Being Dense Has Its Advantages
How compactors apply machine weight determines maximum compaction densities.
At its root, the landfill business can be drawn down to a simple mathematical calculation. An operation receives a permit for a certain number of acres and is relegated to how deep it can dig and how high it can go. The result is the number of cubic yards of space that an operation has available to fill at the site. There is one key remaining variable—how the space is filled—and this has a profound impact on the life expectancy and the profitability of the site.
A landfill must consider the type of compactor selected and the technique of how its weight is applied—how the face is compacted, the layer thickness of the waste stream and number of passes—to maximize the amount of material that can be placed in this finite asset. A wrong equipment decision or practice can be costly.
Calculated on estimated waste stream and anticipated densities, landfills are designed with a projected life expectancy. Choosing a compactor that delivers high pound-per-cubic-yard (lb/cy) densities can lengthen the life of a landfill by years. Extending the life by 5 or 10 years can lead to millions, if not tens-of-millions, of dollars in additional revenue for a landfill.1 Not to mention, it delays the high development costs and multiple challenges associated with siting a new landfill.
Much of the industry incorrectly believes that compaction is all about the weight of the machine. It’s actually about how the machine applies this weight and the techniques used by the operator that will help to optimize densities.2
Tips on Technique
A well-trained landfill compactor operator can be one of the operation’s biggest assets. Knowing how many machine passes, the proper way to spread the waste, and the best way to compact the face can all add to (or subtract from) compaction densities.
Four to six is a rule of thumb for the number of passes to effectively compact the waste stream to the best of the compactor’s ability. Once you go beyond six passes, the law of diminishing returns takes over and the effect the compactor has on density plateaus.3 Going beyond six passes starts to waste fuel, increases wear and tear on the machine and decreases production.
As the layer of trash spread by the blade increases, compaction densities begin to suffer. A machine will deliver the most compaction at a 1.0-foot layer thickness. Densities begin to suffer significantly as the material is spread more than 2.5 feet deep. However, operators need to balance layer thicknesses with productivity requirements. While thinner lifts improve densities, operators should avoid the ‘smear’ method (spreading material over a wide area extremely thin) as this slows production. It is recommended that compaction pass all the way to the edge to avoid waste “cliffing” as a way to increase densities.4
Using a compactor that offers full-width compaction in a single pass offers both productivity and density advantages. Full-width coverage eliminates the need for the operator to index the machine over to compact the center tunnel of material left uncompacted on the first past.5 While the final landfill slope is typically regulated by state agencies, material placed at the lowest slope possible is more densely compacted. When working a high-degree slope, the full weight of the machine cannot be directly exerted on the lift. Therefore, as slope angle increases, the machine’s compactive effort decreases.
If working a steep slope is a must, running at a 45-degree angle up and down the slope is recommended. Running sideways across the slope is ill-advised, as this exerts more pressure on the lower wheels and much less weight on the upper wheels. Operators should also follow through with pushing material down the face so as to avoid furrowing uncompacted trash.6
A Weighty Issue
Sizing the right landfill compactor for an operation depends on a number of factors, including daily tonnage received, budget, desired densities and sometimes just preference. Landfills should consider having a second or back-up compactor when taking in more than 250 tons of material per day.7 As a general rule, a compactor in the 80,000-pound range delivering a high pound-per-linear-inch (PLI) compaction force will deliver 1,200 to 1,400 lb/cy density. A high PLI compactor in the 110,000-lb class delivers densities reaching and sometimes exceeding 1,700 lb/cy.
It is the machine’s PLI and not just the weight alone that makes achieving these densities possible.8 Consider dozers, a popular piece of landfill equipment, as an illustration of this point. There are dozers working in landfills with weights approaching 70,000 lb, but they will only pack in roughly half as much material in the same space as a 70,000-lb landfill compactor.9 The reason being: the dozer’s weight is spread over a wider area.
In other words, dozers are built for flotation and not compaction. This is why it’s very rare today to see a dozer being used as a compactor. Space is at a premium, and it’s very costly to site and develop a new landfill.
Flotation vs. density vs. productivity is a balancing act that landfill compactor manufacturers address carefully and uniquely. Most manufacturers, especially of models with wheels at the corners that leave an uncompacted center channel, must balance a narrow wheel width to achieve density against a wider width so as not to sink too far into the waste and get stuck. Compactors offering full-width compaction do not face this issue to as great of a degree, since there is no uncompacted center trough.10
The balancing act is all about PLI to maximize air-space use. The theory is the smaller the area that the machine’s full weight is distributed, the higher the compaction densities. PLI is calculated by dividing total machine weight by the combined width of all four wheels (in inches).11 Generally speaking, the higher the PLI, the more material will be compacted in the same space, resulting in a greater density and higher profit potential. There is a proportional relationship between PLI and density. A 20-percent increase in PLI will produce virtually a 20-percent increase in compaction densities (see Weight and PLI sidebar).12
Considering the balance between profitability, density and budget, be sure to look at all factors when sizing the right compactor for the landfill. The choice is up to the individual operation. However, carefully choosing a compactor that offers high PLI compaction force and having the operator follow proper compaction techniques will maximize compaction densities and air space use; thereby extending the life and profitability of the landfill.
Terex has a wide rang of products in the recycling and waste management industry, including specially-designed landfill compactors, material handlers, crushers, trommels and screens. For more information, call (203) 222-7170 or visit www.terex.com.
Rick Zettler is owner of Z-Comm, a company specializing in freelance writing and photography for the construction and landfill compaction industries. He can be reached at (319) 265-0052 or firstname.lastname@example.org.
Fred Heath, President of Road Machinery Services, Inc., Statesville, NC, Terex® For decades, Road Machinery has specialized in providing service, aftermarket parts support, rebuild services, and used and rental equipment to its landfill customers. The company became the Terex® Trashmaster landfill compaction equipment distributor in 2001 for the Carolinas in order to provide more equipment solutions for its customers.
Mark Hiatt, Vice President of Road Machinery Services
Mike Rodriguez, Product Specialist for Terex landfill compaction.
Charles King, Road Machinery Services’ Landfill Sales Coordinator
Heath and Hiatt
Weight and PLI
A recent field experience illustrates the difference between weight and PLI. A public landfill in the Southeast U.S. uses two different models of 80,000-lb-class compactor—one offering slightly over 600-PLI compaction force and the other approximately 430 PLI.
Originally, the new 600-PLI machine was used as the primary compactor, but operators logged nearly 2,000 hours on the machine in the first year of operation. Concerned that the compactor would reach the hour limit on the extended warranty before the five years was up, management decided to increase usage 430-PLI compactor.
The landfill alternated using the two machines, parking the 600-PLI compactor for extended periods of time in order to save hours. It would then try to run the 600-PLI machine to see if compaction densities could be improved. The higher PLI compactor would sink to the belly pan in the many layers of compacted material, since the waste stream was originally compacted at a lower density.
While landfill management was upset that the machine got stuck, they fail to realize the amount of lb/cy density and air space sacrificed to keep the new machine in warranty. The 600-PLI machine offers nearly 30-percent higher PLI, which results in roughly 20 to 25 percent more compaction.13 By not maximizing compaction, you are losing air space and profits.