1.7. Components of shear strength of soils

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 in soil is fundamentally derived from two main sources: cohesion and frictional resistance. Cohesion refers to the natural forces that bond particles together, which does not change with the amount of stress applied to the soil. For example, sand grains may stick together through cementation or electrostatic forces in clay. On the other hand, frictional resistance comes into play when particles rub against each other under stress, which can change based on the compressive forces acting on the soil. Thus, understanding both sources is crucial when assessing the stability of soil structures.

Think of cohesion like a group of friends who are tightly holding hands while facing a windstorm. Their grip (cohesion) prevents them from being blown away despite the wind's pressure (external stress). Frictional resistance, however, is like the shoes they wear; if they're wearing grippy shoes, they can stand firm against sliding. If they wear slippery shoes instead, they might just slide around, indicating that the conditions have changed.

Cohesion (C), is a measure of the forces that cement particles of soils.

Cohesion in soil represents the attraction between soil particles that holds them together. This attraction can be due to various factors, including water content and the type of soil. For example, clay soils tend to have higher cohesion due to their small particle size and high surface area, allowing them to hold more moisture and create stronger bonds. Understanding cohesion is key for predicting how much load a soil can bear without shearing.

Imagine making a ball of dough from flour and water. The water acts like the cohesive forces between clay particles, allowing the flour particles to stick together well when mixed. If you were to try to roll it out as dry flour, it would fall apart, similar to how loose soils without cohesion cannot hold their shape under pressure.

Internal Friction angle (f), is the measure of the shear strength of soils due to friction.

Internal friction refers to the resistance to deformation that occurs when soil particles try to slide past one another. The angle of internal friction is denoted as 'f' and is a measure of how effectively the soil can resist this movement under shear stress. Higher angles of internal friction indicate a greater resistance to sliding. Factors that affect this angle include the size and shape of the soil particles; rough, angular particles can interlock more effectively than smooth, round ones.

Consider a pile of rocks on a slope. If the rocks are jagged and interlock well, the slope is stable and less likely to slide down. Now imagine replacing those rocks with smooth pebbles; they can easily slide past each other, making the slope more prone to failure. This illustrates how internal friction affects the shear strength of soil.

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 loose material can remain in place without sliding. This angle varies based on the size and shape of the particles in the material; larger or more angular particles tend to form a steeper slope because they interlock better than smaller or rounder particles. The higher the overall shear strength of the material, the greater the angle of repose.

Picture a pile of sand versus a pile of gravel. If you were to shovel both into a heap, you'd notice that the sand (fine particles) can't form a steep angle and tends to create a more gentle slope. Gravel, being larger and more angular, can form a much steeper pile without sliding down. This visual can help you understand how particle characteristics influence 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).

In soil mechanics, stresses are forces that act on soil particles, critical for understanding how soil behaves under loads. Normal stress compresses the soil, potentially leading to changes in volume, while shear stress causes particles to slide over one another, which can lead to failure. The balance of these two stresses helps engineers determine how much weight the soil can support safely before failure occurs.

Imagine a stack of books. When you add more books on top, you're applying a normal stress that compresses those below, potentially causing them to buckle if the load is too heavy. Now, imagine pushing the stack sideways. That motion introduces shear stress, which could cause the books to slide off each other. Understanding these stresses helps us figure out what’s happening inside the ground when we build on it.

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