What Is Subsoil Drainage?

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Pipes, soil and a shovel.
Pipes, soil and a shovel. (Image: winnond/iStock/Getty Images)

The layer of earth below the top surface soil is called the subsoil. It lacks most of the organic elements and the rich soil flora and fauna contained in the surface soil, and is made up primarily of minerals and leached compounds. Clay accumulations can occur. Subsoils can contain moisture that could negatively interfere with ground stability, structures or horticulture. Install subsoil drainage systems to conduct moisture away from a given area.

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French Drains

French drains use gravel-filled trenches and clay roof tiles. The tiles are spaced apart from each other over the gravel to let water filter through from the subsoil into the gravel, which allows water flow into a collecting channel, which then carries the water away. This system was invented by the American Henry French, author of Farm Drainage, who lived from 1813 to 1885. French drains are still commonly used around homes and in landscaping situations such as behind retaining walls.

Polyethylene Fabric

Some drain systems use polyethylene or polypropylene fabric to line the trench. The fabric is permeable to moisture in just one direction, which prevents backflow into the trench and filters out silt. A common applications is for roadside trenches lined with fabric and filled with rubble. Such geotextiles can also cover perforated drainage pipes to prevent silt and other particulates from clogging the interior of the pipes.

Pipes Within Gravel or Rubble

Depending on how much of the subsoil needs water drainage, dig trenches of varying depths, fill the bottoms with gravel or rubble and install pipes within the rubble to more freely conduct moisture to an outflow. Unperforated pipes can be open-jointed or porous clay or porous concrete. Perforated pipes come in clay, concrete or unplasticized polyvinylchloride, called uPVC. The perforations are on the bottom of the pipe so the water rises into the pipe and is conducted along it.

Outflow

Pipe layouts are configured in patterns that suit the drainage needs, with common patterns being grid iron, herringbone and fan. Lateral pipes feed into a collecting pipe or main drain which conducts the subsoil water away from the site. If water is discharged onto another property or into public systems, permission or permits are required. For outflows that can cause erosion, erosion protection is required.

Installation

Subsoil drainage systems are best installed when the structure or landscaping feature is being installed. Subsoil drainage can be needed around paths or roadways, in raised garden beds, behind walls or around foundations. Subsoil drainage systems can require the expertise of a trained professional with experience in water table levels, slope of the land, subsoil analysis and local building regulations.

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