This was the original problem we found in the alleyway. Previous Contractors installing a new foul drain smashed through an existing surface water drain taking the roof run off. They then proceeded to connect every rain water down pipe into the foul drainage system.

New 225 to existing 375 chamber.

Problems in the alley are the amount of services and the fact that no machinery can get down it - everything dug by hand.

The trapped rectangular hopper detail worked on paper, but varying size of existing foundations mean this detail can't be used at all rwp's.

Overall depth 550. RWP in the back, paved area drains to the front. Trap brings the pipe back up to a higher level to connect to the shallow drainage in the alley.