Unexpected manhole

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So...

groundworks due to commence on my double storey extension in place of conservatory. Removed the conservatory carpet to see some crumbling self levelling screed and some DPM sticking through. A tap with an iron bar and soon found I have a sealed manhole there.

The pipe runs straight through, but the invert is 1700mm deep. About 3 metres back (and outside of the line of our foundations is an inspection cover at 500IL with some pipes joining from a gully and ensuite.

What is missing is some point at which the pipe drops to such a depth, and also where the SVP from the main bathroom joins. (Yes, I've done loo-paper checks and it appears in the deep one, but not the shallow 1!

Biggest problem is that the manhole is close the foundation line and the room will be a kitchen. Even if I retained the sealed cover, it would be half way under a run of units. As it is a straight through, can the manhole be filled in?

Will I have to have another manhole under the floor whereever the missing joint / deepening is?

The floor will be block and beam for heave protections. Luckily not ordered this yet, how do you incorporate a sealed drain into that?
 
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I'd want to know where the 1.2m drop is before getting rid of anything, could be the drop is immediately outside the deeper chamber, and removing this chamber could leave you potentially unable to access the deep run in future. I would expect where a backdrop was fitted, to find an access point into the horizontal section of the higher run to allow rodding along it from the chamber, but anything is possible!

If the ensuite and gulley connections were later than the original drainage then I would suspect they were run at a shallower depth for speed, cost and convenience, only dropping at the last moment to enter the manhole at the correct depth. If they were happy to 'lose' a manhole then drainage clearly wasn't the top priority....

I think you may have to do some further investigation to ascertain exactly what goes where (a CCTV survey is a good start), as this may well govern your next move. If there is a sudden change in invert levels, then B.C.O. is probably going to insist on a chamber at that point, omitting that is just storing up trouble for a later date in the event of problems.
 
Thanks.

I am going to try and get a survey done this week. I think I am no resigned to needing to have something inside of the extension, though my biggest question mark is how this will work below a block and beam floor.

With the insulation, screed and UFH pipes going down on top, it would be impossible to access in future.

I suppose the only good news is that my groundworkers are already resigned to digging the footings to 2m deep anyway, so will be going down a long way if things need moving!
 

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