What is a perched water table?
A perched water table (Dutch: schijnwaterstand or schijnwaterspiegel) is a body of groundwater held up on a layer of low permeability above the main (phreatic) water table. It is caused by a local feature in the soil profile: clay, loam or a dense hardpan, such as the iron pan in a podzol, stops the water from draining downwards. Between the perched water and the true water table there is unsaturated ground.
A perched water table can be temporary, for example after heavy rain, or more or less permanent, as with some heathland pools whose water level sits above the surrounding groundwater because of an impermeable layer.
Application
Perched water matters in construction because it causes problems you would not expect from the normal groundwater level:
- Wet basements and crawl spaces: water trapped on a clay layer can press against basement walls, even where maps show a deep water table.
- Soakaways that fail: a soakaway or infiltration crate in or above a low-permeability layer fills up and stays full. Have a percolation test carried out first (in the UK, BRE Digest 365 describes the standard soakaway test).
- Waterlogged gardens: puddles that linger for days often point to an impeding layer below.
- Excavations: water can flow unexpectedly into trenches from a perched water table.
Tips:
- Hand auger: a simple auger hole shows whether there is a clay or loam layer in the topsoil.
- Break through: a thin impeding layer can sometimes be broken by deep digging or subsoiling so the water can drain away.
- Drainage: where that is not possible, land drains can take the water to a ditch or sewer (check local rules).
- Basements: design the waterproofing for the highest expected water level, including any perched water.
Related Terms
- Water table
- Groundwater level
- Seepage
- Ground investigation
- Drainage
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