Drainage pipe and foundation

[mysterio619]

Hello hope this is the right forum.

So I'm planning on doing a small extension to an existing side extension. I applied for building control and they flagged that there might be drain pipe to be checked before the foundation is dug.

My architect/structural engineer took a look at the man hole (the pipes were 2.4m deep) and said that the existing side extension (built at least 20 years ago) might be built over the sewer pipe and that if that's the case then Thames water might have the authority to break the existing side extension.

He asked me to get builder to check the depth of the foundation of the existing structure to see if it needs underpinning. When the builders dug it seems like there's some sort of step concrete foundation that stretches to my fence.

I've contacted local council to see if there was building regulations from when the side extension was originally done since if it does then it must surely comply (build over agreement wasn't in place when the extension was done).

I'm now reconsidering whether I should even do the extension since moving the sewer pipe will set me back by £8000 apparently. What would be the ideal next step here (would a CCTV drain survey help in figuring out if it's running straight?)


I've attached pictures of the stepped concrete foundation, the front inspection chamber which shows the location and the drain running parallel and directly underneath the edge of the existing side. Is it possible that the direction of the pipe changes and back to divert around the pipe and if so what could the lower foundation step be for?

 

[GeoEng]

Hi,

With regard to the sewer you need to know where it goes.
Does it go beyond your property?
The next inspection cover if there is one will be on the direction the pipe is going.
Bends have inspection covers usually so it should relatively easy to locate.
From experience, a long time ago, if you have not found it after fifty meters then you have probably miss it or you are end of the line.

 

[GeoEng]

The foundation looks like a strip foundation.
You should draw a plan to scale of the existing build and all services.
From there its easier to know where your going and to explain to others.
I use to do that, surveyed most of the Peckham Estates before they were redeveloped in the 1990s.

 

[mysterio619]

Thank you so there is an inspection chamber 3m behind the house and it seems directly in line with the inspection chamber at the front.

I guess then my follow up question is given the sewer pipe is 2.5m below ground what would the easiest way be to determine if the existing foundation that runs lateral to the pipe (and likely directly above it) is not putting excessive force on the pipe. Would the strip foundation have helped remove the pressure on the sewer pipe or given that the pipe is 2.5m down is it safe to think that it wouldn't be affected.

 

[GeoEng]

If the pipe was going to fail, it is most likely to be at the point it passes under the foundation. If the local geology is stable it should be OK, but running directly under a foundation rather than passing directly under does add risk. A good test is, how long has it been under the foundations and has there been movement. Normally most movement occurs soon after construction, settlement.

Position is best sorted by measuring the walls assuming its 90 degree walls. Diagonal measurements to the manhole covers. Check the pipe direction. That should be accurate enough to know if its under the foundations. Draw up on a free cad software.