1.30 BMP C205 - Subsurface Drains
To intercept, collect, and convey groundwater to a satisfactory outlet, using a perforated pipe or conduit below the ground surface. Subsurface drains are also known as “French drains.” The perforated pipe provides a dewatering mechanism to drain excessively wet soils, provide a stable base for construction, improve stability of structures with shallow foundations, or to reduce hydrostatic pressure to improve slope stability.
Use 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.
1.30.3 Design and Installation Specifications
Are used either to lower the water table in large, relatively flat areas, improve the growth of vegetation, or to remove surface water.
Are installed along a slope and drain in the direction of the slope.
Can be installed in a grid pattern, a herringbone pattern, or a random pattern.
Are used to remove excess groundwater from a slope, stabilize steep slopes, and lower the water table immediately below a slope to prevent the soil from becoming saturated.
Are installed perpendicular to a slope and drain to the side of the slope.
Usually consist of a single pipe or series of single pipes instead of a patterned layout.
Depth and spacing considerations for interceptor drains
The depth of an interceptor 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 stormwater system at a depth of 2 to 6 feet.
Subsurface Drain Sizing and Placement
Size subsurface drains to carry the required capacity without pressure flow. The minimum diameter for a subsurface drain is 4 inches.
The minimum velocity required to prevent silting is 1.4 feet per second. Grade the line to achieve this velocity at a minimum. The maximum allowable velocity using a sand-gravel filter or envelope is 9 feet per second.
Use filter material and fabric around all subsurface drains for proper bedding and filtration of fine materials. Envelopes and filters should surround the subsurface drain to a minimum of 3-inch thickness.
Construct the trench on a continuous grade with no reverse grades or low spots.
Stabilize soft or yielding soils under the subsurface drain with gravel or other suitable material.
Backfill immediately after placement of the subsurface drain pipe. Do not allow sections of the subsurface drain pipe to remain uncovered overnight or during a rainstorm. Place backfill material in the trench in such a manner that the subsurface drain pipe is not displaced or damaged.
Do not install permanent subsurface drains near trees as tree roots may clog the lines. Use solid pipe with watertight connections where necessary to pass a subsurface drainage system through a stand of trees.
The quantity and quality of discharge needs to be accounted for in the receiving stream (additional detention may be required).
The capacity of a subsurface drain is determined by calculating the maximum rate of groundwater flow to be intercepted. Therefore, it is good practice to make complete subsurface investigations, including hydraulic conductivity of the soil, before designing a subsurface drainage system.
Ensure that the outlet of the subsurface drain empties into a channel or other watercourse above the normal water level.
An adequate outlet for the subsurface drainage system must be available either by gravity or pumping.
Empty the outlet of the subsurface drain into a sediment trapping BMP through a catch basin. If free of sediment, it can then empty into a receiving channel, swale, or stable vegetated area adequately protected from erosion and undermining.
Secure an animal guard to the outlet end of the pipe to keep out rodents.
Use at least 10 feet of corrugated metal, cast iron, or heavy-duty plastic without perforations outlet pipe. Do not use an envelope or filter material around the outlet pipe, and bury at least two-thirds of the pipe length.
When outlet velocities exceed those allowable for the receiving stream, provide outlet protection.
Check the subsurface drains periodically to ensure that they are free-flowing and not clogged with sediment or roots.
Keep the outlet clean and free of debris.
Keep surface inlets open and free of sediment and other debris.
Trees located too close to a subsurface drain often clog the system with their roots. If a subsurface drain becomes clogged, relocate the subsurface drain or remove the trees as a last resort. Plan the placement of the subsurface drain to minimize this problem.
Where subsurface drains are crossed by heavy vehicles, check the line to ensure that it is not crushed and use pipe material that can handle traffic loads.