How does capillary rise affect effective stress in soils?

Capillary rise is the phenomenon where water rises in a narrow tube against the force of gravity due to the adhesive forces between water molecules and the tube walls. In soils, capillary rise can have a significant impact on effective stress and soil behavior.

Effective stress is defined as the difference between the total stress acting on a soil and the pore water pressure. The total stress is the weight of the soil and any external loads, while the pore water pressure is the pressure exerted by the water in the soil pores.

When capillary rise occurs, the water in the soil pores is pulled upwards by the adhesive forces between water molecules and the soil particles. This creates a negative pore water pressure, meaning that the pore water pressure is less than atmospheric pressure.

The negative pore water pressure due to capillary rise has a number of effects on effective stress and soil behavior.

1. Increased Shear Strength: The negative pore water pressure due to capillary rise increases the effective stress in the soil. This is because the negative pore water pressure acts to hold the soil particles together, increasing the soil's shear strength.

2. Improved Bearing Capacity: The increase in effective stress due to capillary rise improves the soil's bearing capacity. This is because the higher effective stress increases the soil's resistance to deformation under load.

3. Soil Consolidation: Capillary rise can also contribute to soil consolidation. When the soil dries out, the negative pore water pressure due to capillary rise increases, which increases the effective stress and causes the soil to consolidate.

In summary, capillary rise can have a significant impact on effective stress and soil behavior by creating a negative pore water pressure that increases the effective stress, resulting in increased shear strength, improved bearing capacity, and soil consolidation.