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1 <br />BMP C205: Subsurface Drains <br />Purpose <br />To intercept, collect, and convey ground water to a satisfactory outlet, using a perforated <br />pipe or conduit below the ground surface. Subsurface drains are also known as "french <br />drains." The perforated pipe provides a dewatering mechanism to drain excessively wet <br />soils, provide a stable base for construction, improve stability of structures with shallow <br />foundations, or to reduce hydrostatic pressure to improve slope stability. <br />Conditions of Use <br />Use when excessive water must be removed from the soil. The soil permeability, depth <br />to water table and impervious layers are all factors which may govern the use of sub- <br />surface drains. <br />Design and Installation Specifications <br />Relief drains are used either to lower the water table in large, relatively flat areas, <br />improve the growth of vegetation, or to remove surface water. <br />Relief drains are installed along a slope and drain in the direction of the slope. <br />They can be installed in a grid pattern, a herringbone pattern, or a random pattern. <br />Interceptor drains are used to remove excess groundwater from a slope, stabilize <br />steep slopes, and lower the water table immediately below -a slope to prevent the <br />soil from becoming saturated. <br />Interceptor drains are installed perpendicular to a slope and drain to the side of the <br />slope. <br />They usually consist of a single pipe or series of single pipes instead of a patterned lay- <br />out. <br />. Depth and spacing of interceptor drains -The depth of an interceptor drain is <br />determined primarily by the depth to which the water table is to be lowered or the <br />depth to a confining layer. For practical reasons, the maximum depth is usually lim- <br />ited to 6 feet, with a minimum cover of 2 feet to protect the conduit. <br />• The soil should have depth and sufficient permeability to permit installation of an <br />effective drainage system at a depth of 2 to 6 feet. <br />• An adequate outlet for the drainage system must be available either by gravity or <br />by pumping. <br />• The quantity and quality of discharge needs to be accounted for in the receiving <br />stream (additional detention may be required). <br />• This standard does not apply to subsurface drains for building foundations or deep <br />2414 Stormwater Management Manual for Western Washington <br />I <br />1 <br />1 <br />1 <br />1 <br />�L <br />L <br />1 <br />1 <br />1 <br />1 <br />1 <br />1 <br />1 <br />Volume 11 - Chapter 4 - Page 346 <br />1 <br />