2.2. Shear strength

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

When soil is compacted, particles are pressed closer together, resulting in more contact points between them. This increased number of contacts means that the soil can resist sliding forces better, which effectively increases its shear strength. Granular soils, like sand and gravel, benefit significantly from this densification process, as the friction between the closely packed particles holds them together more securely.

Imagine a pile of marbles. If you shake them gently, they can roll and slide over each other easily, much like loosely packed sand. However, if you tightly pack the marbles in a container, they can't move around as freely, and the entire structure becomes more stable. This is similar to how granular soil behaves when it's compacted.

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 various factors including how dense the soil is when dry, the amount of water added during compaction, and the way the soil structure is formed. For instance, if clay is compacted too wet, it can become too loose and lose strength. Conversely, if it’s compacted dry, it can hold strength better due to the arrangement of particles, known as the flocculated structure, which resists sliding.

Think of how wet and slippery playdough behaves compared to how it acts when it's dry. When playdough is wet, it's mushy and prone to deformation; however, when it's dry and crumbly, it can hold its shape but might crack under pressure. This mirrors how clay reacts to moisture and compaction.

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

Optimum moisture content refers to the ideal amount of water needed for compaction. Cohesive soils, such as clays, have better strength characteristics when they are compacted dry of optimum. This is because a flocculated structure forms, where particles are clustered together, creating more force resistance against shear. In contrast, if compacted wet of optimum, the structure disintegrates into a more dispersed form, leading to lower shear strength and increased susceptibility to failure.

Consider a sponge wet with water versus one that is slightly damp. The wet sponge is soft, easier to compress, and less effective at holding weight. Conversely, a dryer sponge maintains its shape better and can support more weight. This illustrates how the moisture condition of soil affects its overall strength.

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