Drainage systems below ground

By Vollie Brink

A sanitary drainage system below ground (in ground under the floor of a building or structure) is treated differently in SANS 10400-P and also by the plan approver in the office of the building control officer and the building inspector.

I always go back to the original purpose of the National Building Regulations (NBR): It was intended to render ‘affordable housing’ in the first instance, but office buildings were also included.

If you carefully study SANS 10400-P, then it does not include buildings such as hospitals and shopping centres or factories. However, the ‘principles’ in part P can be applied for the design of the sanitary drainage of other buildings.

In the past, the sanitary drainage piping in the house was taken out of the house above ground and then dropped into a gully. From there, the drain ran underground up to the septic tank or municipal sewer connection if there was a waterborne system sewer connection.

Piping inside the house

The principle that applied was that all piping inside the house had to be above ground and only the external piping had to be below ground. This principle was also carried forward into the new NBR of 1977.

One of the important rules that came with the NBR of 1977 was that there must not be any manholes or rodding eyes inside a building — any building.

The reason for this was, and still is, that it was envisaged that there will be no ‘drains’ inside a building underground, in ground, and that it will not be interconnected.

The idea behind this thinking was that all such underfloor drainage must exit separately from each fixture of the building and all be connected outside the building.

Modern buildings have changed, and all the fixtures are scattered ‘inside’ the buildings and, in many cases, are situated far from the external walls — it is virtually impossible to take each waste and soil pipe out of the building separately.

Pipes must be interconnected

Modern buildings necessitated that the pipes be interconnected ‘inside’ the buildings and are then taken out of the building to external connections. This necessitated internal access into the piping under the floor in ground.

The new 1977 NBR did not address this new development and did not provide a solution other than to leave it for a ‘rational design’.

In ‘the past’, it was a requirement that all underground drainage piping must have an inspection cover on all bends and junctions — this is directly in contradiction with the design of a municipal sewer.

The problem with such inspection covers on the bends and junctions is that it causes, or can cause, blockages, and we engineers would rather have the pipes and bends and junctions as smooth as possible inside. Such covers on pipes underground under a floor is of no use, as it is situated under a concrete floor in a building.

The question is, why must you have access into the piping situated above ground and inside the building? Is it not the same as having a rodding eye in the building?

What is not allowed under a floor

The present situation is that you are not allowed to have the following under a floor in ground inside a building:

  1. No bend
  2. No change of direction
  3. No change of gradient
  4. No interconnections.

These four ‘not allowed’ restrictions can only be overcome by means of a rational design by a Pr Eng or a Pr Tech Eng.

The question is, how can it be ‘rationalised’?

It must firstly be understood that the hydraulic activity inside a pipe, whether it be hanging from a slab above or inside a pipe situated in the ground under the building, is the same.

The effluent in the piping does not know where it is, above ground or underground.

How to rationalise the piping under floors

The following are some ‘tips’ on how to ‘rationalise’ the piping under floors:

  1. Keep soil and waste separate.
  2. Keep the ground floor separate from the floors above.
  3. Provide overflow facility on the wastewater for backflow from the soil water.
  4. Keep piping under floors in the building and to exit the building as short as possible.
  5. Do not use elbows; use only long radius bends, r=800 or two 45 bends with straight pieces of 600 in-between.
  6. Junction with 1 000 length of pipe and then 45 bend.
  7. All piping to be at least 100mm in diameter, also the wastewater, and provide a floor drain for the connection of waste fittings such as basins. The floor drain is to act as access to drainage below floor.
  8. No inspection eyes on underground piping.
  9. Provide stub-stack as access and access from above the floor.
  10. Do not encase plastic piping under floor.

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