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Concept
Pore water pressure
Summary
Pore water pressure (sometimes abbreviated to pwp) refers to the pressure of groundwater held within a soil or rock, in gaps between particles (pores). Pore water pressures below the phreatic level of the groundwater are measured with piezometers. The vertical pore water pressure distribution in aquifers can generally be assumed to be close to hydrostatic.
In the unsaturated ("vadose") 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 are measured with tensiometers, which operate by allowing the pore water to come into equilibrium with a reference pressure indicator through a permeable ceramic cup placed in contact with the soil.
Pore water pressure is vital in calculating the stress state in the ground soil mechanics, from Terzaghi's expression for the effective stress of the soil.
Pressure develops due to:
Water elevation difference: water flowing from a higher elevation to a lower elevation and causing a velocity head, or with water flow, as exemplified in Bernoulli's energy equations.
Hydrostatic water pressure: resulting from the weight of material above the point measured.
Osmotic pressure: inhomogeneous aggregation of ion concentrations, which causes a force in water particles as they attract by the molecular laws of attraction.
Absorption pressure: attraction of surrounding soil particles to one another by adsorbed water films.
Matric suction: the defining trait of unsaturated soil, this term corresponds to the pressure dry soil exerts on the surrounding material to equalise the moisture content in the overall block of soil and is defined as the difference between pore air pressure,, and pore water pressure, .
The buoyancy effects of water have a large impact on certain soil properties, such as the effective stress present at any point in a soil medium. Consider an arbitrary point five meters below the ground surface.
Official source
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