Pore Water Pressure

Pore water pressure (u) is the pressure of the water filling the voids of a saturated soil. Because water has no shear strength, it does not contribute to the soil's strength directly; instead it reduces the effective stress (σ′ = σ − u) that the grain skeleton carries, and is therefore the controlling variable in stability and settlement. It has a static (hydrostatic) component from the water table and an 'excess' component generated when soil is loaded or unloaded faster than water can drain.

Excess pore pressure is central to time-dependent geotechnics: it builds up under rapid loading (embankment on soft clay), during pile driving and in earthquakes (the trigger for liquefaction), and dissipates over time as consolidation/settlement occurs. Managing it — via drainage, staged construction, preloading with vertical drains, or dewatering — is how soft-ground and excavation problems are solved. It is measured by piezometers and inferred in IS 2720 triaxial/consolidation testing, and is fundamental to earth-dam, embankment (IS 7894) and slope-stability design.