Foundations for double storey extension

richyje

Hi,

I have planning permission for a double storey side extension and a single rear kitchen extension. I had a full plans application approval this week which included a build over agreement with Thames Water when I discovered after digging a couple of trial holes the 9" Thames Water surface water sewer pipe running closer to my foundations than I had expected. I initially informed Thames Water it was 600mm away from my foundations, but have since informed them of the new situation. My builder has since suggested that instead of having 600mm width foundations we would be able to have 800mm width and have our wall on the outside of the foundation itself (on the 600mm foundation the wall would have been in the centre of the foundation - leaving 150mm foundation on the other side)
Generally, does anyone know if having a 800mm width foundation is acceptable on a double storey extension ?
My foundations will be approx 2m in depth due to some trees which are close by. The depth of the pipe is approx 1.4m so we are going well below the invert level of the pipe.

Thanks,

Richard

ormus

you can have deep strip foundations of only 400mm, if they are reinforced.
with rebar.

iamcornholio

richyje wrote: »

800mm width and have our wall on the outside of the foundation itself

Generally, does anyone know if having a 800mm width foundation is acceptable on a double storey extension ?
My foundations will be approx 2m in depth due to some trees which are

You can't just plonk the wall on the outside of a wide foundation as this creates potential issues with load distribution and shear cracking, and differential settlement. This type of foundation will need to be designed by a structural engineer.

800 wide by 2000 deep are monster foundations, so you might want to consider mini-piling. In which case get your builder to quote on these large foundations for comparision.

The probelm with a narrow 400mm strip foundation is that load distribution to the soil is limited, so rebar wont help at all, as it's not the strength of the concrete which is the main issue but rather how the load is spread