SOIL MATERIAL
SOIL PROPERTIES
The following terms refer to properties and characteristics of soil materials. Knowledge
of these industry terms will provide a better understanding of compaction.
Compressibility. Compressibility is the reduction in soil volume that occurs when force
is applied to it. Compressibility is achieved as air and water are expelled from the voids
between the soil particles forcing the particles closer together, thus occupying less
volume.
Cohesion. A soil is cohesive or non-cohesive depending on the degree to which the
particles stick to one another. The inner-particle molecular attraction of one particle for
another, increases with a decrease in particle size. Granular materials have very low
cohesion because the particles are large and rough. Clay particles are small and
smooth and have high cohesion.
Fig. 2 – INTERNAL FRICTION
Shear Resistance. Soil particle resistance to movement when pressure, impact or
vibration is applied is shear resistance. The resistance comes from the friction between
the soil particles when they slide by each other. Therefore, the higher the shear
resistance the greater the force required to compact the soil. Clay has a high shear
resistance, while granular material has a low shear resistance.
Elasticity. As the term implies, elasticity is a soil’s ability to return in part to its original
form after a compressive load is removed. Soils of this type are very undesirable in
construction and road building. For example, a continually flexing road surface will
fatigue and break under traffic loads.
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