Pore Water Pressure

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.

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. In dry soil, particles at this point experience a total overhead stress equal to the depth underground (5 meters), multiplied by the specific weight of the soil. However, when the local water table height is within said five meters, the total stress felt five meters below surface is decreased by the product of the height of the water table in to the five meter area, and the specific weight of water, 9.81 kN/m^2. This parameter is called the effective stress of the soil, basically equal to the difference in a soil’s total stress and pore water pressure. The Pore water pressure is essential in differentiating a soil’s total stress from its effective stress. A correct representation of stress in soil is necessary for accurate field calculations in a variety of engineering trades.

JK Mitchell of the University of California, Berkeley reports that pore water pressure develops as a result of four scientific phenomena: Water elevation difference, hydrostatic water pressure, osmotic pressure, and absorption pressure. It is a fundamental notion in fluid mechanics that water flows from higher elevation to lower elevation. This elevation difference causes a velocity head, or with water flow, as exemplified in the Bernoulli’s energy equation. Hydrostatic water pressure exists for any water body, as the water particles exert force in all directions due to applied pressures along with self-weight. A congregation of water is unlikely to be homogeneous in terms of ion concentration. This variance also causes a force in water particles as they attract by the molecular laws of attractions. The last factor affecting the development of pore water pressure is the absorption pressure of water and surrounding soil particles.

At any point above the water table, in the vadose zone, the effective stress is approximately equal to the total stress, as proven by Terzaghi’s principle. Realistically, the effective stress is greater than the total stress, as the pore water pressure in these partially saturated soils is actually negative. This is primarily due to the surface tension of pore water in voids throughout the vadose zone causing a suction effect on surrounding particles. This capillary action is the “upward movement of water through the vadose zone” (Coduto, 266). Capillary effects in soil are more complex than in free water due to the randomly connected void space and particle interference through which to flow; regardless, the height of this zone of capillary rise, where negative pore water pressure is generally peaks, can be closely approximated by a simple equation. The height of capillary rise is inversely proportional to the diameter of void space in contact with water. Therefore, the smaller the void space, the higher water will rise due to tension forces. Sandy soils consist of more coarse material with more room for voids, and therefore tends to have a much shallower capillary zone than do more cohesive soils, such as clays and silts.