In the plumbing trade when we refer to the term "underground" it encompasses all the piping for both the water supply and sanitary drainage that is installed below grade. In this article I will cover the rules and good trade practices for installing underground drainage piping as it relates to residential plumbing. Click here for information on Installing a Water Service.
Grade(slope) for Underground Drainage Pipe
Every drainage pipe installed within the building's plumbing system must have a minimum grade of 1:50 (or 1/4" per foot) with the exception of the building drain when it's larger than 3 inches. A 4" building drain or building sewer is permitted to be sloped at 1:100 (or 1/8" per foot).
*caution must be taken whenever working in dug trenches deeper than 1.2M(4 feet). Use properly sloped side walls or shoring to prevent wall collapse.
Underground piping from the municipal system into your home is installed in dug trenches to a depth below the frost line for your climate, which obviously could vary a great deal depending on your geographical location. The trench should be constructed as narrow as is reasonable to allow proper joining of the pipes and avoid digging below the depth that will be required to support the pipe, as the undisturbed soil is naturally compacted and will not settle after time.
When laying the piping in the trench, the writing that indicates the type and class of pipe should face up and all the joints must be visible until an inspection by the local authority has been done.
The base supporting the underground pipe must be free of large rocks, jagged material, frozen soil and organic material(ashes, wood, plant matter). Generally loads of sand or crushed rock is used to line the trench because it can easily be manipulated and provides an excellent support on which to grade the pipe. Once the inspection has taken place, the underground pipe must be covered with a minimum of 300mm(1 foot) of clean fill (sand, crushed rock, suitable soil) and tamped to prevent settling.
If the pipe will be below any high traffic area; such as a drive way, the backfill should be tamped repeatedly as each layer is added to ensure solid compaction of the earth and heavier duty pipe may also be required.