In soil science and agronomy, soil compaction is usually a combination of both engineering compaction and consolidation, so may occur due to a lack of water in the soil, the applied stress being internal suction due to water evaporation as well as due to passage of animal feet.

Air void mean space where air is absence. Somewhere where is lack of air means there is void space so when void space will reduce it give rise to compaction

Compaction is the process of mechanically increasing the density of soil by reducing the air voids between its particles. It is an essential step in construction and engineering projects to ensure the stability and strength of the soil. Compacted soil has improved load-bearing capacity and reduced susceptibility to settlement.

There are several factors that contribute to compaction, but the main reason is the reduction of air voids within the soil mass. This is achieved by expelling the pore water present in the soil. Let's explore these factors in more detail:

1. Expulsion of pore water:
- Pore water refers to the water that fills the spaces between soil particles.
- During compaction, external forces such as mechanical compaction equipment or heavy machinery are applied to the soil.
- These forces cause the water to be expelled from the voids, resulting in a reduction in the water content of the soil.
- As the water content decreases, the soil particles come closer together, reducing the air voids and increasing the soil density.

2. Reduction of air voids:
- Air voids are the spaces between soil particles that are not filled with water.
- Compaction techniques aim to reduce these air voids and increase the density of the soil.
- As the soil particles are compressed and brought closer together, the air voids are eliminated or minimized.
- The reduction of air voids improves the soil's strength, stability, and resistance to deformation.

3. Effect of compaction effort:
- The effectiveness of compaction is influenced by factors such as the compaction effort, soil type, moisture content, and compaction method.
- The compaction effort refers to the energy applied to the soil during the compaction process.
- Higher compaction efforts result in greater expulsion of pore water and reduction of air voids, leading to increased soil density.
- The appropriate compaction effort depends on the soil type and desired level of compaction.

In conclusion, compaction is primarily due to the reduction of air voids within the soil mass. This is achieved by expelling the pore water present in the soil. The process of compaction improves the soil's load-bearing capacity, stability, and resistance to settlement, making it an essential step in civil engineering and construction projects.