SOIL MATERIAL
SOIL TYPES
The unconsolidated material that is lying above the bedrock is soil. Specifically, soil is
made up of disintegrated rocks and other material that have been broken down into
smaller pieces by nature’s mechanical and chemical processes. This material is
available to the contractor as building material. Soils can be classified into three groups
– granular, clays, and organic.
Granular. If the breakdown process from solid bedrock was mechanical, such as by
water or wind erosion, glacial action, freezing and thawing, etc., the soil will have the
same composition as the base rock from which it came. Granular soil particles are non-
cohesive, sand or gravel consisting of grains as small as .002 inches (.05 mm).
Clay. If the breakdown included some chemical reactions due to heat, pressure, and
moisture over long periods of time, the composition of the base rock changes and tiny
plate-like or scale shaped particles result. These very fine particles are regarded as a
cohesive, clay soil.
Organic. Plant growth also contributes to soil formation. Decayed plants form loam or
peat, which is desirable for agricultural purposes. This type of soil has a spongy residue
that holds water and air, which is very detrimental for construction applications.
SOIL MIXING
Because of all the varying soil constituents, there is a wide variation of soils. When the
soil particle size range extends over more than two size classes, the soil is termed a
mixed soil. And this is common, since most soils in the field are mixtures of various
sizes of granular and cohesive material. In fact, soil with the highest bearing capacity
contains sufficient quantities of gravel and sand to contribute to high internal friction and
enough fine-grained soil such as clay to provide adequate cohesion.
Mixing soils at the job is closely allied with moisture. It is a key step that makes
compaction easy or difficult.
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