The process of consolidation is often confused with the process of compaction. Compaction increases the density of an unsaturated soil by reducing the volume of air in the voids (see Fig.1). However, consolidation is a time-related process of increasing the density of a saturated soil by draining some of the water out of the voids (see Fig.1).
Consolidation is generally related to fine-grained soils such as silts and clays. Coarse-grained soils, such as sands and gravels, also undergo consolidation but at a much faster rate due to their high permeability. Saturated clays consolidate at a much slower rate due to their low permeability.
Consolidation theory is required for the prediction of both the magnitude and the rate of consolidation settlements to ensure the serviceability of structures founded on a compressible soil layer.