DESIGN: DEAR MR PLUMBER
57
Drainage systems below ground
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.
By Vollie Brink
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.
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’?
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. 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.
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. 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. PA
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.
www.plumbingafrica.co.za
Vollie Brink
Vollie Brink is one of the
industry’s longest serving
wet service engineers.
He continues to serve on
SABS committees and
has been involved in the
Green Building Council
Star rating system.
Brink continues to consult
for various organisations
while enjoying a
well-earned retirement.
October 2018 Volume 24 I Number 8