1.3.3. Capillary Rise and Partial Saturation

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 can move upwards through small pores or spaces in soil due to surface tension. This rise occurs above the water table, where soil is still saturated. In saturated soil, the water pressure often becomes negative, which indicates that the water in the soil is under tension, rather than simply sitting at atmospheric pressure. The actual height that water can rise in the soil depends on the size of soil particles and the size of the pores; finer soils can retain water better and thus have a higher capillary rise compared to coarser soils that have larger pores.

Imagine a sponge placed in water; when you pull the sponge out, water clings to the sponge and may rise up even higher than the water level it was in. This is similar to how water can rise in smaller soil pores through capillary action. Finer soils like clay have smaller pores and retain water more effectively, so they can exhibit a higher 'water rise' compared to sandy soils, which allow water to pass through quickly and don't hold much water above the water table.

Between the top of the saturated zone and the ground surface, the soil is partially saturated, with a consequent reduction in unit weight. The pore pressure in a partially saturated soil consists of two components: Pore water pressure = u_w, Pore air pressure = u_a. Water is incompressible, whereas air is compressible. The combined effect is a complex relationship involving partial pressures and the degree of saturation of the soil.

In the zone above the water table, the soil is not fully saturated but instead is partially saturated. This means that both water and air occupy the pore spaces. This mixture leads to a reduction in the soil's overall weight because not all the pores are filled with water, unlike in fully saturated soil. The forces in these partially saturated soils consist of pore water pressure (the pressure from the water in the pores) and pore air pressure (the pressure from the air occupying the remaining pore spaces). Understanding how water and air pressures interact is essential for grasping how soil behaves under various conditions, especially in terms of stability and strength.

Think of a slice of bread that has only absorbed a bit of water as opposed to being fully soaked. The bread can get a bit heavier, but it won't weigh as much as a completely soggy piece of bread. In this analogy, the 'pore water pressure' is like the moisture in the bread, while the 'pore air pressure' represents the air that remains in the bread's texture. This balance between air and water affects how the bread (or soil) holds together, its ability to support weight, and how it reacts to additional stresses.

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