FEATURE 21
It is all about the drain
By
Rory Macnamara
In our last feature on Drainage , we did not go into detail on the aspect of sub soil drainage which a starting point is the French drain as referred to in SANS 10400 : P .
More of that later when we look at drainage types in relation to sub soil drainage . Initially , we must accept that the plumber ’ s role in sub soil drainage is part of a larger picture in terms of expelling excess water from surrounding soil in buildings . The topic itself reaches everywhere like roads , dams , railways , etc . For this feature we will focus on general building applications related to the plumber and plumbing .
The design considerations are ,
• Placing an artificial conduit or flow path below the water table . The hydraulic head of the conduit is less than that of the soil to be drained . This creates a gradient toward the flow channel depressing the free water surface . Water is removed from the conduit at its downstream end .
• In evaluating the need to remove subsurface water one must consider the surface water runoff from the buildings , pavements , and other places where due to impervious soil conditions the water is unable to seep into the ground . One must consider the soil conditions that prevail e . g ., clay where the long-term soil moisture can manifest itself at the building foundations . Finally , groundwater levels must be considered . These can rise and be reduced depending on conditions , so design needs to consider all these aspects .
Sub soil drainage is neither a luxury , nor a neat add on , but should only be built if necessary .
Bear in mind that excess water from neighbouring buildings adds to the water flow thus retaining walls built for the purpose of controlling such water becomes a sub soil nightmare .
Types of Drains In South Africa we tend to work with four basic variations whilst there are up to eight types applied around the world .
All photos by © TeMa South Africa
Materials required for a successful installation .
The sub soil drains should be installed on flat ground , in a sag or depression , or on slopping ground . Whilst the design is the same for all four drains which is a trench with fill or filter material , sand , gravel and / or stones .
One uses stone or high-void material to create a greater flow capacity than the in-situ material and a hydraulic gradient to drive liquid movement into drain and out of the area .
“…. any aspect beyond what the standard expresses , specialist advice should be sought .”
August 2022 Volume 28 I Number 6 www . plumbingafrica . co . za