What is the phreatic surface?
The phreatic surface is the upper boundary of the phreatic (shallow, unconfined) groundwater, at the level where the water pressure in the ground is exactly equal to atmospheric pressure. Above this surface the soil is unsaturated; below it the pores are completely filled with water. In practice the phreatic surface coincides with the water table measured in an open standpipe.
Position and behaviour
The level of the phreatic surface is not constant. It rises after rainfall or during high river levels and falls in dry periods or through extraction. The surface slopes with the terrain and the flow towards ditches and rivers, and is influenced by upward seepage and infiltration.
Importance in construction
The phreatic surface is decisive when designing foundations, basements and underground structures:
- A high water table requires watertight basements and resistance to uplift (buoyancy).
- Excavating below the phreatic surface calls for dewatering to keep the building pit dry.
- Timber pile foundations that end up above the phreatic surface are prone to pile rot from oxygen exposure.
Related terms
- Water table
- Dewatering
- Seepage
- Foundation
- Pile rot
- Building pit
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