Answer:

Introduction:
Soil compaction is the process of densifying the soil by reducing the air voids present in the soil mass. The percentage of voids filled with air in soil compacted under the optimum moisture content (OMC) is an important parameter that determines the degree of compaction achieved in the soil.

Percentage of Voids Filled with Air:
The percentage of voids filled with air in a soil compacted under OMC depends on the soil type, compaction energy, and the compaction method used. Generally, the percentage of voids filled with air in a soil compacted under OMC shall be around 5-10%. This means that the remaining voids in the soil mass shall be filled with water.

Reason behind 5-10%:
The reason behind the 5-10% air voids is that the presence of air voids in the soil mass helps in improving the soil's permeability and reduces the risk of soil cracking and deformation due to changes in moisture content. However, if the percentage of air voids is too high, it can significantly reduce the soil's strength and stability, making it susceptible to erosion and settlement.

Conclusion:
In conclusion, the percentage of voids filled with air in a soil compacted under OMC shall be around 5-10%. This ensures that the soil is adequately densified while maintaining its permeability and stability. However, the actual percentage of air voids may vary depending on the soil type and the compaction method used.