Introduction:
The maximum density of cohesionless soil is achieved when the soil is fully saturated. This means that the soil is filled with the maximum amount of water that it can hold.

Explanation:
Saturation:
- Saturation refers to the state of a soil when all the void spaces are filled with water.
- When a soil sample is fully saturated, there is no air present in the void spaces between the soil particles.
- Saturation is an important factor in determining the density of cohesionless soil.

Density:
- Density is defined as the mass of a substance per unit volume.
- In the case of soil, density is typically measured as dry density or wet density.
- Dry density is the mass of solids per unit volume of soil, while wet density is the mass of solids and water per unit volume of soil.

Maximum Density:
- The maximum density of cohesionless soil is achieved when it is fully saturated.
- When the soil is fully saturated, the void spaces between the soil particles are filled with water.
- This water fills the void spaces and helps to compact the soil particles closer together, resulting in increased density.
- The presence of water in the soil also increases the interparticle forces and enhances soil cohesion, further contributing to the maximum density.

Effects of Water Content:
- When a soil is partially saturated or has zero water content, the soil particles are not as closely packed together.
- In partially saturated soil, there are air voids present between the soil particles, which reduces the density.
- In zero water content soil, the absence of water eliminates the interparticle forces and cohesion, resulting in lower density.

Conclusion:
The maximum density of cohesionless soil is achieved when the soil is fully saturated. Saturation ensures that the void spaces between the soil particles are filled with water, leading to a closer packing of particles and increased density. Partial saturation or zero water content reduces the interparticle forces and cohesion, resulting in lower density.