1.4. Shear Failure under Foundation Load

At failure, shear stress along the failure surface reaches the shear strength of soil. 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 is crucial to understanding how soil behaves under stress. At the point of failure, the amount of stress that the soil can withstand (shear stress) matches its shear strength. This means that when the applied forces on the soil exceed its ability to resist sliding, failure occurs. The definition of shear strength indicates its role in preventing failure - it's the soil's ability to keep particles from sliding past one another.

Think of a stack of books on a table. If you push sideways on the top book gently, it might stay in place. However, if you push too hard, the top book will slide off the stack. The point at which the book slides is similar to the shear strength of soil; the table (ground) can only hold so much before it gives way.

Soil derives its shear strength from two sources: – Cohesion between particles (stress independent component) • Cementation between sand grains • Electrostatic attraction between clay particles – Frictional resistance and interlocking between particles (stress dependent component)

Shear strength arises from two primary components. The first is cohesion, which is due to the forces that bind soil particles together, such as cementation and electrostatic attraction. This means that some soil naturally sticks together without external forces. The second component is friction, which depends on the physical interaction and interlocking of particles; this is influenced by the stress acting on the soil. When soil particles are pushed together with more force, they create more friction, increasing strength.

Imagine a pile of sand on a beach. If you gently try to slide your hand through, it flows easily due to low cohesion. However, if you wet the sand, the particles stick together more, showing increased cohesion. The way grains interact and lock together in the presence of more force mirrors how shearing occurs in soils.

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

Cohesion is a key factor contributing to the strength of soil, providing resistance to slipping due to internal forces amongst soil particles. Internal friction, on the other hand, is the resistance that soil offers against sliding due to frictional forces when external load is applied. Together, these factors determine how stable soil is under various loads and conditions.

Consider two friends trying to move a sofa together. If they push against it and it remains stable, that illustrates friction. If one friend holds onto the arm of the sofa while the other pushes, the arm's grip (cohesion) prevents sliding. This scenario presents how cohesion and internal friction work together to provide stability.

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).

Stress is a critical aspect of soil mechanics, comprising two main types. Normal stress compresses soil and affects its volume, while shear stress tries to slide soil particles past each other. The balance of these stresses determines soil stability and can lead to failure if shear stress exceeds shear strength.

Picture a loaded bookshelf. The vertical weight of the books creates pressure on the shelves (normal stress), while if someone pulls on the front of the shelf, that action causes the shelves to slide (shear stress). Understanding these forces helps us ensure that the bookshelf remains stable and does not collapse.

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