French drains

French drains are primarily used to prevent ground and surface water from penetrating or damaging building foundations and as an alternative to open ditches or storm sewers for streets and highways. 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.

History and construction

The earliest forms of French drains were simple ditches that were pitched from a high area to a lower one and filled with gravel. These may have been invented in France[2] but Henry Flagg French (1813–1885) of Concord, Massachusetts, a lawyer and Assistant US Treasury Secretary described and popularised them[3] in Farm Drainage (1859).[4] French's own drains were made of sections of ordinary roofing tile that were laid with a 1⁄8 in (0.32 cm) gap in between the sections to admit water. Later, specialised drain tiles were designed with perforations. To prevent clogging, the size of the gravel varied from coarse in the center to fine on the outside and was selected contingent on the gradation of the surrounding soil. The sizes of particles were critical to prevent the surrounding soil from washing into the pores, i. e., voids between the particles of gravel and thereby clogging the drain. The later development of geotextiles greatly simplified this technique. 

Ditches are dug manually or by a trencher. An inclination of 1 in 100 to 1 in 200 is typical. Lining the bottom of the ditch with clay or plastic pipe increases the volume of water that can flow through the drain. Modern French drain systems are made of perforated pipe, weeping tile surrounded by sand or gravel, and geotextile or landscaping textile. Landscaping textiles prevent migration of the drainage material and also prevent soil and roots from entering and clogging the pipe. The perforated pipe provides a minor subterranean volume of storage for water, yet the prime purpose is drainage of the area along the full length of the pipe via its perforations and to discharge any surplus water at its terminus. The direction of percolation depends on the relative conditions within and without the pipe. 

Subsurface drainage systems have been used commonly for centuries. They have many forms that are similar in design and function to the traditional French drain.