French drain
A French drain or weeping tile (also blind drain,rubble drain,rock drain,drain tile, perimeter drain, land drain, French ditch, sub-surface drain, sub-soil drain or agricultural drain) is a trench filled with gravel or rock or containing a perforated pipe that redirects surface water and groundwater away from an area. A French drain can have perforated hollow pipes along the bottom (see images) to quickly vent water that seeps down through the upper gravel or rock.
French drains are primarily used to prevent ground and surface water from penetrating or damaging building foundations. Alternatively, French drains may be used to distribute water, such as a septic drain field at the outlet of a typical septic tank sewage treatment system. French drains are also used behind retaining walls to relieve ground water pressure.
The earliest forms of French drains were simple ditches, pitched from a high area to a lower one and filled with gravel. These may have been invented in France but were described and popularised by Henry Flagg French (1813–1885) of Concord, Massachusetts, a lawyer and Assistant US Treasury Secretary, in his 1859 book Farm Drainage. French's own drains were made of sections of ordinary roofing tile laid with a 1⁄8 in (0.32 cm) gap left in between the sections to admit water. Later, specialised drain tiles were designed with perforations. To prevent clogging, the gravel size varied from coarse at the center to fine at the outside and was designed based on the gradation of the soil surrounding the drain. The particle sizing was critical to keep the surrounding soil from washing into the voids in the gravel and clogging the drain. The development of geotextiles greatly simplified this procedure.
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