduction and to be self-cleaning. Each tooth has a tungsten-hardened puncture point. Terra has recently introduced the Scissor Cleat, specifically designed for C&D application, and also offers the HDT Wedge cleat for backup machines and cleaner-bar requirement sites.
Terra standard warranty is a four-year, 10,000-hour, flat-rate exchange warranty, and additional warranty programs are available, including the Premium Wheel Package three-phase warranty with unlimited hours and unlimited years.
HJ Industries
HJ Industries is a manufacturer and distributor of customized products for the waste industry, including compaction wheels for a wide variety of makes and models (Caterpillar, Bomag, Aljon, and Terex). The company has recently developed an optional, patent-pending, inner vertical barrier, known as the Wire Wall, which not only helps stop waste and wire from wrapping around the machine’s wheel and axle areas, but it also performs added compaction by incorporating an exclusive intermittent compaction cleat and hardened, angular, knife-blade design. This exclusive HJ Industries’ design allows equal wear between the Wire Wall and Trac-Pac cleats, provides added stability to the machine when traveling on sloped areas, and draws a line in the waste material, allowing operators to see their last pass with the machine, thereby preventing wasteful, redundant passes.
The Trac-Pac cleats incorporate a patented traction pocket design with the tips resembling a stepped pattern, half the cleat extending twice as far as the other half. Use of this cleat improves compaction rates by 10% to 15%. Being able to dig deeper into the waste not only increases compaction but also improves cross-slope stability and overall traction. This design also results in longer wear life and a more even wear across the cleats. The wheels are available in several cleat configurations, with wire wall guarding available, surpassing the 10,000 operational-hour wear-life duration.
HJ Industries also offers a remanufactured exchange wheel program from two locations, one in Ohio to serve the eastern states and one in Arizona to serve the western states. They will deliver a remanufactured, work-ready wheel sent to the site in exchange for the site’s existing, worn wheels. This can minimize landfill compactor downtime for wheel replacement to just hours versus days.
Caterpillar Inc.
Caterpillar’s waste-compaction wheels come equipped either with abrasion-resistant Plus Tips for long operational life or with chopper blades for more efficient waste shredding and for providing effective waste compaction along with traction and stability on waste slopes. Each model of waste compactor has its own wheel specifications. The 816F Series 2 compactor comes equipped with compactor wheels having a width of 3.33 feet, a diameter of 4.25 feet, and 20 tips each. These tips can be abrasion-resistant teeth, or they can be chopper blades.
The larger 826H landfill compactor comes with proportionally larger compaction wheels measuring 3.94 feet in width and 5.03 feet in diameter and having 24 tips per wheel. These tips measure 0.53 of a foot in length, increasing the overall wheel diameter by 1.04 feet to 6.07 feet. Finally, Cat’s heaviest compactor, the model 836H, has compaction wheel with a 4.59-foot width, 5.64-foot diameter, and 35 tips per wheel. These tips measure 0.55 of a foot in length, making the overall wheel diameter 6.73 feet.
The Caterpillar Plus Tips get their long operational lifetime from a design that utilizes extended-life, abrasion-resistant materials. Deployed in a widely spaced tip pattern, the configuration is designed for less clogging. These extra-long cleats provide extended lifetimes between replacements. The design of the tip maximizes the compaction effort, allowing for fewer tips per surface area of the wheel drum. This allows Caterpillar to guarantee these tips for 10,000 operating hours or four years, greatly reducing the per-hour cost of the tips. As an option to the extended-life tips, Caterpillar offers a self-cleaning chopper blade configuration. These blades are designed to maximize the compaction effort even with fewer blades per surface area of the wheel drum. Arranged in a staggered chevron pattern, the blades are evenly distributed. Blades on the front wheels are arranged differently than on the rear wheels, allowing for consistent compaction moving both forward and backward.
AL-jon
The design of Al-jon compactor wheels and cleats emphasizes long operational lifetimes and a unique shape that efficiently chops and grinds the waste as it is being compacted. Cleat spacing minimizes clogging, maximizes traction, and improves compaction.
The cleat design allows for smooth entry and exit from the waste without fluffing or clinging, making the Al-jon wheels self-cleaning. Customized wheels and cleaner bars are available for use in heavy-clay applications. Al-jon’s unique frame design, wheels, and cleats also reduce wire wrap and eliminate the need for supplemental axle guarding that blocks access to the axle area.
This purpose-built design allows Al-jon to provide a five-year or 10,000-hour non-prorated cleat and exclusive wire-wrap-damage warranty in MSW applications. Standard wheels for the company’s 80,000- to 115,000-pound units are 48 inches wide and feature 8-inch-tall cleats. Fifty-two-inch-wide wheels with 10-inch-tall cleats are standard on the 120,000-plus-pound Advantage 600 compactor.
Terex
Terex is normally thought of as a manufacturer and supplier of earthmoving equipment. It also has wide range of waste-handling equipment, including waste compactors. These feature a unique triangular wheel configuration with the front two wheels closer to the center of the machine and covering the gap between the more widely spaced rear wheels. This allows for complete compaction coverage with each pass, and so no movement is wasted. Compactors with a more conventional wheel alignment leave a significant gap between the wheels, requiring additional passes to ensure proper coverage. Equal distribution of the vehicle’s weight results in greater compaction densities. Terex wheels are equipped with Big Dog cleats configured so that weight application per cleat is maximized.
The company’s Terex TC400 Trashmaster landfill compactor has compactor front-rear wheel widths of 30/36 inches and a front-rear wheel surface of 1,173/1,408 square inches, resulting in an applied pressure of 15.5 psi. Its triangular wheel configuration gives total wheel width coverage of 132 inches (11 feet). The larger Terex TC550 has a front-rear wheel width of 35/40 inches with a surface of 1,446/1,652 square inches respectively, applying a pressure of 17.75 psi. It offers total wheel width coverage of 150 inches (12.5 feet).
The company’s compactor wheels are designed to minimize clogging of waste between the cleats. By minimizing clogging, cleat replacement costs are reduced, as is damage caused by wire wrapping around the wheels. These are large (75-inch diameter for the TC400 and 85-inch diameter for the TC550) self-cleaning wheels with greater width in the rear to allow for better operations on slopes without digging into the waste. The replaceable cleats are mounted on the wheel drum in a chevron pattern that prevents waste from being stuck between the cleats. This reduces buildup and the need for raker bars. The individual cleats have a contour design that chops the waste surface and prevents fluffing of the waste under the wheels.
Caron Compactor Co.
Caron Compactor is a major supplier of compactor wheels and teeth. An innovator in the field, Caron pioneered the use of compaction wheels with high-alloy, field-replaceable, pin-on wheel cleats. The company’s current line of compactor teeth is marketed to all the major players in the industry (Al-Jon, Bomag, Caterpillar, Komatsu, and Rexworks). The company’s pinned cleat design allows for easy replacement and repair out in the field without the need for lost productivity because of shop time.
Caron’s wheels come in two basic configurations for most compactors, the standard width to match the original manufacturers and narrower, high-density wheels for increased pounds per linea