Foundation design - drain crossing at corner

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mfarrow

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Hi

I'm in the middle of a discussion with the BCO regarding the proposal to cross the corner of an extension where an existing 4" drain sits.

The drain sits at a centreline of 530mm below ground level, so around 470mm above the bottom of any trench.

I proposed a full fill foundation with 50mm compressible material around the drain:
Rear Extension - original proposal.png

This was rejected by the BCO, who requested a 1m^2 pad foundation with lintels in a T-shape, which I felt would be too busy and confuse the build process. I therefore proposed the design below, i.e. increasing the ground bearing area in the corner, so the load would be spread equivalent to having a strip foundation built. Loads would be nicely spread around the drain using standard C20/GEN3 concrete:
Rear Extension - revised proposal.png

BCO has stated they believe this is fundamentally wrong, and has asked for the design as below:
Rear Extension - BCOs proposal.png

Personally, I don't see the point of an eccentric lump of concrete outboard of the corner, but I would value your opinion.

Thanks
Mike
 
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Begs the question.....why on earth don't you extend that IC over to the corner, if the only reason for present positioning is that poxy kitchen gully?
 
Good point, but I'm not sure I could lose the step in the IC, it was very close when installed originally (ie small block of wood under a 1.8m level to get level).
 
Have you or the bco considered Ancon stainless steel ladder reinforcement in the bed joints of the first few courses above the concrete?
This would turn the below-ground brick/blockwork into a beam capable of cantilevering the wall over the pipe; the distance the wall would project beyond the pipe does not look to be very much.

https://www.ancon.co.uk/products/masonry-reinforcement/
 
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Just to close the thread, which I forgot to do, the Ancon solution was considered a "nice idea" but ultimately they needed calculations to justify it, so big old lump of concrete it is.

On the "new chamber in corner" idea, yes this would be good except I know the falls in the sewer pipe are around 1:200, so this isn't practicable with a plastic manhole (where you get a step between side entry and exit) unless the builders can find a step-free one and have their wits very much about them when installing the inlet/outlet (and an even smaller block of wood on the level than I used originally).

Or the pipe would need to be actually level, as opposed to practically level... if you catch my drift.
 
There are step free ones to go round a bend and with a single junction.
 
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