1.2. Pore Water Pressure

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 the pressure exerted by water that fills the spaces, or pores, within soil. Its value is influenced by how deep the water table is beneath the ground surface, as well as whether water is moving through the soil (seepage flow). The deeper the water table, the higher the pore water pressure because of the weight of the water above.

Think of a sponge sitting in water. The deeper the sponge is submerged, the more water fills its pores, and thus the more pressure is felt at the bottom of the sponge due to the weight of the water above.

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.

Hydrostatic conditions refer to a state where water is static and not flowing. Under these conditions, the pore water pressure at any point within the soil can be calculated using the formula u = γ.h, where γ is the weight of water per unit volume, and h is the depth of the water above that point. This relationship helps us to understand how pressure varies with depth in the soil.

Imagine a sealed tube filled with water. The pressure you feel at the bottom of the tube increases as you go deeper because of the added weight of the water above. Similarly, in soil, as you go deeper, the pore water pressure increases due to the weight of the water above.

The natural level of ground water is called the water table or the phreatic surface. Under conditions of no seepage flow, the water table is horizontal. The magnitude of the pore water pressure at the water table is zero. Below the water table, pore water pressures are positive.

The water table is the boundary between saturated soil and unsaturated soil. It represents the level at which the soil is fully saturated with water. At the water table itself, the pore water pressure is zero because water pressure exactly balances the weight of the water column above it. Below this level, the pore water pressure becomes positive as the weight of the water increases.

If you think of a glass of water filled to the brim, the water level is like the water table. If you were to dive below the surface, you would feel the pressure from the water above you; this is akin to the positive pore water pressure found in saturated soil below the water table.

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

Both the total stress and pore water pressure can be measured at any point.

The principle of effective stress is fundamental in soil mechanics and was established by Karl Terzaghi. It states that the effective stress within a soil mass can be calculated by subtracting pore water pressure from total stress. This concept is critical because it shows how the stresses contribute to soil behavior, particularly in saturated soils.

Consider a balloon filled with water tied to a weight. The weight represents the total stress acting downwards due to gravity. The water inside the balloon exerts an upward pressure (pore water pressure). The net effect of the weight (total stress) minus the upward pressure (pore water pressure) determines how much the balloon can stretch or compress. This net pressure is the effective stress.

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 a load is applied to saturated soil, the total stress increases, which leads to an initial increase in pore water pressure. This pressure increase temporarily pushes water out of the soil’s pores. As water drains, the solid grains begin to carry more of the load, thereby increasing the effective stress, which affects how compact and stable the soil becomes.

Imagine a sponge that you are weighing down with a heavy object. At first, the sponge can't absorb the extra weight all at once, and the water quickly tries to escape from the pores. As some water squeezes out, the sponge becomes firmer and can support the weight better, similar to how soil reacts when stressed.

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 partially saturated soils, which lie above the water table, the pore water pressure can actually be negative. This occurs because air is also present in the pore spaces, and the effective suction forces can create a negative pore pressure. Capillary rise refers to how high water can move upward through the soil against gravity, influenced by the size of soil grains—smaller grains can draw water higher than larger grains.

Think of a straw placed in a glass of water. If you suck on the straw, the water rises due to the difference in pressure created inside the straw. Similarly, in soils, smaller particles can create a suction effect that allows water to rise above the water table.

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