Pore water pressure

Pore water pressure is the pressure exerted by water within the voids of soil or rock. It affects how much stress the solid grains carry and how readily the material deforms or fails.

Effective stress: The stress carried by the solid skeleton of a saturated porous material after pore pressure is subtracted from total stress. Pore pressure reduces the stress carried between grains, directly changing strength and deformation.
Piezometer: An instrument that measures groundwater pressure or hydraulic head at a particular point. Piezometers provide direct measurements of pore pressure in soil and rock.
Total stress: The stress at a point from all applied loads, including the weight of overlying material. Effective stress is calculated by subtracting pore pressure from total stress.
Quick clay: Sensitive marine clay that can lose much of its strength when disturbed or remolded. Changes in pore pressure and soil structure contribute to its sudden flow failures.
Darcy's law: A law relating fluid flow through a porous medium to hydraulic gradient, permeability, and fluid viscosity. Pressure differences drive the seepage that Darcy's law quantifies.
Slope stability: The assessment of whether a natural or engineered slope can resist sliding or collapse. Rising pore pressure can lower effective stress and trigger slope failure.
Pore space: The connected or isolated voids between and within grains in a porous material. Water occupies pore space, where its pressure acts on the surrounding solid skeleton.
The 1964 Alaska earthquake: A magnitude 9.2 earthquake that struck southern Alaska on March 27, 1964. Earthquake shaking generated excess pore pressure and widespread ground failures.
Hydraulic head: The mechanical energy per unit weight of groundwater, expressed as an equivalent height of water. Pressure head is one component of hydraulic head and helps determine groundwater flow.
Consolidation (soil mechanics): The gradual reduction in saturated soil volume as excess pore water drains under applied load. Dissipation of excess pore pressure controls the rate of settlement.

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