COMPACTION - Grading Work.

Grading work usually involves fill compaction, which is defined as the densification of soil by mechanical means. This physical process of getting the soil into a dense state can increase the shear strength, decrease the compressibility, and decrease the permeability of the soil.

There are many different types of fill, such as hydraulic fill, structural fill, dumped or uncompacted fill, debris fill, and municipal landfill. Table 15.2 presents a discussion of the typical characteristics, uses, and possible engineering problems for these types of fill.

It is always desirable to construct foundations on structural fill. For example, structural fill is used for all types of earthwork projects, such as during grading operations in order to create level building pads, slope buttresses, and shear keys. Structural fill is used for highway embankments, earth dams, and for backfill material of retaining walls and utility trenches. Structural fill is also used to create mechanically stabilized earth retaining walls and road subgrade. If it is not possible to support a foundation on structural fill, then a deep foundation system that penetrates the compressible soil and bears on solid material would be required.

As previously discussed, heavy compaction equipment is commonly used to create structural fill for mass grading operations. But many projects require backfill compaction where the construction space is too small to allow for such heavy equipment. Hand-operated vibratory plate compactors, such as shown in Fig. 15.14, are ideally suited for compacting fill in small or tight spaces. The hand-operated compactors can be used for all types of restricted access areas, such as the compaction of fill in utility trenches, behind basement walls, or around storm drains. For many projects having small or tight spaces, the backfill is simply dumped in place or compacted with minimal compaction effort. These factors of limited access, poor compaction process, and lack of compaction testing frequently lead to backfill settlement.

FIGURE 15.14 Hand-operated vibratory plate compactor used for compacting fill in tight spaces.

There are four basic factors that affect compaction of structural fill as follows:

1. Soil type. Nonplastic (i.e., cohesionless) soil, such as sands and gravels, can be effectively compacted by using a vibrating or shaking type of compaction operation. Plastic (i.e., cohesive) soil, such as silts and clays, are more difficult to compact and require a kneading or manipulation type of compaction operation. If the soil contains oversize particles, such as coarse gravel and cobbles, then they tend to interfere with the compaction process and reduce the effectiveness of compaction for the finer soil particles. Typical values of dry density for different types of compacted soil are listed in Table 15.1.

2. Material gradation. Those soils that have a well-graded grain size distribution can generally be compacted into a denser state than a poorly graded soil that is composed of soil particles of about the same size. For example, a well-graded decomposed granite (DG) can have a maximum dry density of 137 pcf (2.2 Mg/m3), while a poorly graded sand can have a maximum dry density of only 100 pcf (1.6 Mg/m3, modified Proctor).

3. Water content. The water content is an important parameter in the compaction of soil. Water tends to lubricate the soil particles thus helping them slide into dense arrangements. However, too much water and the soil becomes saturated and often difficult to compact. There is an optimum water content at which the soil can be compacted into its densest state for a given compaction energy.

Typical optimum moisture contents (modified Proctor) for different soil types are as follows:

a. Clay of high plasticity (CH). Optimum moisture content ≥ 18 percent
b. Clay of low plasticity (CL). Optimum moisture content = 12 to 18 percent
c. Well-graded sand (SW). Optimum moisture content = 10 percent
d. Well-graded gravel (GW). Optimum moisture content = 7 percent

Some soils may be relatively insensitive to compaction water content. For example, open-graded gravels and clean coarse sands are so permeable that water simply drains out of the soil or is forced out of the soil during the compaction process. These types of soil can often be placed in a dry state and then vibrated into dense particle arrangements.

4. Compaction effort (or energy). The compaction effort is a measure of the mechanical energy applied to the soil. Usually the greater the amount of compaction energy applied to a soil, the denser the soil will become. There are exceptions, such as pumping soils that cannot be densified by an increased compaction effort. Compactors are designed to use one or a combination of the following types of compaction effort:

a. Static weight or pressure
b. Kneading action or manipulation
c. Impact or a sharp blow
d. Vibration or shaking

TABLE 15.2 Different Types of Fill

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