Compressibility and its causes

Compressibility is the property of soil by which it decreases in volume when it is subjected to a compressive load (an increase in effective stress). It is measured by the coefficient of compressibility $a_v$, the coefficient of volume compressibility $m_v$ and the compression index $C_c$.

Causes of compression in soil:

1. Compression of the solid particles (negligible).
2. Compression of water and air in the voids (negligible for water; air is compressible in partly saturated soils).
3. Expulsion of water (and air) from the voids, which is the main cause in saturated soil (consolidation).
4. Rearrangement and reorientation of the particles into a denser packing, and crushing or bending of grains.
5. Compression of the adsorbed layers and plastic (creep) deformation of the particle contacts (secondary compression).