It is, as the name implies, a drainage system that is located underground. The drainage system is made up of a variety of pipe lengths and fittings such as bends, junctions, and other fittings that allow the pipe to change direction or adapt to another system. Underground drainage is commonly known as the sewage system since it is primarily used to convey waste water (grey water) or foul water straight to the sewage treatment plant. This technique can also be used to drain rainwater into a soakaway or watercourse.
These days, most systems are composed of PVC. This is because PVC is a lightweight yet sturdy material that lasts a long time and is less expensive to manufacture than clay or concrete. It is also easier to install and almost completely maintenance-free due to its lightweight. The inner wall is smooth, which facilitates the flow of wastewater, and PVC has a higher “flow rate” than clay or concrete.
An underground drainage system is easily identified because it is terracotta in color, which is the industry standard for this system. Surprisingly, dirt pipes and fittings are nearly identical, having been manufactured on the same machinery. The only difference is that these will be black, white, or grey in color and contain a UV additive to prevent discoloration from sunlight.