Subsoil drains are used to improve the run off of surface water and the drainage of ground water to maintain the water table at some depth below the surface for the following reasons:
(1) to improve the stability of the ground
(2) to avoid surface flooding
(3) to alleviate or avoid dampness in basements
(4) to reduce humidity in the immediate vicinity of buildings.
Ground water, or land or field, drains are either open jointed or jointed, porous or perforated pipes of clayware, concrete, pitch fibre or plastic (see Volume 5). The pipes are laid in trenches to follow the fall of the ground, generally with branch drains discharging to a ditch, stream or drain.
On impervious subsoils, such as clay, it may be necessary to form a system of drains to improve the run off of surface water and drain subsoil to prevent flooding. Some of the drain systems used are natural, herring bone, grid, fan and moat or cut-off.