4. Influence on Shear strength

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

In granular soils, when compaction occurs, the particles are forced closer together. This increases the number of points where the particles touch each other, creating more contacts. As a result, these additional contact points provide greater resistance against shearing forces, contributing to higher shear strength in the soil.

Think of a box of marbles. If you have them scattered loosely, they can easily roll past each other (low shear strength). However, if you compact them tightly in a box, they create friction against each other, making it harder for them to move (high shear strength).

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

The shear strength of clay soils is influenced by several factors. Firstly, dry density affects how tightly packed the clay particles are; higher density usually means higher shear strength. Secondly, the amount of water during compaction (moulding water content) can affect soil structure. Too much water makes the soil more dispersed and weaker, while appropriate water content can enhance bonding between particles. The method of compaction and how strain is drained also play roles in determining the overall strength.

Imagine making a snowball. If the snow is just right, it sticks together well and forms a strong snowball (high shear strength). However, if the snow is too wet, it becomes slushy and falls apart when you try to roll it (low shear strength).

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

When cohesive soils are compacted with less water than the optimum level, the particles maintain a flocculated structure, which is a more stable configuration that provides higher shear strength. Conversely, when compacted with too much water, the particles become dispersed, reducing their inter-particle bonding and thus lowering the shear strength.

Think of building a sandcastle. When you use just the right amount of water, the sand sticks together well, making a sturdy castle. If you add too much water, the sand becomes too wet and soft, causing the castle to collapse easily.

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