AFrench drain[1] (also known by other names includingtrench drain,blind drain,[1]rubble drain,[1] androck drain[1]) is atrench filled withgravel orrock, or both, with or without a perforated pipe that redirectssurface water andgroundwater away from an area. The perforated pipe is called aweeping tile (also called adrain tile orperimeter tile[2]). When the pipe is draining, it "weeps", or exudes liquids. It was named during a time period when drainpipes were made fromterracotta tiles.
French drains are primarily used to prevent ground and surface water from penetrating or damagingbuilding 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 aseptic drain field at the outlet of a typicalseptic tank sewage treatment system. French drains are also used behindretaining walls to relieve ground water pressure.
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 inFrance,[3] butHenry Flagg French (1813–1885) ofConcord, Massachusetts, a lawyer andAssistant U.S. Treasury Secretary, described and popularized them[4] inFarm Drainage (1859).[5] French's own drains were made of sections of ordinary roofing tile that were laid with a1⁄8 in (0.32 cm) gap in between the sections to admit water. Later, specialized 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 surroundingsoil. 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 ofgeotextiles greatly simplified this technique.
Subsurface drainage systems have been used for centuries. They have many forms that are similar in design and function to the traditional French drain.
Ditches are dug manually or by atrencher. 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, for example weeping tile surrounded by sand or gravel, andgeotextile or landscaping textile. Landscaping textiles prevent migration of the drainage material and preventsoil 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.
Variations of French drains include:
A French drain can end, i.e., open at a downhill slope,dry well, orrain garden where plants absorb and hold the drained water. This is useful if city water systems or other wastewater areas are unavailable.
Depending on the expected level and volume of rainwater or runoff, French drains can be widened or also fitted on two or three underground drainpipes. Multiple pipes also provide for redundancy, in case one pipe becomes overfilled or clogged by a rupture or defect in the piping. A pipe might become overfilled if it is on a side of the drain which receives a much larger volume of water, such as one pipe being closer to an uphill slope, or closer to a roofline that drips near the French drain. When a pipe becomes overfilled, water can seep sideways into a parallel pipe, as a form of load-balancing, so that neither pipe becomes slowed by air bubbles, as might happen in a full-pipe with no upper air space.
Filters are made from permeable materials, typicallynon-woven fabric, may include sand and gravel, placed around the drainage pipe or envelope to restrict migration ofnon-colloidal particles[clarification needed] from the surrounding soils. Envelopes are the gravel, stone, rock, or surrounding pipe. These are permeable materials placed around pipe or drainage products to improve flow conditions in the area immediately around the drain and for improving bedding and structural backfill conditions.
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French drains are often installed around a home foundation in two ways:
In most homes, an external French drain or drain tile is installed around the foundation walls before the soil is backfilled. It is laid on the bottom of the excavated area, and a layer of stone is laid on top. A filter fabric is often laid on top of the stone to keep fine sediments and particles from entering. Once the drain is installed, the area is backfilled, and the system is left alone until it clogs.[7]
French drains can be used in farmers' fields for thetile drainage of waterlogged fields.[2] Such fields are called "tiled". Weeping tiles can be used anywhere that soil needs to be drained.
Weeping tiles are used for the opposite reason inseptic drain fields forseptic tanks. Clarifiedsewage from the septic tank is fed into weeping tiles buried shallowly in the drain field. The weeping tile spreads the liquid throughout the drain field.
In the US, municipalities may require permits for building drainage systems asfederal law requires water sent tostorm drains to be free of specific contaminants and sediment.[8]
In the UK, local authorities may have specific requirements for the outfall of a French drain into a ditch orwatercourse.