Introduction
To select the most appropriate type of compaction equipment, a project manager must know the characteristics, capabilities, and limitations of the different types of rollers. Generally, compaction equipment uses one or more of the following methods to accomplish soil densification:
1. Static weight (pressure)
2. Kneading (manipulation)
3. Impact (sharp blow)
4. Vibration (shaking)
Factors to be considered when selecting compaction equipment
i. Type and properties of the soil
ii. Density desired to be achieved
iii. Placement layer thicknesses
iv. Size of job
v. Compaction equipment available
Spectrum of capabilities of different rollers
See Table 1 for the spectrum of capabilities for different types of rollers and the type of associated compactive effort as summarized in the following lines of sentences:
Sheeps-foot rollers are best for predominantly fine-grained cohesive materials such as clays and sandy clays. Steel-drum rollers are best for larger particle materials such as gravel or cobble.
Vibratory rollers are ideal for well-graded or gap-graded materials because the shaking action causes the smaller particles to fill voids around the larger grains.
Table 1: Spectrum of capabilities of different rollers
Considerations for the selection of most suitable compaction equipment
The considerations for the selection of the most suitable compactor among the three common types of compactors used in construction works are outlined below.
A. Sheeps-foot Roller
This type of roller has feet that are square or angular and taper down away from the drum. This design allows the roller to achieve better penetration on the initial pass, resulting in a thorough, uniform compaction throughout the layer. This roller compacts the material from the bottom of the layer to the top and walks out after achieving the desired density. It is suitable for compacting all fine-grained materials but is generally not suitable for use on cohesionless granular materials. The layer thickness for the sheeps-foot roller is limited to 200 mm in compacted depth. If the material is loose and reasonably workable (permitting the roller’s feet to penetrate into the layer on the initial pass), it is possible to obtain a uniform density throughout the full depth of the layer.
The sheeps-foot roller does not adequately compact the upper 50 to 75 mm of a layer. Therefore, it is necessary to follow up the sheeps-foot roller with a pneumatic-tyred or smooth-drum roller to complete the compaction or to seal the surface if not placing a succeeding layer. The sheeps-foot roller compacts from the bottom up and is particularly appropriate for plastic (moistened) materials. It is ideal for working soils that have moisture contents above the acceptable moisture range since it tends to aerate the soil as it compacts.
B. Pneumatic-tyred Roller
Pneumatic-tyred rollers are suitable for compacting most granular materials. They are not effective in compacting fine-grained clays. Pneumatic-tyred rollers compact using two types of compaction effort—static-load and kneading. The contact pressure is controlled by adjusting the tyre pressure and the wheel load. Contact pressure is affected by tyre pressure and wheel load. The contact Pressure of the tyre is determined primarily by the tyre pressure. The tyre side walls carry about 10% of the load and the trapped air supports 90% of the load. Contact Area = (0.9 x wheel load)/tyre pressure. The gross contact area of the tyre is used in the analysis because the contact pressure neglects the raised portions of the tyre. Therefore, Contact Pressure = wheel load/contact area.
Wheel load: wheel load is significant for compacting at the required depth or in test rolling to detect subsurface defects. Research has shown that an increase in wheel load is advantageous in compacting thick layers.
Surface coverage: the wheel arrangement and tyre deflection determine the surface coverage. The arrangement is as shown in Figure 1 below (for 7 tyred pneumatic rollers). The arrangement is to ensure complete coverage in one pass.
C. The Dual-drum Vibratory Roller
This type of roller with its smooth steel drum can compact a wide variety of materials from sand to cobble. It can be used to compact asphalt paving, cohesionless subgrade, base course, and wearing surfaces. It is very effective when used immediately behind a grader blade to create a smooth, dense, and watertight subgrade finish. Because it has a relatively low unit pressure and compacts from the top down, it is normally used for relatively shallow layers (less than 100 mm). The smooth steel drum is ideal for base- or wearing-course finish work. In using this roller care must be taken to ensure that base course materials are not crushed. This is the roller of choice for asphalt paving.
D. The Smooth-drum Vibratory Roller
This type of roller can be an effective and economical means of attaining the desired density for cohesionless materials. This roller is very effective in compacting non-cohesive/nonplastic sands and gravel, which are often used in subbase and base course applications. Because this roller is relatively light, the recommended maximum loose-layer depth (the fill material measured before compaction) is 225 mm.
Note
a. Average compaction requirements should be gotten from the rolling test.
b. Vibration has two measurements—amplitude (the measurement of the movement or throw) and frequency (the number of repetitions per unit of time). The amplitude controls the depth to which the vibration is transmitted into the soil, and the frequency determines the number of blows or oscillations that are transmitted in a period of time.
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Reference
FM 5-434 (2000): Earthmoving operations. Department of Army, Washington, USA.
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