4.1. Increase the number of contacts resulting in increased shear strength.

Increase the number of contacts resulting in increased shear strength, especially in granular soils.

Shear strength refers to the ability of soil to resist shear stress, which is the force that causes layers of soil to slide past one another. When soil particles are compacted together more tightly, there are more contact points between them. This increase in contacts helps to distribute loads better and provides greater resistance against sliding. Granular soils, like sand and gravel, benefit greatly from increased contact, as this leads to a higher shear strength overall.

Think of a pile of marbles. If you have them scattered loosely, they can easily roll away from one another. However, if you push them together, they form a compact mass. When you try to apply a force to push the mass, it's much harder to move than when they were loose. The same concept applies to soil particles!

In clays, shear strength depends on dry density, moulding water content, soil structure, method of compaction, strain drainage condition etc.

Unlike granular soils, cohesive soils (like clays) have their shear strength influenced by a number of factors. The dry density refers to how compact the soil is when all the water is removed. The water content affects how the soil particles fit together—more water might help them slide past one another, while less water can cause them to bind more strongly. The structure of the soil and how it is compacted also play critical roles, since different arrangements of particles can lead to different strengths.

Imagine trying to mold clay with your hands. If your hands are dry, the clay holds together better, but if your hands are wet, the clay becomes slippery and harder to shape. The way you compact the clay—how hard you press and for how long—also affects its firmness and strength.

Shear strength of cohesive soils compacted dry of optimum (flocculated structure) will be higher than those compacted wet of optimum (dispersed structure).

Soil can be compacted in various ways, and the method used can significantly change its strength. 'Optimum moisture content' is the ideal water level at which soil can be compacted to the highest density. When clay is compacted dry of this optimum level, it tends to form a flocculated structure, which is interlocking and stable. On the other hand, if it is compacted when there is too much moisture, it forms a dispersed structure that is less stable, ultimately resulting in lower shear strength.

Consider packing a suitcase. If you try to stuff a suitcase when your clothes are dry and folded neatly, they fit perfectly and are packed tightly. But if you try to pack when the clothes are wet and wrinkled, they don't fit as well, and the suitcase might even pop open. Just like this, achieving the right moisture level in soil is crucial.

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