Factors affecting compaction

The dry density achieved by compaction depends on:

1. Water content: at low water content the soil is stiff and resists compaction. Water lubricates the particles, so density increases up to the optimum moisture content (OMC) where $\gamma_d$ is maximum. Beyond OMC, water occupies space and density falls.
2. Compaction effort (energy): more effort (more blows, heavier rammer, more passes) gives a higher maximum dry density and a lower OMC. Increasing the effort beyond a limit gives little gain.
3. Type of soil: well-graded coarse soils give a high $\gamma_{d,max}$ and low OMC with a steep curve; clays have a low $\gamma_{d,max}$, high OMC and a flat curve. Poorly graded sands give a low density.
4. Method of compaction: impact (hammer), kneading, vibration, and static pressure give different densities. Vibration is best for sands; kneading for clays.
5. Lift thickness and number of passes in the field.
6. Type of roller and speed: sheep-foot for clay, vibratory for sand.
7. Admixtures: lime, cement or salts change OMC and density.
8. Temperature, and initial moisture condition.