Pore Water Pressure

Published: December 3, 2021 | Last updated: July 5, 2023

What Does Pore Water Pressure Mean?

Pore water pressure is the pressure experienced by water trapped in the voids in a saturated soil mass. This occurs due to either the location of the soil or external forces. The position of the groundwater table and flow-through seepage also influences the pore water pressure. In trenchless construction, understanding pore water pressure is important in the selection of the correct trenchless method and design of the pipeline.

The geotechnical investigation involves the calculation of various soil factors such as pore water pressure, effective stress and total stress and is, therefore, an essential part of a trenchless project. Piezometers are instruments installed in small diameter boreholes in construction sites to monitor the piezometric water level or pore water pressure.


Trenchlesspedia Explains Pore Water Pressure

Measuring pore water pressure using a piezometer works on the simple principle of a riser pipe with a perforated filter tip. In trenchless technology, the piezometer is used to perform tests to determine the feasibility of carrying out projects in difficult areas. Installing piezometers is especially important in offshore projects that require slope stability to prevent hazards. This method can discover sub-soil geohazards before beginning subsurface construction using trenchless technology.

Pore water pressure can be calculated using the formula:

u = γw × zw


  • u is the pore water pressure.
  • γw is the density of water.
  • zw is the depth of the water in the soil.

Seasonal variations can cause the water table to move up or down, resulting in a change in the pore water pressure. When the water level below ground rises, pore water pressure increases, causing a decrease in effective stresses. A dip in the water level, however, decreases pore water pressure, increasing the effective stress of the soil. The total stress and pore water pressure, therefore, depend on the level of the groundwater.


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