Curtain Drains, Select Fill
CURTAIN DRAINS are sometimes used to lower the groundwater levels in the leaching area. They consist of a trench of crushed stone with a perforated pipe near the bottom. The water they intercept is piped "to grade" at a lower area of the lot, or piped to a storm drain or catch basin in the road. The curtain drain should lower the ground water at least two feet below the bottom of the system.
Curtain drains are required to be at least twenty-five feet upslope of the leaching area. If downslope of the system the setback is fifty feet.
Curtain drains can be used in conjunction with leaching systems that are elevated with "select" fill.
SELECT FILL is clean sand or sand and gravel that is used to elevate a properly prepared area in order to install the leaching system high enough above groundwater or ledge. It has the added benefit of spreading the soakdown area of the leaching units by saturating the soils between rows. It must be able to meet the specifications of a complex "sieve analysis" that is detailed in the code and which is mainly designed to severely limit the amount of fine particles (fines) in the fill material.
Occasionally select fill is also used for "wick" holes that penetrate the bottom of a leaching area. This is sometimes done to allow saturation of better draining soils below while keeping the bottom of the system the requisite height.