Classifying Earth Materials

For the purposes of foundation design,  earth materials are classified  according to particle size, the presence  of organic content, and, in the case of finer grained soils, sensitivity to moisture content:

• Rock is a continuous mass of solid mineral material, such as granite  or limestone, which can only be re- moved by drilling and blasting. Rock  is never completely monolithic, but is  crossed by a system of joints  (cracks)  that vary in quantity and extent and that divide the rock into irregular blocks. Despite these joints, rock is generally the strongest and most stable material on which a building can be founded.

• Soil is a general term referring to any earth material that is particulate.
•  If an individual particle of soil is too large to lift by hand or requires two hands to lift, it is a boulder.
•  If it takes the whole hand to lift a particle, it is called a cobble.
•  If a particle can be lifted easily between thumb and forefinger, the soil is  gravel. In the Unified  Soil

Classification System (Figure 2.2), gravels are classified as having more than half their particles larger than 0.19 inch (4.75 mm) in diameter but none larger than 3 inches (76 mm).

The Unifi ed Soil Classifi cation System, from ASTM D 2487. The group symbols are a universal set of abbreviations for soil types.
Figure 2.2
The Unifi ed Soil Classifi cation System, from ASTM D 2487. The group symbols are
a universal set of abbreviations for soil types.
•  If individual soil particles can be seen but are too small to be
picked up individually, the soil is sand. Sand particles range in size from about 0.19 to 0.003 inch (4.75–0.075 mm). Both sand and gravel are referred to as coarse-grained soils.
•  Individual silt particles are too small to be seen with the unaided eye and range in size from 0.003 to 0.0002 inch (0.075–0.005 mm). Like coarse-grained soil particles, silt particles are roughly spherical, or equidimensional, in shape.
• Clay particles are plate-shaped rather than spherical (Figure 2.3) and smaller than silt particles, less
than 0.0002 inch (0.005 mm) in size.

Silt particles (top) are approximately equidimensional granules, while clay particles (bottom) are platelike and much smaller than silt. (A circular area of clay particles has been magnifi ed to make the structure easier to see.)
Figure 2.3
Silt particles (top) are approximately
equidimensional granules, while clay
particles (bottom) are platelike and much
smaller than silt. (A circular area of clay
particles has been magnifi ed to make the
structure easier to see.)
Both sands and silts are also referred to as fine-grained soils.

•  Peat, topsoil, and other  organic soils are not suitable for the support of building foundations. Because of their high organic matter content, they are spongy, they compress easily, and their properties can change over time due to changing water content or biological activity within the soil.

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