1.1. Total Stress

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 refers to the overall pressure that exists at a certain depth in the soil. This stress comes from all the weight that is above that specific point. When you think about the earth, it’s not just one layer but multiple layers of soil and water. Therefore, when a load is applied, such as from a building or a pile of earth, both the solid particles and the water within the soil pores help to bear that load. As we go deeper into the soil, the weight increases, leading to higher total stress.

Imagine stacking books on a table. The books on top add weight to those below. If you place a stack of heavier books on the table, the pressure they exert is similar to the total stress experienced by soil. The deeper into the stack you go, the more pressure you'll feel from all the books above.

Total stress thus increases with depth and with unit weight. Vertical total stress at depth z, s = g.Z Below a water body, the total stress is the sum of the weight of the soil up to the surface and the weight of water above this. s = g.Z + g .Z The total stress may also be denoted by s or just s. It varies with changes in water level and with excavation.

The equation for total stress helps us calculate how much pressure is being exerted at a specific depth (denoted as 'z'). The variable 'g' represents the unit weight of the soil or water. As depth increases, or when you’re dealing with heavier materials, the total stress increases accordingly. For example, if you’re underwater, you must also account for the weight of the water above the point of interest, which adds to the overall stress. Changes in water level or excavation can affect this total stress, showing how dynamic soil conditions can be.

Think of a diver going deeper into a swimming pool. The deeper the diver goes, the more pressure they feel from the water above them. Similarly, as you go deeper into soil, the added weight of the soil and water contributes to the total stress at that depth.

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 is a critical component of total stress. It reflects the pressure exerted by water in the tiny spaces between soil particles. This pressure increases with depth since the column of water above any given point increases the force on it. Conditions like seepage flow, which is the movement of water through the soil, can also affect this pressure. Understanding pore water pressure is essential for predicting how soil will behave under different conditions.

Consider a sponge submerged in water. The deeper the sponge is in the water, the more water it absorbs, leading to higher pressure within its pores. In a similar fashion, soil below the water table experiences increased pore water pressure as more water fills the voids between soil particles.

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
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.

Karl Terzaghi's principle of effective stress revolutionized our understanding of soil mechanics. It indicates that the stress carried by the soil skeleton (the solid particles) can be calculated by subtracting the pore water pressure from the total stress. Changes in soil stress impact how soil behaves, especially in terms of its ability to resist sliding or deformation. This principle is crucial for engineers when designing foundations and other structures to ensure stability and safety.

Imagine trying to hold a heavy box underwater. The box feels lighter because of the buoyancy created by the water. In soil, effective stress is the load that the soil structure can bear, taking into account the lifting effect of the water pressure in its pores.

In a saturated soil system, as the voids are completely filled with water, the pore water pressure acts equally in all directions. The effective stress is not the exact contact stress between particles but the distribution of load carried by the soil particles over the area considered. It cannot be measured and can only be computed.

In saturated soils, the presence of water in the pores impacts stress distribution. While pore water pressure acts uniformly, effective stress represents how much load the soil can withstand. This is not an exact measure but rather a calculation that helps engineers understand and predict behavior under loads. Although it cannot be directly measured, it is essential for understanding soil stability.

Think about a trampoline. When you jump on it, the way the fabric stretches and distributes your weight is similar to how effective stress operates. The loads (your weight) are distributed across the surface, even though each point on the trampoline fabric feels the pressure from above.

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