Pore water pressure refers to the pressure exerted by groundwater trapped in the soil or rock, in spaces between the aggregate called pores. The pressure of the pore water below the water table is measured by piezometers. Generally the vertical pressure can be assumed distributed in a similar way to the hydrostatic pressure.

The pore water pressure is fundamental in calculating the stress state of the soil, using the Terzaghi expression for the effective stress.

The buoyancy effects of water have an important effect on certain soil properties such as the effective stress present at any point in a medium soil. Let us consider an arbitrary point 5 meters below the ground surface. In a dry soil, the particles would be subject to a total stress at the depth at which they are found multiplied by the specific weight of the soil. However, when the water table falls by those 5 meters, the pressure between particles decreases as much as the pressure exerted by the water, which is multiplied by the own weight of the water and by the height of the water table measured from the point. The difference between total pressure and water pressure is called effective tension:.

Usually, for simple examples.

u in this case would be the pore water pressure.