MANHOLES, ROCKER PIPES & BENCHING
In this Bitesize CPD we will be looking at manholes, rocker pipes and benching. Rocker pipes are very often missed, forgotten or unknown by the groundworkers. Benching is often built too low or with flat tops for rats to nest. These are among the most common drainage defects picked up by FCM’s Clerks of Works during their site inspections.
Concrete ring manhole with rocker pipes on all connections:
Rocker pipes are short lengths of pipe which will permit differential settlement between the pipe and chamber.
In recent years this practise has spread to DI (ductile iron) water pipes and consultants are now detailing rocker pipes on ductile iron pressure pipes.
Sewers for Adoption states that you should allow for any differential settlement between the manhole and pipeline, with short lengths of either spigot/socket butt pipes being built into the manhole wall.
A rocker pipe should then be laid connecting the short spigot/socket butt concrete pipe to the incoming/outgoing pipe run, thereby incorporating a flexible joint close to the manhole.
See diagram below:
If rockers are not fitted, the chamber can settle and the connection pipe can be damaged or broken or pulled out of its connection to the chamber. The result is the same – pipe surround and waste fines washed into the chamber and a developing void behind the damaged pipe openings.
Manhole benching is the sand and cement haunching that sits between the manhole walls and the channel pipes - it is there to stop rats from leaving the system and to stop the splashing of waste and water from incoming branch lines as this causes waste to build up in the manhole leading to blockages.
For this reason the benching has to be installed at a minimum pitch to ensure that any overspill of waste and water slides back into the channel pipe and goes on its way.
Most domestic manholes are benched using a sand and cement mix. On the bigger sites a sand, cement and grano mix is used as this gives a superior smooth finish.
Correctly benched manhole:
If the benching is not steep enough, paper and solids will build up on the benching itself before falling back into the channel pipe - more often than not this will cause a blockage on the outlet of the chamber.
I will not add a photo or video of a blocked drain as we have all seen too many and this blog goes out at breakfast time!
Fox Curtis Murray are a building consultancy specialising in providing Clerks of Works services and Quality Control inspections.
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