The lowdown on drainage installation
Stevie Fairbairn looks at the most common mistakes made when installing drainage systems – and how to avoid them
I
f there is one area of a new build project where the expression ‘once it’s in, there is no going back’ applies, it’s the drainage system. Drainage is one of the first things to be done
in a build, and much of the subsequent construc- tion sits on top of it, whether it’s the building itself or the hard landscaping. As such, making a mis- take with the drainage system often only becomes apparent once the building is com- plete, and can be painful and expensive. That’s why every effort should be made to get
things right first time. Yet, there is a key problem, which can be summed up as ‘out of sight, out of mind.’ Because systems are hidden from view, some builders may be tempted to see it as an area of the project where corners can be cut. It is a temptation that many have found too
great to resist, but the results can be very expen- sive to put right. They can also result in substan- dard drainage performance, meaning annoy- ance, if not misery, for the property owners perhaps for years to come.
Common installation
blunders So, what are the most common mistakes (or examples of negligence)? And how can they be avoided?
1: Incorrect sizing of pipework The architect, civil engineer and surveyor on any project should establish the correct type and diameter of pipework to install. Yet subsequent drainage surveys regularly find this does not lead, for whatever reason, to the right sized pipes going into the ground. If the pipes do not have the hydraulic capac-
ity for the demands placed on them during peri- ods of peak flows, problems occur. Blockages can increase, pipes can surcharge, backing up into manholes, toilets and sinks, and they will also be at greater risk of structural failure.
2: Incorrect pipe gradient Drainage systems should be self-cleansing – the force of the water through the system from grav- ity should be strong enough to carry waste out into main sewers. For this to happen however, the pipes have to be installed at the right
gradient. If they are too shallow, or a necessary pumping system is not installed to reduce build costs, there will be a higher risk of blockages.
3: Incorrect backfill The trenches which drains are installed into are backfilled with material called ballast. If the wrong type is used, or it is not properly distrib- uted or compacted, pipes can settle over time, causing ‘flat spots’ which hamper the self-cleans- ing process, resulting in a build-up of debris and blockages. Normally the solution is excavating and re-bedding the pipe but this is costly, par- ticularly if something has been built over the top of it.
4: Cross-connections These occur when a surface water and foul water drain are wrongly connected together, which if it is from foul to surface water can result in waste being discharged into open water courses, causing potentially serious pollution. This is surprisingly common, even with relatively large housing developments. This will eventually result in prosecution by the Environment Agency – and a lot of expense from fines, court costs and remedial action. If it is a surface water to foul, and not accounted for in the engineer’s hydraulic calculations, the system can be overloaded, causing blockages, surcharging and sewer floods.
THE RESULTS OF A STAKE THROUGH A PIPE [ 38 selfbuilder & homemaker
www.sbhonline.co.uk
Making the wrong initial choices can lead to problems later
]
5: Poor drainage design Drainage systems are under more pressure now than ever before, so designing them to cope with the added demands placed on them is essential. Unfortunately many people put wet wipes, sanitary products and other items down toilets, sinks and drains, yet systems are often installed with inadequate maintenance access, such as manholes and rodding points. Different types of pipes have different strengths and weaknesses. For example, plastic pipes need more bedding, and can be less resilient to modern maintenance and cleaning techniques. Clay pipes on the other hand are more susceptible to being invaded by roots. Making the wrong initial choices can lead to problems later.
6: Poor drainage construction This is one of the biggest issues, as badly installed drainage systems often perform badly.
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