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.
Defective Manhole Benching
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.
The image above shows signs or erosion to the benching where it meets the channel pipes this will allow water loss into the sub-soil undermining the chamber and leading to possible movement, tree root ingress is common in the base of manholes due to washed out or poorly installed benching,
Repairing Manhole Benching
If at all possible you should break out the defective benching and start again so as to ensure you get a complete sealed manhole base, failing that you should rake out any defective sections to ensure that the patched repairs can key into the existing benching.
Ideally this should be done when the system is out of use to make sure everything has set before it comes into contact with water again, there are however products on the market that speed up the curing process of the sand/cement/grano if this is not possible.