Soil Sizes

Figure 15-58.—Soil profile. Soil Classification The soils you normally work with in earthwork operations are classified as follows: . Coarse grained . Fine grained l Organic 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. Soil Sizes Soils are grouped by the size of their particle grains. One method used to distinguish sizes is through the use of sieves (fig. 15-59). A sieve is a screen attached across the end of a cylindrical metal frame. The screen allows particles smaller than its openings to fall through and retains larger particles. Sieves with screen openings of different sizes allows you to sort soil into particle groups, based on size. Sieve sizes are designated by the screen opening size; for example, a 3-inch sieve has a screen with openings 3 inches square. A No. 4 sieve has four openings per linear inch, thus having 16 openings per square inch. Figure 15-59.—Standard sieve set. 15-35