Better Roads

July 2014

Better Roads Digital Magazine

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Better Roads July 2014 13 speed of 1 to 2.5 km/h (0.6-1.6 mph) for rock and clay, and 2 to 5 km/h (1.2 to 3 mph) for gravel and sand will provide the best results. Smooth drum vibratory compactors were the fi rst vi- bratory machines introduced. They are most effective on granular materials with particle sizes ranging from large rocks to fi ne sand. They are also used on semi-cohesive soils with up to 50 percent cohesive soil content. Lift thicknesses vary according to the size of the compactor. Whenever large rock is used in the fi ll, the lifts may be very thick—up to 1.2-m (4-ft.) lifts are not unusual. One thing to remember when large rocks are in the fi ll is that the thickness should be about 300 mm (12 in.) more than the maximum rock size. This permits lift consolidation without having large rocks protrude above the surface. Padfoot drum machines expand the material range to include soils with more than 50 percent cohesive material and a greater percentage of fi nes. When the pad penetrates the top of the lift, it breaks the natural bonds between the particles of cohesive soil and achieves better compaction results. The pads are involuted to walk out of the lift with- out fl uffi ng the soil and tapered to help them stay clean. The typical lift thickness for padded drum units on cohesive soil is in the 150- to 460-mm (6- to 18-in.) range. Caterpillar has two pad shapes available: square faced pads and oval-faced pads. Square pads perform well on semi-cohesive soils and thinner lifts of less than 150 mm (6 in.). Square pads do a good job of sealing the surface. Oval pads have less surface area than square pads, so they apply greater ground pressure than square pads. This allows the pad to penetrate deeper into the lift. Oval pads perform better on cohesive soils and thicker lifts of 150 to 460 mm (6 to 18 in.). Oval pads do not seal the surface as well as square pads. Tamping Foot Compactors Tamping foot compactors are high-speed, self-propelled, non-vibratory compactors. They usually have four padded steel wheels and are equipped with a dozer blade. Their pads are tapered with a rectangular face. Tamping foot compactors compact from the bottom of the lift to the top. Because the pads are tapered, they can walk out of the lift without fl uffi ng the soil. Therefore, the top of the lift is also being compacted and the surface is relatively smooth and sealed. Tamping foot compactors are capable of speeds in the 16-32 km/h (10-20 mph) range, but they typically operate in the 10-15 km/h (6 to 10 mph) range. Generally, two to four cycles (4 to 6 machine passes) will achieve desired densities in 200- to 300-mm (8- to 12-in.) lifts although four cycles may be needed in poorly graded plastic silt or very fi ne clay. Tamping foot compactors are effective on all soils except clean sand. Tamping foot compactors leave a fairly smooth, sealed sur- face so hauling units are able to maintain a high speed when traveling over the fi ll. Also, since dozer-equipped tamping compactors do both spreading and compacting, the contrac- tor may be able to reduce the number of track-type spreaders. Tamping foot compactors are best suited for large projects. They need long, uninterrupted passes to build up speed that generates high production. On lifts greater than 300-mm (12-in.) thick, tamping foot compactors are about two to three times more productive than single-drum vibratory compactors. Sheepsfoot Compactors Sheepsfoot compactors derived their name from the fact that early Roman road builders would herd sheep back and forth over base material until the road was compacted. The word "sheepsfoot" became a generic term to describe all types of padded drums. In reality, a sheepsfoot compactor is very dif- ferent from a padded drum or tamping foot compactor. A sheepsfoot pad is cylindrical, usually 200-mm (8 inches) long. The pad is circular and will range in diameter from 76 to 127 millimeters (3 to 5 inches). The pads on tamping foot or padded drums are tapered with an oval or rectangular shape. And the pad face is smaller than the base of the pad—that's an important difference. The pads on sheepsfoot drums penetrate through the top lift and actually compact the lift below. When a pad comes out of the soil, it kicks up or fl uffs the material. The result is a Based on the principle of particle rearrangement to decrease voids and increase density, vibratory compactors come in either smooth or padfoot drums. (Read a companion piece that ran in the May 2014 issue, "Size Matters in Mix Grades and Aggregate Shapes," at

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