Build over sewer & Foundations

[VPP]

I'm in the process of purchasing a house in London with a timber garage that is like to knock down and replace with a single or even double story brick structure.

Unfortunately there is a 150mm public sewer pipe running alongside the house near the middle of this 8ft wide timber garage. Even more annoyingly Thames water say foundations need to be at least 600mm from the pipe. The question is can the foundations be less that 600mm underneath the invert of the sewer? If not then how would one build wide enough foundations without asking the neighbours to borrow their land.

The BCO mentioned (over the phone) something about cantilever slab foundation although I can't quite understand what that is.

Any help on this would be appreciated!

(See attached pictures)

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[noseall]

Have you sought professional advice i.e. from a designer or architect?

 

[VPP]

No not yet, it may take a few weeks. I just wanted a second opinion from this forum first.

 

[ants1]

I'm in the process of purchasing a house in London with a timber garage that is like to knock down and replace with a single or even double story brick structure.

Unfortunately there is a 150mm public sewer pipe running alongside the house near the middle of this 8ft wide timber garage. Even more annoyingly Thames water say foundations need to be at least 600mm from the pipe. The question is can the foundations be less that 600mm underneath the invert of the sewer? If not then how would one build wide enough foundations without asking the neighbours to borrow their land.

The BCO mentioned (over the phone) something about cantilever slab foundation although I can't quite understand what that is.

Any help on this would be appreciated!

(See attached pictures)

[/i]

Do you have your answer to this yet? If not I will explain....

 

[VPP]

No I didn't get any responses; I'd appreciate any advice you could give.

Thanks

 

[ants1]

No I didn't get any responses; I'd appreciate any advice you could give.

Thanks

Would you be able to make it clear on your drawings exactly where you propose the new structure to go and also where the boundarys are..

I have plenty of these come through my desk and there has always been a solution so far. Thames water very rarely refuse build overs as long as there done properly, one thing they will never allow is covering over manholes though!

Cantilevered slabs are very useful when the drain runs directly down the line of where the foundation is to be dug.. What you do is still dig the trench to the side of the drain at a suitable distance and then extend (cantilever) a re-inforced slab over the top of the drain, so you can build directly over it but all the load is taken back to the side where you dug the trench.. (hard to visualise without a drawing)

I would say for you (as you said it runs down the middle) digging trenches down to invert level of drains and shuttering off the concrete 0.3 metres either side of the drain and then spanning conc. lintels over the top could be the answer.

Mark on the drawing exactly where the structure is and we will see

Thanks

A

 

[VPP]

Thank you for you reply, I understand it has something to do with "moments" from school Physics! But like you said, it's difficult to visualise.

I have uploaded a new image in my album with green dashed showing where the extension might go. I'm thinking of getting a new manhole built nearer to the neighbours where I wouldn't want to build. Do you think Thames Water be OK with that?

Thanks again

 

[ants1]

Thank you for you reply, I understand it has something to do with "moments" from school Physics! But like you said, it's difficult to visualise.

I have uploaded a new image in my album with green dashed showing where the extension might go. I'm thinking of getting a new manhole built nearer to the neighbours where I wouldn't want to build. Do you think Thames Water be OK with that?

Thanks again

Ok.. another few questions first!

Are you certain this is a public sewer? or are you just assuming it is because its 150mm dia. pipe? Is it a sewer shown on the thames water maps or is it an 'adopted sewer'?

So the rear wall and the flank wall to the extension will be right on the boundary? How close are neighbouring buildings to these boundarys?

I don't quite understand where you are thinking of moving the m/h too nearer your neighbours.. Thames water would never allow a 45 bend under an extension with no access which is what you would have if you build over it as they don't allow internal sealed m'h's.

 

[VPP]

Thanks again for the reply,

I am sure it's a public sewer because it is a 1932 built house, and the pipes pick up waste from the neighbouring homes. The side pipe doesn't show on Thames Water pipes however.

The house is an "end of rung" house so my neighbours next to the proposed extension are at a 90 degree angle to me and they haven't built any outhouses at the back of their gardens yet.

The rear wall will not be a boundary wall though, the garden is much larger than 6m.

 

[ants1]

Thanks again for the reply,

I am sure it's a public sewer because it is a 1932 built house, and the pipes pick up waste from the neighbouring homes. The side pipe doesn't show on Thames Water pipes however.

The house is an "end of rung" house so my neighbours next to the proposed extension are at a 90 degree angle to me and they haven't built any outhouses at the back of their gardens yet.

The rear wall will not be a boundary wall though, the garden is much larger than 6m.

Ok so the boundary to the side is at the bottom of the neighbouring garden. I was just thinking of any party wall issues, but don't have to worry about that so that's good.

I will tell you what I would suggest you put forward as a proposal to Thames water, but each case is taken on its own merit so its obviously not going to be guaranteed but I cant see any reason for them to no accept this.

What you will need to do is re-route the drain run coming in from the left to a new m/h at the rear of the new extension and then have a straight run from the rear to the m/h at the front. That way there is no internal m/h and no 45 bend under the extension. I have seen cases where they have allowed this before so I cant see why they wouldn't.

I would also suggest you put a suspended floor in the extension and not a ground bearing slab, so all the floor load is taken across to the foundations and there is no weight over the pipe.

As for the foundations its hard to say for sure without doing a site survey as it depends on ground conditions, bearing capacity, near by trees and all sorts. And also what your local building control will accept.

First option - I know for a fact they don't always stick to the 600mm away from the pipe rule but speak to them and see what they say.. Anyway, you can try and stick to the strip foundation route, shuttering concrete either side of pipe to an agreed distance and span lintels over the drain pipe.. As for the boundary you can whats called - eccentrically load the foundation so you dig the trench right up to the boundary and build the wall off the edge of the foundation - It just needs to be wide enough to make sure you wont get any over turning.. This depends of ground conditions etc.

Alternatively - you can dig pad foundations along the footprint of the building (say 1 on each corner and 1 mid span on flank wall) and then span groundbeams - steel encased in conc. or conc. beams between the pads. This way you keep the foundation as far away from the drain as you like! This would require specialist design. Or you could use a combination of both- beams to span drain at rear and strip along flank wall.

Lots to think about, too much to go through over this forum. You need to consult with Thames water and get a surveyor/engineer to look it over. Its defiantly possible though.

A

 

[VPP]

Thank you for the advice!

I had thought of moving the pipe back actually, I'll get an architect on it as soon as I can.