1.5. Slope Stability Failure as an Example of Shearing Along Internal Surface

Thus shear strength of soil is

"The capacity of a soil to resist the internal and external forces which slide past each other".

Shear strength refers to the ability of soil to withstand forces that would cause its particles to move relative to one another. It is crucial in geotechnical engineering because it determines how much load the soil can support without failing due to shearing.

Imagine a stack of books on a table. If you push one book sideways, it may slide off. The friction between the books is similar to shear strength; the more friction, the harder it is for them to slide, just as soil needs sufficient shear strength to support structures without collapsing.

• The shear strength of a soil is its resistance to shearing stresses.
• It is a measure of the soil resistance to deformation by continuous displacement of its individual soil particles.
• Shear strength in soils depends primarily on interactions between particles.
• Shear failure occurs when the stresses between the particles are such that they slide or roll past each other.

This definition breaks down the concept of shear strength into more understandable components. It highlights that shear strength is not just about visuals but is related to how soil particles interact with one another. When the forces on these particles exceed their bonding strength, failure occurs, manifesting as a slide or roll of particles.

Think of a group of people holding hands to form a line. If everyone is connected well (good shear strength), they won't break apart when someone pushes. However, if you start pushing hard enough, they might let go of each other's hands (shear failure) and fall apart.

Soil derives its shear strength from two sources: – Cohesion between particles (stress independent component) – Frictional resistance and interlocking between particles (stress dependent component)

Shear strength comes from two main sources: cohesion and friction. Cohesion is the attraction between particles which keeps them together regardless of pressure, while friction arises when particles rub against each other and can vary with the load applied to them.

Consider a wet sandcastle on the beach. The moisture (cohesion) holds the grains together despite the weight of the castle. As the tide rises (increasing load), the sand may start to shift due to friction but can still hold its shape until the water pushes it too far.

Cohesion (C), is a measure of the forces that cement particles of soils. Internal Friction angle (ϕ), is the measure of the shear strength of soils due to friction.

Cohesion provides a baseline strength of the soil due to the attractive forces between its particles, while the internal friction angle indicates how the arrangement of particles affects their ability to resist sliding. Together, these factors define how stable a soil mass is under stress.

Cohesion is like glue that sticks two pieces of paper together, while internal friction is like the resistance felt when you try to slide one paper over another. The effectiveness of both determines if they will stay together or slip apart.

Gravity generates stresses (force per unit area) in the ground at different points. Stress on a plane at a given point is viewed in terms of two components:

• Normal stress (σ): acts normal to the plane and tends to compress soil grains towards each other (volume change)
• Shear stress (τ): acts tangential to the plane and tends to slide grains relative to each other (distortion and ultimately sliding failure).

In any soil structure, stress is a fundamental concept dictated by the weight of materials above. Normal stress compresses soil and can contribute to settling, while shear stress can lead to slippage and failure. Understanding these stresses helps engineers determine soil behavior under loads.

Think about a book on a pile of other books. The weight of the books above exerts normal stress, pushing down and potentially compressing the bottom book. Meanwhile, if you try to pull just one book on the side, you apply shear stress, which could cause the books to shift if the friction isn't enough.

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