Effect of compaction on soil properties

Compaction is the process of densifying soil by reducing the air voids using mechanical energy (rolling, ramming or vibration), at constant water content.

Effects on engineering behaviour

1. Shear strength increases: particles interlock more closely and the dry density rises.
2. Compressibility and settlement decrease: a dense, stiffer soil settles less under load.
3. Permeability decreases: voids are reduced, so seepage through dams and embankments is lower.
4. Bearing capacity increases: the safe load for foundations and pavements is higher.
5. Swelling and shrinkage depend on the placement water content: soil compacted wet of optimum swells less but shrinks more on drying; soil compacted dry of optimum swells more but shrinks less.
6. Liquefaction potential is reduced in sands because of the denser state.
7. Slope stability improves because of the higher strength and lower pore pressure.
8. Water absorption and frost damage decrease.

Compacting wet of optimum gives lower permeability and a more flexible soil (good for dam cores); compacting dry of optimum gives higher strength and stiffness (good for road subgrade) but more swelling.