What happens to the effective stress at any point below the groundwater table if the groundwater table rises?

The effective stress at any point below the groundwater table is a critical concept in soil mechanics and geotechnical engineering. Effective stress is defined as the stress that contributes to the strength of the soil and is calculated as the total stress minus the pore water pressure.

When the groundwater table rises, the pore water pressure in the soil increases because the additional water adds pressure to the voids in the soil matrix. As a result, even if the total stress (from the weight of the soil above) remains constant, the increase in pore water pressure reduces the effective stress. The relationship can be expressed mathematically:

Effective Stress ( \sigma' = \sigma - u )

Where ( \sigma ) is the total stress, and ( u ) is the pore water pressure. As the groundwater table rises, ( u ) increases, leading to a decrease in the effective stress ( \sigma' ).

While surface loads can also influence effective stress, they are not a necessary condition for the decrease in effective stress caused by the rise in groundwater table. Thus, the key factor is the increase in pore water pressure due to the rising water level, which directly results in a decrease in effective stress in the soil at points below the groundwater table.