How can the water table influence soil stability?

The influence of the water table on soil stability primarily relates to its effect on pore water pressure and effective stress. The water table represents the upper level of groundwater, and its depth can significantly affect the stress balance within soil layers.

When the water table rises, the pore water pressure in the soil increases. This increase in pore water pressure reduces the effective stress, which is the stress that contributes to the soil skeleton's ability to resist shear forces. Effective stress is calculated as the difference between the total stress acting on the soil and the pore water pressure. When effective stress decreases, the soil becomes more susceptible to failure or instability, particularly in saturated conditions.

Conversely, when the water table drops, the pore water pressure decreases, leading to an increase in effective stress, which generally enhances soil stability. Thus, understanding the relationship between the water table, pore water pressure, and effective stress is critical for assessing soil stability and potential failure mechanisms in geotechnical engineering.