Case Study 49 – Sub Surface Drains
Sub surface drains are installed in permeable and porous soil to collect and convey drainage water. They are especially installed deeply underneath the ground to evacuate excess water from permeable soil. If the drains are installed carefully with proper planning, then they will provide problem free service for several years with minimum maintenance.
Although subsurface drains are expensive and costly, they are effective in controlling stormwater as well as lowering groundwater table. They are extensively used to improve soil atmosphere for vegetative growth and avoid water seepage in wet areas. They are helpful in controlling ground water in irrigated areas and removing excess water from ponds, high-rise buildings, parks, sports stadiums, railway stations and airports.
They will reduce the soil erosion and improves stability in soil. They are also known as relief drain, tile, underground drain, drainage tile and interceptor drain. They are not limited to wet lands, but can be used in all types of lands, but the soil must have sufficient deepness and permeability to allow easy installation and maintenance of subsurface drains.
Subsurface drains are usually installed, where excess ground water in the soil causes soil erosion and landslides. In order to stabilize slopes, toughen construction foundation and strengthen retaining walls, subsurface drains are ideal. They usually consist of subsurface main drain and outlet. Water is collected and conveyed through the main drain and sometimes, water is collected through laterals.
The installation should be carefully planned to limit the damage caused by soil infiltration, floating ice, rodents, etc. Base drainage layer, filter layer, collector pipe and outlet pipe are the components of the subsurface drains. In addition, they are of four types:
- Corrugated and PVC slotted subsurface pipes
- Mole drainage
- Interceptor drains
- Ground water pumps
The minimum diameter of the subsurface drains should be 4 inches and maximum diameter depends of the condition of the soil and particular area, where the drains are supposed to be installed. Filter material like geo-textile fabric is used around the drains for proper blockade of filtration of soils. The outlet drain should have 10-foot section of cast iron or PVC pipe without perforations to limit water clogging and jamming.
The subsurface drains should be inspected on regular basis for any accidental pipe breaks, soil infiltration and water clogging. The debris should be removed immediately and if the pipe is broken, replace the particular section right away. If they are installed beneath highways, where heavy vehicles passes regularly, then they should be inspected on regular basis for proper functioning.