A French drain is a trench filled with rock or gravel which can hold a perforated pipe. The perforated hollow pipes along the bottom quickly vent water that seeps down through the gravel or rock bed.
French drains are primarily used to prevent ground and surface water from penetrating or damaging building foundations but can also be used to distribute water, such as a septic drain field at the outlet of a typical septic tank sewage treatment system. French drains can also used behind retaining walls to relieve ground water pressure.
