9. Effect on Soil Structure

On dry side of optimum, the structure is flocculated. The particles repel and density is less.

When the soil is on the dry side of the optimum moisture content, the soil particles tend to clump together in a flocculated structure. This means that the particles are grouped in a way that they are not as tightly packed. In this state, the soil has lower density because there is more space or voids between the particles. The repellent forces between the particles prevent them from coming into closer contact, which is why flocculated soil is less dense.

Think of a bowl of dried grains. When they are dry and not mixed, the grains clump up but have bubbles of air between them, making the bowl appear less full. This is similar to how soil behaves when it is dry—it has more voids and thus a lower density.

Addition of water increases lubrication and transforms the structure into dispersed structure.

When water is added to the dry soil, it acts as a lubricant. This lubrication helps the soil particles to slide past each other more easily, leading to a transformation from the flocculated structure to a dispersed structure. In this state, the soil particles are more spread out, leading to a more uniform distribution without as many voids between them, which significantly increases the density of the soil.

Imagine how sand behaves when it is dry versus when it is wet. Dry sand runs through your fingers and doesn’t hold shape well. However, when you add water, it sticks together and can be shaped into sandcastles because the water helps the grains to bond with each other.

In coarse grained soil, single grained structure is maintained.

Coarse grained soils, such as sand and gravel, maintain a single-grained structure. This means that the individual soil grains do not clump together like in finer soils. Instead, they remain separate, leading to larger voids between the particles. This single-grained structure contributes to the properties of coarse soils, such as higher permeability and lower cohesion compared to finer soils.

Think of marbles in a jar. Each marble stays separate, and there's a lot of empty space between them. This is similar to how coarse grained soils behave, allowing air and water to pass through more easily compared to a packed mass of fine dust.

In composite soil, behaviour depends on composition.

Composite soils are made up of a mixture of different types of soil. The structure and behavior of this type of soil will depend on the proportions of each component. For example, if a composite soil contains a lot of coarse grains, it may behave more like coarse grained soil, while a predominance of fine particles could lead to behavior similar to fine grained soils. This complexity can affect how the soil responds to compaction and moisture.

Consider a salad composed of various ingredients—lettuce, tomatoes, croutons, and dressing. Depending on the amounts of each ingredient, the salad can be crunchy, soggy, or well-mixed. Similarly, the behavior of composite soil is determined by the mix of different soil types in it.

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