1.6. Shear Strength in Soils

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 is how much force a soil can withstand before it starts to deform or slide. It measures the ability of the soil's particles to hold together and resist forces that would cause them to slip or move apart from each other.

Think of a pile of sand. When you push on it, the particles can shift, but if you push too hard, they all start to slide apart. The point just before that sliding is the shear strength of the sand.

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.

The shear strength of soil is not just about how strong the particles are individually, but also how they interact with each other. If the forces between particles become too great, they will start to move relative to one another and cause a failure. This is called shear failure where particles lose their cohesive grip.

Imagine a stack of marbles. They hold together nicely until you push too hard, causing them to roll over each other. That tipping point is like shear failure in soil – the moment they can no longer hold their position due to the forces applied.

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 components. Cohesion refers to the stickiness of the particles to each other, while frictional resistance depends on how much force is applied. As stress increases, the friction between the particles helps maintain their position against sliding.

Consider a pile of clay. The wet clay sticks together (cohesion), allowing it to hold its shape. If you push harder on it (increasing stress), the particles will start to slide, but the friction they generate still fights against that sliding, maintaining some structure for a while.

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

Cohesion is the force of attraction that holds soil particles together, like glue. The internal friction angle indicates how easily soil particles can slide against each other. Together, these two factors define the overall shear strength of the soil.

Imagine a piece of sticky tape. The adhesion of the tape (cohesion) keeps items stuck together, while how rough or smooth the surface it’s stuck to is affects how easily you can pull it off (internal friction).

Angle of Repose determined by:

• Particle size (higher for large particles)
• Particle shape (higher for angular shapes)
• Shear strength (higher for higher shear strength)

The angle of repose is the steepest angle at which soil can remain stable without sliding. Larger, angular particles can maintain a steeper slope compared to smaller, round particles, because they interlock better and resist sliding.

Think of how a pile of gravel (with angular stones) holds its shape much better compared to a pile of sand (which is more rounded). The gravel can maintain a steeper slope, representing a higher angle of repose.

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).

Stresses in soils come from the weight of the soil above, represented as normal and shear stress. Normal stress squishes soil particles together, while shear stress tries to make them slide past each other. Both pressures are important in understanding shear strength.

Consider a stack of books. The weight of the books on top compresses the ones below them (normal stress), while trying to slide a book out causes it to experience shear stress – if you pull too hard, you could topple the stack.

--