1.3. Effective Stress Principle

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 or σ' ) is related to total stress (σ) and pore water pressure (u) as = σ - u Both the total stress and pore water pressure can be measured at any point.

The effective stress principle, formulated by Karl Terzaghi, is a foundational concept in soil mechanics. It states that the stress carried by soil particles is affected by both the total stress (which includes the weight of the soil, water, and any additional loads) and the pore water pressure (the pressure exerted by water in the soil's pores). The effective stress can be calculated by subtracting the pore water pressure from the total stress. This relationship is only applicable in saturated soils, where all void spaces are filled with water.

Imagine a sponge (the soil) fully soaked in water. When you press down on the sponge, the water inside pushes outward (pore pressure), but the actual compressive force that can be transmitted through the sponge (effective stress) is the force remaining after accounting for the water pressure. If you squeeze the sponge and some water escapes, the sponge can then support more weight without collapsing, demonstrating how effective stress allows it to bear loads.

2. 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 change in stress applied to saturated soil results in changes to its effective stress, which in turn affects how the soil behaves. When the effective stress increases, the soil becomes more compact (compression occurs) and can resist greater shear forces (shear strength increases). Therefore, any observable changes in the soil's physical properties, such as compression and shear resistance, are solely due to the variations in effective stress.

Think of a packed suitcase. When you apply pressure on the suitcase (like adding more clothes), the items inside compress, making it easier to pack more. This pressure reflects the effective stress on the items. If you were to remove some clothes (reducing the pressure), it becomes less compact, and less pressure is needed to add more items. The same principle applies to soil under varying loads.

If the total stress is increased due to additional load applied to the soil, the pore water pressure initially increases to counteract the additional stress. This increase in pressure within the pores might cause water to drain out of the soil mass, and the load is transferred to the solid grains. This will lead to the increase of effective stress.

When additional load is applied to saturated soil, the total stress increases. In response, the pore water pressure rises as well because the water in the soil's pores resists the added load. Initially, this can cause the soil to behave less effectively under load due to increased pore pressure. However, if the water can drain out, the pressure will stabilize, allowing the solid grains of soil to carry more of the load, which increases the effective stress.

Consider a sponge sitting in a bucket of water. When you press down on the sponge, the water tries to push back, increasing pressure in the sponge (pore water pressure). If you continue to press down and the sponge allows some water to escape, the sponge can then support more weight effectively without losing its shape, similar to how effective stress works in soil.

Above the water table, when the soil is saturated, pore pressure will be negative (less than atmospheric). The height above the water table to which the soil is saturated is called the capillary rise, and this depends on the grain size and the size of pores.

Capillary rise occurs when water is drawn up into the soil above the water table due to surface tension and adhesion forces. The extent of this rise varies depending on the soil's grain size; finer soils (like clay) support greater capillary action compared to coarser soils (like sand), which have larger pore spaces and less effective capillary rise. Above the water table, the pore water pressure becomes negative as it resists atmospheric pressure.

Imagine a straw submerged in a cup of water. When you take the straw out, water tends to cling inside the straw due to surface tension (just like capillary action in soil). If the straw is thin, it holds more water compared to a thicker straw. This illustrates capillary rise's dependence on pore size: finer soils can hold more 'water' through this action than coarser ones can.

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