Pore water pressure refers to the pressure of groundwater

Groundwater

Groundwater is water located beneath the ground surface in soil pore spaces and in the fractures of rock formations. A unit of rock or an unconsolidated deposit is called an aquifer when it can yield a usable quantity of water. The depth at which soil pore spaces or fractures and voids in rock...

 held within a soil

Soil

Soil is a natural body consisting of layers of mineral constituents of variable thicknesses, which differ from the parent materials in their morphological, physical, chemical, and mineralogical characteristics...

 or rock

Rock (geology)

In geology, rock or stone is a naturally occurring solid aggregate of minerals and/or mineraloids.The Earth's outer solid layer, the lithosphere, is made of rock. In general rocks are of three types, namely, igneous, sedimentary, and metamorphic...

, in gaps between particles (pores). Pore water pressures in below the phreatic

Phreatic

The term phreatic is used in Earth sciences to refer to matters relating to ground water below the water table . The term 'phreatic surface' indicates the location where the pore water pressure is under atmospheric conditions...

 level (see also groundwater

Groundwater

Groundwater is water located beneath the ground surface in soil pore spaces and in the fractures of rock formations. A unit of rock or an unconsolidated deposit is called an aquifer when it can yield a usable quantity of water. The depth at which soil pore spaces or fractures and voids in rock...

) are measured in piezometer

Piezometer

A piezometer is either a device used to measure static liquid pressure in a system by measuring the height to which a column of the liquid rises against gravity, or a device which measures the pressure of groundwater at a specific point...

s. The vertical pore water pressure distribution in aquifer

Aquifer

An aquifer is a wet underground layer of water-bearing permeable rock or unconsolidated materials from which groundwater can be usefully extracted using a water well. The study of water flow in aquifers and the characterization of aquifers is called hydrogeology...

s can generally be assumed to be close to hydrostatic.

In the unsaturated zone the pore pressure is determined by capillarity

Capillary action

Capillary action, or capilarity, is the ability of a liquid to flow against gravity where liquid spontanously rise in a narrow space such as between the hair of a paint-brush, in a thin tube, or in porous material such as paper or in some non-porous material such as liquified carbon fiber, or in a...

 and is also referred to as tension

Tension

Tension may refer to:* The Void , also known as Tension in some regions* Tension , a factor that affects knitting gauge...

, suction

Suction

Suction is the flow of a fluid into a partial vacuum, or region of low pressure. The pressure gradient between this region and the ambient pressure will propel matter toward the low pressure area. Suction is popularly thought of as an attractive effect, which is incorrect since vacuums do not...

 or matric pressure. Pore water pressures under unsaturated conditions (vadose zone

Vadose zone

The vadose zone, also termed the unsaturated zone, is the portion of Earth between the land surface and the top of the phreatic zone i.e. the position at which the groundwater is at atmospheric pressure . Hence the vadose zone extends from the top of the ground surface to the water table...

) 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

Soil mechanics

Soil mechanics is a branch of engineering mechanics that describes the behavior of soils. It differs from fluid mechanics and solid mechanics in the sense that soils consist of a heterogeneous mixture of fluids and particles but soil may also contain organic solids, liquids, and gasses and other...

, from Terzaghi

Karl von Terzaghi

Karl von Terzaghi was an Austrian civil engineer and geologist, called the father of soil mechanics.-Early life:...

's expression for the effective stress

Effective stress

Karl von Terzaghi first proposed the relationship for effective stress in 1936. For him, the term ‘effective’ meant the calculated stress that was effective in moving soil, or causing displacements...

 of a soil .

ISO 11276:1995

BS

BS

BS, B.S., Bs or bs may refer to:-Academics:*Bachelor of Science degree, also abbreviated BSc*Bachelor of Surgery*Behavioural sciences-Standards:*Venezuelan bolívar's currency sign*Bolivian boliviano's currency sign...

 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 sum of matric and pneumatic pressures

Matric pressure

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.

Pneumatic pressure

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.