2.6. Shear Strength of Soils

τ=c+σ⋅tan(ϕ)

• c: Cohesion
• φ: Angle of internal friction

The Mohr-Coulomb failure criterion is a mathematical model that helps to determine when a soil will fail under shear stress. In the equation, τ represents the shear strength of the soil, c represents the cohesion (the inherent strength of the soil particles to stick together), and σ is the normal stress acting on the surface. The angle ϕ is the angle of internal friction, which indicates how much resistance the soil provides against sliding. When you apply stress to the soil, if the resulting shear stress exceeds the limit set by this equation, the soil will fail or slide.

Imagine a stack of books (soil layers) stacked on top of each other. The cohesion is like the sticky notes used to keep them together, while the angle of internal friction is how the rough surfaces of the book covers help prevent them from sliding off each other. If you push down with too much force (stress), the stack will collapse.

• Direct Shear Test
• Triaxial Shear Test (UU, CU, CD)
• Unconfined Compression Test (for cohesive soils)

Shear tests are conducted to measure the shear strength of soil. The Direct Shear Test involves placing a soil sample in a device that allows it to be sheared under controlled pressure, providing direct measurements of shear strength. The Triaxial Shear Test is more complex and can control drainage conditions, testing different scenarios, including unconsolidated undrained (UU) and consolidated drained (CD) conditions. The Unconfined Compression Test is specific for cohesive soils and measures the compressive strength without any lateral confinement. This is particularly important for understanding how soils will behave under loads once constructed.

Think of the shear tests as different ways to test how a stack of cards behaves under pressure. The Direct Shear Test could be like pulling the cards sideways until they slip, while the Triaxial Test is like pushing from the sides while carefully lifting to see how much force they can take before falling apart. The Unconfined Compression Test is simply pressing down on the top of a stack of cards without any support, to see how much pressure they can handle before collapsing.

• High shear strength → good load support capacity
• Low shear strength soils may require stabilization

Shear strength is crucial in determining if the soil can support the weight of the infrastructure above it, such as roads and buildings. Soils with high shear strength can effectively support heavy loads without failing, making them ideal for pavement support. On the other hand, soils with low shear strength may not support such loads adequately, leading to potential pavement failure or deformation. To address this, stabilization methods can be employed to improve their strength.

Consider how a solid table can hold many books (high shear strength), while a stack of notebooks might buckle if too many are placed on it (low shear strength). When the notebooks are stacked, you could place a solid board underneath to support them better, just like how low shear strength soils can be stabilized to handle the weight of the pavement.

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