Sanitary Drainage, Soil and Waste Systems: Fall, Stack and Inspection
What this domain tests
Drainage questions test the difference between soil and waste, the layout of stacks and vents, gradients, inspection chambers and gully traps. You must show that you can design a drain that flows by gravity without blockages or smell.
Core rules and concepts
- Soil carries sewage from WCs; waste carries sullage from sinks, basins and baths. They may join in a one-pipe or two-pipe system.
- A soil stack carries discharge vertically; a vent stack protects trap seals; a waste stack carries non-fecal waste.
- Drains must fall continuously. A common design range for 100 mm pipe is about 1:40 to 1:80; confirm the exact figure in the applicable code.
- Self-cleansing velocity is about 0.6 m/s; too flat a drain silts up.
- Use inspection chambers at changes of direction, junctions and long runs; gully traps receive surface water and sink waste.
- Junctions should be swept, not square, and no drain should have a sag.
Worked example
A 100 mm drain runs 12 m at a gradient of 1:60. Fall = length ÷ gradient denominator = 12 ÷ 60 = 0.2 m = 200 mm. The reasoning: the invert at the lower end must be 200 mm below the upper end. Check that the resulting velocity is adequate for self-cleansing and that the inspection chamber has a benched channel.
Traps that cost marks
- Laying a drain flat or with a backfall.
- Using a square junction instead of a swept junction.
- Omitting a vent and allowing trap seals to siphon.
- Forgetting an inspection chamber at a change of direction.
- Connecting a WC into a waste pipe that is too small.