History and Construction
The earliest forms of French drains were simple ditches, pitched from a high area to a lower one and filled with gravel. These were described and popularised by Henry Flagg French (1813–1885) a lawyer and Assistant U.S. Treasury Secretary from Concord, Massachusetts in his 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.
Ditches may be dug by hand or with 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 can be made with perforated pipe (weeping tile) surrounded by sand or gravel and geotextile or landscaping textile. Landscaping textiles are used to prevent migration of the drainage material as well as preventing dirt and roots from entering and clogging the drainage pipe. The perforated pipe provides a minor underground storage volume but the prime purpose is for the perforations to drain the area along the full length of the pipe and to discharge any surplus water at its end. The direction of percolation will depend on the relative conditions inside and outside the pipe.
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