Compaction is as much about how a machine applies weight — pound-per-linear-inch (PLI) — as how much the machine weighs. Calculate PLI by dividing weight by the combined width of all four wheels in inches. The higher the PLI, the more material will be compacted in the same space. Photo: Terex Corp.
After six passes with a landfill compactor — regardless of size — density levels plateau. Photo: Terex Corp.

Look at PLI, not weight

Sizing a compactor depends on daily tonnage received, budget, desired densities — and sometimes just preference.

“I've had customers who could be running smaller compactors buy larger ones to increase densities and extend landfill life,” says Road Machinery Services Landfill Sales Coordinator Charles King. “I've also seen operations that should be running larger compactors order smaller ones due to budgetary reasons.”

The machine's pound-per-linear-inch (PLI) is just as important to effective compaction as its weight. PLI is calculated by dividing total machine weight by the combined width of all four wheels, in inches. Generally speaking, the higher the PLI, the more material will be compacted in the same space, resulting in a greater density and more revenue.

“There's a proportional relationship between PLI and density,” Hiatt says. “A 20% increase in PLI will produce virtually a 20% increase in compaction densities.”

As a general rule, a compactor in the 80,000-pound range delivering a PLI compaction force will deliver 1,200 to 1,400 lb./cu. yd. density. A high-PLI compactor in the 110,000-pound class delivers densities reaching and sometimes exceeding 1,700 lb./cu. yd.

An 80,000-pound dozer, however, packs in roughly half as much material in the same space as an 80,000-pound landfill compactor. The reason? The dozer's weight is spread over a wider area. In other words, dozers are built for flotation — not compaction.

Balancing act

Flotation vs. density vs. productivity is a balancing act that manufacturers address carefully and uniquely. Compactors with wheels at the corners leave an uncompacted center channel, so manufacturers 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. This isn't an issue with full-width compactors like the Terex TC550 and TC400 because the design doesn't produce an uncompacted center trough.

The experience of one Road Machinery Services customer illustrates the difference between weight and PLI.

The landfill operates two different models of 80,000-pound class compactors: one offering slightly more than 600-PLI compaction force and the other approximately 430 PLI.

The higher-PLI compactor had more hours on it and was reaching the hourly cap on the extended warranty. To limit the machine hours logged, facility managers purposely ran the 430-PLI compactor for up to two weeks straight without running the other compactor. When they ran the 600-PLI machine to see if it could improve densities, it got stuck in the many layers of compacted but less-dense material.

Managers were more upset about that than about the massive amounts of lb./ cu. yd. density they'd sacrificed to keep the higher-PLI machine in warranty. “If you don't maximize compaction, you're squandering air space,” Hiatt says.

To increase compaction densities, some manufacturers offer high-density wheel options with approximately a 20% narrower wheel width than the standard wheel. This concentrates more weight over a smaller area. While these wheels increase PLI, they also increase the center gap that's not compacted on the first pass. More machine indexing is required, which sacrifices productivity.

—Rick Zettler of Z-Comm specializes in PR management, freelance writing, and photography for the waste, construction, and aggregate industries. For more information on Terex landfill compaction equipment, contact Aron Sweeney at 605-987-2603, 605-987-5023 (fax), or