Answer:

Introduction:
Cohesive soil is a type of soil that has cohesive properties due to the presence of clay particles. The compactive effort refers to the level of energy applied to compact the soil. Optimum moisture content (OMC) is the moisture content at which a soil can be compacted to achieve maximum dry density.

Explanation:
When it comes to cohesive soil and increasing compactive effort, the optimum moisture content decreases. This can be explained by the following factors:

1. Increased compaction:
As the compactive effort increases, the energy applied to the soil increases as well. This leads to increased compaction of the soil particles. In cohesive soils, the clay particles have a tendency to stick together, forming a cohesive matrix. When the soil is compacted, the clay particles are compressed and forced closer together, reducing the void space between them.

2. Reduction in void space:
As the clay particles are compressed, the void space between them decreases. This reduction in void space means that less water is required to fill the gaps between the particles. Therefore, the optimum moisture content decreases with increasing compactive effort.

3. Increased density:
Increasing the compactive effort leads to an increase in the density of the soil. The soil particles become more closely packed, resulting in a higher dry density. With higher density, there is less room for water to be absorbed by the soil, leading to a decrease in the optimum moisture content.

4. Increased cohesion:
Cohesive soils have a high degree of cohesion, which is the ability of the soil particles to stick together. As the compactive effort increases, the cohesive forces between the clay particles become stronger. This cohesion between the particles reduces the need for additional moisture to provide lubrication and allows for effective compaction at lower moisture content.

Conclusion:
In summary, for cohesive soil, the optimum moisture content decreases with increasing compactive effort. This is due to increased compaction, reduction in void space, increased density, and increased cohesion between the clay particles. Understanding these factors is essential in achieving effective compaction and engineering properties of cohesive soils.