When the water table increases, what happens to the effective stress?

The effective stress is defined as the stress carried by the soil skeleton and is calculated as the total stress minus the pore water pressure. When the water table increases, the pore water pressure within the soil also increases because there is additional water exerting pressure in the voids of the soil.

As the pore water pressure increases, it directly affects the effective stress, which is given by the equation:

Effective Stress = Total Stress - Pore Water Pressure

When the pore water pressure increases due to a rising water table, the term for pore water pressure in the equation becomes larger. Since the total stress (also known as the overburden pressure) generally remains constant in the short term, the increase in pore water pressure results in a decrease in effective stress. This decrease in effective stress can impact the strength and stability of the soil, making it a critical factor in geotechnical engineering and foundation design.

Therefore, an increase in the water table leads to a reduction in effective stress, which aligns with the understanding that effective stress is essential in assessing soil behavior under varying groundwater conditions.