The Reason for the Decrease in Dry Density in Cohesionless Soil with an Increase in Water Content: Bulking of Sand

Introduction:
Cohesionless soils, such as sands, exhibit a peculiar behavior when water is added to them. One of the significant changes observed is the increase in volume or bulk of the soil. This increase in volume, commonly known as "bulking of sand," leads to a decrease in the dry density of the soil. Several factors contribute to this behavior, including capillary rise, degree of saturation, and water content. However, the primary reason for the decrease in dry density is the bulking of sand.

Bulking of Sand:
When water is added to cohesionless soils like sand, it fills the void spaces between the soil particles. Due to the surface tension of water, the particles become surrounded by a thin film of water, causing them to repel each other. This repulsion leads to an increase in the interparticle spacing, resulting in an expansion in the volume of the soil mass. This phenomenon is known as bulking of sand.

Reasons for Dry Density Decrease:
The increase in volume due to the bulking of sand results in a decrease in the dry density of the soil. This decrease occurs due to the following reasons:

1. Expansion of Soil: As water fills the void spaces and forms a film around the soil particles, the interparticle distance increases. This expansion causes an increase in the overall volume of the soil mass.

2. Decrease in Particle Packing: With the increase in interparticle spacing, the particles lose their close arrangement and packing. The rearrangement of particles in a more open structure leads to a decrease in the soil's density.

3. Reduced Particle Interlocking: In cohesionless soils, the particles rely on interlocking to provide shear strength. However, the presence of water disrupts this interlocking mechanism by reducing the contact forces between particles. This reduction in interlocking contributes to the decrease in dry density.

4. Increased Porosity: The expansion of soil and decrease in particle packing result in an increase in porosity, which refers to the volume of void spaces in the soil. As porosity increases, the amount of solid material (particles) decreases relative to the total volume, leading to a decrease in dry density.

Conclusion:
In cohesionless soils, the addition of water causes an increase in volume, known as bulking of sand. This expansion leads to a decrease in the dry density of the soil due to the expansion of soil, decrease in particle packing, reduced particle interlocking, and increased porosity. Understanding these factors is essential in geotechnical engineering to properly assess and analyze the behavior of cohesionless soils when they come into contact with water.