Equipment for Grading and Compaction of Soil

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Common types of equipment used during the grading operations are as follows:

1. Bulldozer (Fig. 9). The bulldozer is used to clear the land of debris and vegetation (clearing, brushing, and grubbing), excavate soil from the borrow area, cut haul roads, spread out dumped fill, rip rock, and compact the soil.

Figure 9: The bulldozer is in the process of spreading out a layer of fill for compaction. 

2. Scraper (Fig. 10). The scraper is used to excavate (scrape up) soil from the borrow area, transport it to the site, dump it at the site, and the rubber-tires of the scraper can be used to compact the soil. Push-pull scrapers can be used in tandem in order to provide additional energy to excavate hard soil or soft rock.

Figure 10: The scraper is used to excavate material from the borrow area, transport it to the site, dump it at the site, and then compact the soil.

3. Loader (Fig. 11). Similar to the scraper, the loader can be used to excavate soil from the borrow area, transport the soil, and then compact it as structural fill.

Figure 11: The loader can be used to move soil about the job site and compact the soil as structural fill.

4. Excavator (Fig. 12). This type of equipment is ideally suited to excavating narrow trenches for the construction of utilities such as storm drain lines and sewer lines. This equipment is also

used to excavate footings and other foundation elements.

Figure 12: The excavator is ideally suited to excavating narrow trenches for the construction of utilities such as storm drain lines and sewer lines.

5. Dump Trucks and Water Trucks (Fig. 13). If the borrow area is quite a distance from the site, then dump trucks may be required to transport the borrow soil to the site. Dump trucks are also needed to transport soil on public roads or to import select material.

Figure 13: The water truck is adding water to fill that is in the process of beingcompacted.

Especially in the southwestern United States, the near surface soil can be in a dry and powdery state and water must be added to the soil in order to approach the optimum moisture content. A water truck, such as shown in Fig. 13, is often used to add water to the fill during the grading operation.

The Caterpillar Performance Handbook (1997), which is available at Caterpillar dealerships, is a valuable reference because it not only lists rippability versus types of equipment, but also indicate types and models of compaction equipment, equipment sizes and dimensions, and performance specifications.

Compaction equipment can generally be grouped into five main categories, as follows:

1. Static weight or pressure. This type of compaction equipment applies a static or relatively uniform pressure to the soil. Examples include the compaction by the rubber-tires of a scraper, from the tracks of a bulldozer, and by using smooth drum rollers.

2. Kneading action or manipulation. The sheepsfoot roller, which has round or rectangular shaped protrusions or feet, is ideally suited to applying a kneading action to the soil. This has proven to be effective in compacting silts and clays.

3. Impact or a sharp blow. There are compaction devices, such as the high-speed tamping foot and the Caterpillar tamping foot, that compact the soil by imparting impacts or sharp blows to the soil.

4. Vibration or shaking. Nonplastic sands and gravels can be effectively compacted by vibrations or shaking. An example is the smooth drum vibratory soil compactor.

5. Chopper wheels. This type of compaction equipment has been specially developed for the compaction of waste products at municipal landfills.

Table 1 presents a summary of different types of compaction equipment best suited to compact different types of soil.

TABLE 1 Characteristics of Compacted Subgrade for Roads and Airfields

A common objective of the grading operations is to balance the volume of cut and fill. This means that just enough earth material is cut from the high areas to fill in the low areas. A balanced cut and fill operation means that no soil needs to be imported or exported from the site, leading to a reduced cost of the grading operation.

When developing a site so that the cut and fill is balanced, consideration must be given to the bulking or shrinkage factor associated with the compaction operation. Bulking is defined as an increase in volume of soil or rock caused by its excavation. For example, very dense soil will increase in volume upon excavation and when compacted, the compacted soil may have a dry unit weight that is less than existed at the borrow area. Conversely, when loose material is excavated from a borrow area and worked into a compacted state, the compacted soil usually has a dry unit weight that is greater than existed at the borrow area. The shrinkage factor is often defined as the ratio of the volume of compacted material to the volume of borrow material (based on dry unit weight).

Fill placement should proceed in thin lifts, i.e., 6 to 8 in. (15 to 20 cm) loose thickness. Each lift should be moisture conditioned and thoroughly compacted. The desired moisture condition should be maintained or reestablished, where necessary, during the period between successive lifts. Selected lifts should be tested to ascertain that the desired compaction is being achieved.

There are many excellent publications on field compaction equipment. For example, Moving the Earth (Nichols and Day, 1999) presents an in-depth discussion of the practical aspects of earth moving equipment and earthwork operations.