Introduction:
In civil engineering, compaction is a process of densifying soil or aggregate materials by applying mechanical energy to reduce the void ratio. The standard compaction test is commonly used to determine the maximum dry density (MDD) and optimum moisture content (OMC) of a soil sample.

Explanation:
When the compactive effort is increased in a standard compaction test, the optimum moisture content (OMC) of the soil decreases. This means that the soil requires less water content to achieve maximum compaction.

Reason:
There are several reasons why the OMC decreases with increased compactive effort:

1. Increased compactive effort: When the compactive effort is increased, it means that higher energy is applied to the soil during the compaction process. This increased energy causes better particle rearrangement and closer packing of soil particles, resulting in a denser soil structure. As a result, the void ratio decreases, and the soil requires less moisture to achieve maximum compaction.

2. Reduced air voids: With increased compactive effort, the air voids in the soil are reduced due to the increased energy applied. The soil particles get closer to each other, displacing the air voids and reducing the overall porosity of the soil. This reduction in air voids leads to a decrease in the optimum moisture content.

3. Improved particle interlocking: Higher compactive effort enhances the interlocking between soil particles, resulting in improved shear strength and stability. The increased particle interlocking reduces the need for excess moisture to lubricate the soil particles, resulting in a lower optimum moisture content.

4. Increased density: Increased compactive effort leads to a higher degree of compaction, resulting in higher soil density. As the soil becomes denser, the voids between the particles decrease, requiring less water content to fill up the voids and achieve maximum compaction.

Conclusion:
In summary, when the compactive effort is increased in a standard compaction test, the optimum moisture content (OMC) decreases. This is because the increased compactive effort improves particle rearrangement, reduces air voids, enhances particle interlocking, and increases soil density, all of which contribute to a decreased water requirement for achieving maximum compaction.