1.3.1. First Part of Effective Stress Principle

When a load is applied to soil, it is carried by the solid grains and the water in the pores. The total vertical stress acting at a point below the ground surface is due to the weight of everything that lies above, including soil, water, and surface loading. Total stress thus increases with depth and with unit weight. Vertical total stress at depth z, s = g.Z.

Total stress is the cumulative force per unit area at a specific location in the soil. It accounts for the weight of all materials above that point, like soil layers, water, and any additional loads on the surface. The formula, s = g.Z, shows that total stress increases with depth because more weight is exerted by the materials above as you go deeper into the ground. Here, 'g' represents the unit weight of the soil.

Think of total stress like stacking books on a table. The more books you add, the heavier the top book becomes due to the weight of all the books beneath it. Just like the book, the soil sees increased pressure with each layer added on top.

The pressure of water in the pores of the soil is called pore water pressure (u). The magnitude of pore water pressure depends on:

• the depth below the water table.
• the conditions of seepage flow.

Pore water pressure refers to the pressure exerted by water within the soil's pore spaces. This pressure is determined by how deep you are below the water table and the flow conditions of the water. As you go deeper, the water pressure increases because there is more water above pushing down. This pressure impacts the soil's strength and stability.

Imagine a sponge submerged in water. The deeper the sponge is, the more water pushes on it from above, increasing the water pressure inside its pores. This is similar to how pore water pressure works in soil.

The principle of effective stress was enunciated by Karl Terzaghi in the year 1936. This principle is valid only for saturated soils and consists of two parts:

1. At any point in a soil mass, the effective stress (represented by σ') is related to total stress (σ) and pore water pressure (u) as σ' = σ - u.

Karl Terzaghi introduced the effective stress principle, which states that the stress that really affects soil behavior is called effective stress (σ'). It's the total stress (σ) minus pore water pressure (u). This relationship shows that when water is present in the soil, it counteracts some of the stress the soil bears. Understanding this principle is crucial for analyzing potential soil failures.

Consider a water balloon as a soil mass. The pressure you feel when squeezing the balloon represents the effective stress. If you push down harder (increasing total stress), but there’s too much water inside (increasing pore water pressure), the effective feeling of pressure against your hand is reduced. The water relieves some of the stress needed to maintain shape.

All measurable effects of a change of stress, such as compression and a change of shearing resistance, are exclusively due to changes in effective stress.

Any changes that occur in the soil's behavior, like how it compresses or its ability to resist sliding (shear strength), are solely based on changes in effective stress. If effective stress increases, the soil becomes stronger and more stable, while a decrease can lead to instability.

Imagine a layer of marshmallows resting on top of one another. If someone adds weight on top (increasing total stress) but the marshmallows are lightly compressed (meaning effective stress is also high), they hold their shape. However, if the weight is too much and the marshmallows become squished (representing a drop in effective stress), they might collapse, illustrating how effective stress influences stability.

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