1. Stresses in the Ground

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 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.

Total stress in the ground refers to the overall pressure exerted at a particular point in the soil due to the weight of everything situated above it. This includes the weight of the soil itself, water, and any surface loads like buildings or vehicles. As you go deeper into the ground, the weight of materials above increases, leading to greater total stress. The formula for calculating this vertical total stress at depth 'z' is given as s = g.Z, where 'g' is the unit weight of the soil. When the point is below a water body, the total stress comprises the weight of the soil and the weight of the water above it. This relationship is crucial in geotechnical engineering to ensure structures can safely withstand the stresses beneath them.

Imagine standing on a pile of books. The pressure you feel on your feet from the books is similar to total stress. The more books piled on top, the greater the pressure on your feet. In the ground, total stress works the same way—it gets heavier the more soil and water you have above a certain point.

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. Under hydrostatic conditions, no water flow takes place, and the pore pressure at a given point is given by u = γ.h, where h = depth below water table or overlying water surface.

Pore water pressure refers to the pressure exerted by water within the soil's pores. This pressure plays a critical role in determining soil behavior, especially underground. The amount of pore water pressure is influenced by how deep you go below the water table, which is the level below which the soil is saturated with water, and by the conditions of water flow through the soil. When water is not flowing (hydrostatic conditions), pore water pressure can be calculated using the formula u = γ.h, where 'γ' is the unit weight of water and 'h' is the depth of soil column water above the point in question.

Think of pore water pressure like the pressure you feel when you dive into a swimming pool. The deeper you go, the more water is above you, and thus the greater the pressure you feel. Similarly, as you go deeper in the ground, the water pressure increases due to the weight of water above.

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.
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.

The effective stress principle is a fundamental concept in soil mechanics introduced by Karl Terzaghi. It states that the effective stress within a soil mass determines how the soil behaves under load. The effective stress (c3') is calculated by subtracting pore water pressure (u) from the total stress (c3) at that point: σ' = σ - u. This principle indicates that any changes in the mechanical properties of soil, such as compression and strength, result solely from changes in effective stress. It is critical for understanding soil stability and behavior, especially in saturated conditions.

Imagine trying to compress a sponge soaked in water. The water inside the sponge pushes against the walls of the sponge while you are pressing down. The effective stress would be the pressure you exert minus the pressure of the water pushing back up. In soil terms, the effective stress translates into how well the soil can bear loads after accounting for any water pressure.

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. In coarse soils, the capillary rise is very small.

Capillary rise refers to the phenomenon where water moves upwards against gravity in saturated soils above the water table. This occurs because of the adhesion of water molecules to soil particles and the surface tension of the water. The height to which water can rise is influenced by the size of the soil particles; finer soils can achieve greater capillary rise than coarser soils, which typically exhibit very limited capillarity. When soil particles are larger, the effect of gravity become dominant, and thus the capillary rise is minimal.

Think of a paper towel dipped in water. The towel absorbs water and draws it up through its fibers even when the lower end is submerged in a bowl of water. The water climbs up into the towel due to capillarity. Similarly, in fine soils, water can rise up through the tiny spaces between particles, but this effect is weaker in sandy soils.

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