1710 W MARINE VIEW DR 2024-01-03

BMP C205: Subsurface Drains Purpose The purpose of subsurface drains is to intercept, collect, and convey ground water to a satisfactory outlet, using a perforated pipe or other conduit below the ground surface. Subsurface drains are also known as "french drains." The perforated pipe provides a dewatering mechanism to drain excess- ively wet soils, provide a stable base for construction, improve stability of structures with shallow foundations, or to reduce hydrostatic pressure to improve slope stability. Conditions of Use Use subsurface drains when excessive water must be removed from the soil. The soil permeability, depth to water table, and impervious layers are all factors which may govern the use of subsurface drains. Design and Installation Specifications Subsurface Drain Type: Relief Drains Relief drains are used to lower the water table in large, relatively flat areas, improve the growth of vegetation, or to remove surface water. Relief drains are installed along a slope and drain in the direction of the slope. Relief drains can be installed in a grid pattern, a herringbone pattern, or a random pattern. Subsurface Drain Type: Interceptor Drains Interceptor drains are used to remove excess ground water from a slope, stabilize steep slopes, and lower the water table immediately below a slope to prevent the soil from becoming saturated. Interceptor drains are installed perpendicular to a slope and drain to the side of the slope. Interceptor drains usually consist of a single pipe or series of single pipes instead of a patterned lay- out. Subsurface Drain Depth and Spacing . The depth of a subsurface drain is determined primarily by the depth to which the water table is to be lowered or the depth to a confining layer. For practical reasons, the maximum depth is usually limited to 6 feet, with a minimum cover of 2 feet to protect the conduit. The soil should have depth and sufficient permeability to permit installation of an effective drainage system at a depth of 2 to 6 feet. 2019 Stormwater Management Manual for Western Washington Volume 11 - Chapter 3 - Page 345