COARSE-GRAINED SOILS.- Soils in this classification are composed of sand and gravel and are in the B-horizon. Coarse-grained soils have 50 percent or less material passing the No. 200 sieve. Their grain shape varies from rounded to angular and has good load-bearing qualities and drains freely.

FINE-GRAINED SOILS.- Fine-grained soils are composed of silt and clay and are in the B-horizon. They have 50 percent or more material passing the No. 200 sieve. Fine-grained soils have good-load bearing qualities when dry; however, these soils drain poorly, and when wet, have little or no load-bearing strength. This characteristic is especially true with clay.

ORGANIC SOILS.- Organic soils, sometimes referred as top soil, are composed mostly of decayed plant and animal matter and are in the A-horizon. These soils retain moisture, are difficult to compact, and are normally used when landscaping a finish project.

Figure ^{15-59.-Standard sieve set.}

If a soil sample passes the 3-inch sieve but does not pass the No. 4 sieve, the larger particle size is less than 3 inches and the smallest size is larger than 1/16 inch. This soil is classified as gravel.

Soils that pass the No. 4 sieve but are retained on the No. 200 sieve are classified as sands. Sands are further broken down as coarse sand or fine sands. Coarse sand passes the No. 4 sieve and is retained on the No. 40 sieve. Fine sand passes the No. 40 sieve and is retained on the No. 200 sieve.

Any soil, passing the No. 200 sieve, is classified as fine-grained.

Soil Gradation

Gradation describes the distribution of different size groups within a soil sample. A well-graded soil (fig. 15-60) is a soil sample that has all sizes of material present from the No. 4 sieve to the No. 200 sieve.

Poorly graded soil may be uniformed-graded (fig. 15-61) or gap-graded (fig. 15-62). If a soil is uniformed-graded, most of its particles are about the same size. An example of this is a sieve analysis in which sand size No. 20 is the only size present.

If a soil is gap-graded, at least one particle size is missing. An example of gap-graded soil is one in which a sieve analysis reveals that sand sizes No. 10 and No. 40 are missing. All other sizes are present.

Soil Compaction

Compaction is pressing together soil particles to form a consolidated mass with increased stability.

Figure 15-60.-Well-graded soil.

Figure 15-61.-Uniform-graded soil.

Figure 15-62.-Gap-graded soil.

Compaction helps the soil to be more resistant to soaking up moisture from below.

Fills are built up in compacted layers. In earthwork operations, these layers are called lifts. Lifts are from 4 inches to 1 foot in depth, depending upon the compaction necessary, compaction equipment available, and material used for the fill.

The fill material must have the right amount of moisture, referred to as optimum moisture content. To obtain maximum compaction, wet the fill, when necessary, before it is compacted. Compaction may be obtained by using a pneumatic, tandem, or vibratory roller.