What is Drainage Tile / Underdrain / French Drain? When should it be used?
What is Drain Tile? Drainage Tile, Underdrain, French Drain are all terms used when referring to a 4” perforated drainage pipe which has a silt sock around it that sits in a trench of ¾” clean stone wrapped in soil separation fabric. The method consists of piping systems used to transport water with the aid of gravity. The pipe is then directed towards an opening in a drainage ditch or connected to a storm sewer system.
When should it be used? Whether it is new construction or re-construction of a parking lot, drainage above and below the surface is a top priority. Drain tile can be used to help dry out or re-direct water infiltration to the sub base.
A parking lot in a low lying area is the main candidate for drain tile installation. If a parking lot is surrounded by areas of higher elevation, e.g. a hill, the perimeter of the parking lot should be lined with drain tile so the natural ground water flow can be caught before entering the sub base of the parking lot.
Soil types are another reason drain tile should be used. If the sub base is made up of a clay mixture, it is likely to hold more moisture than a soil type with finer gradation. Drain tile may be placed in the swale (area of surface water flow) of a parking lot to help dry out the surrounding soils.
When water seepage is apparent, is another reason to install drain tile. If the surface of the asphalt, in a reconstruction project, displays moisture on a dry day, it likely that the sub base is saturated and needs to be dried out.
Minimizing undercuts is another reason to add drain tile. If it is known that the sub base is saturated, due to water seepage or geological data, it may be suggested to install drain tile and let it sit for a period of time. This would be done to help dry out a parking lot area where undercuts are suspected, thus minimizing costs.
Protecting an investment is another reason to install drain tile. If there is room in the budget, drain tile should be placed along the perimeter of a parking lot, especially under curb & gutter and along green space. This practice is an effective way to safeguard against future water infiltration to the sub base of the parking lot.
How is drainage tile installed?
The area is excavated to a depth of 12” – 36” depending the severity of the groundwater issue
A soil separation fabric is installed to prevent sand and other fines from comingling into the ¾” clean stone which is the drainage course.
The trench is then backfilled with ¾” stone and the filter fabric is wrapped around the top in the same manner as you would wrap a Burrito. The drainage tile is then directed into either a catch basin or drainage swale.
The trench is then backfilled with 4” of asphalt and either finished with a milling and paving operation or patched with surface mix asphalt with the seams sealed with hot rubberized crack sealer.
Whether it is new construction or re-construction of a parking lot, drainage above and below the surface is a top priority. Drain tile can be used to help dry out or re-direct water infiltration to the sub base. Drain tile should be considered as an option when constructing or reconstructing a parking lot. While each situation is different, an honest and candid conversation about its uses should take place with your pavement consultant.