Pore water pressure refers to the pressure of groundwater held within a soil or rock, in gaps between particles (pores). Pore water pressures in below the phreatic level (see also groundwater) are measured in piezometers. The vertical pore water pressure distribution in aquifers can generally be assumed to be close to hydrostatic.
In the unsaturated zone the pore pressure is determined by capillarity and is also referred to as tension, suction or matric pressure. Pore water pressures under unsaturated conditions (vadose zone) are measured in with tensiometers. Tensiometers operate by allowing the pore water to come into equlibrium with a reference pressure indicator through a permeable ceramic cup placed in contact with the soil
Pore water pressure (sometimes abbreviated to pwp) is vital in calculating the stress state in the ground soil mechanics, from Terzaghi's expression for the effective stress of a soil .
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BS 7755-5.1: 1996 Soil quality — Determination of pore water pressure — Tensiometer method, which is identical with ISO 11276:1995, defines pore water pressure as
The amount of work that must be done in order to transport reversibly and isothermally an infinitesimal quantity of water, identical in composition to the soil water, from a pool at the elevation and the external gas pressure of the point under consideration, to the soil water at the point under consideration, divided by the volume of water transported.[1]
The amount of work that must be done in order to transport reversibly and isothermally an infinitesimal quantity of water, identical in composition to the soil water, from a pool at atmospheric pressure and at the elevation of the point under consideration, to a similar pool at an external gas pressure of the point under consideration, divided by the volume of water transported.[1]