Trench Fill Foundations - 9 inch Pipe

richyje

Hi,

I have planning permission for a double storey side extension and single rear on my 1920's cottage. The Building Regulation app has been more or less approved with trench fill foundations. I have just dug a trial hole and as I had very much dreaded I have come across a 9 inch surface water sewer which is directly below where my foundations will be.

This 9 inch pipe I was aware of, but i was under the impression it was further away from my foundations than it is. I have a buildover application with Thames Water in progress.

I do not want to reduce the size of my extension, but it has been suggested that it would be possible to cantilever over the pipe in order to keep the original size of the extension. It has also been suggested that pile foundation (or mini piles) which would be cantilevered over the top may be the way to go.

I heard that Pile Foundationing is very expensive... would it be possible to move the trench fill foundation over (to miss the pipe) and cantilever over that.

I dont believe this will cause an issue to Thames Water should they need access to the pipe.

Thanks for all your help.

Richard

Mankysteve

Ask a structural engineer.

dander

It will probably depend where the surface water that's draining through this pipe is coming from. Thames Water will only be interested if it's a public sewer - if it's just a drain from your own property, then it shouldn't be anything to do with them.
I believe each water company's attitudes to building over sewer pipes varies - so there's really no substitute for talking to Thames Water themselves. However I had some similar pipes running through the foundations for my small extension - not public, so only affected building control rather than the water company. The BC approved solution was to build the trench foundations on either side and bridge the pipes with concrete lintels.
The extension itself was a single storey of about 6sqm so not big and heavy - I don't know if this would make a difference to what an approved solution would be.

iamcornholio

Can't you divert the drain to miss the wall and then bring it back in just before if crosses the boundary.

Four inspection chambers will be cheaper than engineers fees, let alone an exotic foundation

Edit:

If this is a public sewer, then you need the water authority's permission to build near or over it, and they will have to approve the design - which will be independant approval to that of the council's building control officer

Tucker

I am not sure how mini piling helps or makes things any better?

When I priced it up, it was about 3 times more expensive than standard strip foundations.

The main thing is, how deep down is this pipe?

I built over my sewer pipe, as it was well above the height of the footings, which are basically about 900mm down and 200mm thick. It was built over with a concrete lintels which is a simple enough process.

richyje

Thanks for all your help..

The pipe is 1.5m below ground and getting shallower to the back of the extension. It is running parallel under my foundations for the whole side of my extension, so dont think bridging is going to be possible =(