Lowering basement floor - sloping?

geoffsussex · 10 Nov 2012

Hi everyone

I would like to have our basement floor lowered by a foot to convert it into a kitchen. As there will be units all the way around the outside, I wondered if there was an alternative to digging out the whole floor and underpinning? I read elsewhere that it is possible to dig out the floor except for about 50cm from the edge, and this means underpinning not required. If this is true, life becomes much easier as the step can be covered by the kitchen units.

Can anyone verify this, or put a name to the process so I can investigate further? Thanks in advance.

^woody^ · 10 Nov 2012

Find out where you read that, and then make a note to never read anything from there again, as it is complete nonsense

gissle · 10 Nov 2012

Could you raise the ceiling level?

geoffsussex · 10 Nov 2012

I found it on this forum! I will dig out the link...

^woody^ · 10 Nov 2012

geoffsussex said:
I found it on this forum! I will dig out the link...

LOL ... not one of mine I hope

RonnyRaygun · 10 Nov 2012

^woody^ said:
Find out where you read that, and then make a note to never read anything from there again, as it is complete nonsense

Not necessarily Woody. If he is only lowering his floor by a foot, and assuming a 45° spread of load, then stepping the wall in by a foot would allow the spread of load to be maintained, without imparting any further forces into the new basement wall, so underpinning shouldn't be required. 500mm would allow him to build his new wall within the 500mm too.

Alternatively, it's possible to design an RC slab and walls. If the walls and slab are designed for the forces imposed by the surcharge from the existing wall, then underpinning can still be avoided and you will end up with a much bigger basement, as the RC walls can be tight to the existing walls.

^woody^ · 10 Nov 2012

I just can't see it being a practical solution, whatever the theoretical musings may indicate

RonnyRaygun · 10 Nov 2012

^woody^ said:
I just can't see it being a practical solution, whatever the theoretical musings may indicate

I agree with that, but it avoids potential party wall issues though, doesn't it?

tony1851 · 10 Nov 2012

A cautionary word of advice for the OP:

About 2 years ago, a regular student landlord-client of mine started doing exactly what the OP wants to do, under the rear outrigger of a Victorian house. I advised against this because the subsoil is pure sand. Undaunted, he went ahead on a building notice and felt confident it would be OK.

During the first night after work, the side walls of the outrigger slipped in, the house was declared dangerous (road closed off etc) and the tenants had to be evacuated. The outrigger (3 floors) had to be completely rebuilt.

If the soil is not cohesive, once you take away lateral support to the foundations, it acts like a fluid. The downward weight of the walls causes the soil to well-up inside the cellar, thus removing support.

As RR suggested, it is not out of the question to do this if the soil is reasonably firm (eg stiff clay) but even then, it would need SE input for a proper design.

RonnyRaygun · 10 Nov 2012

tony1851 said:
A cautionary word of advice for the OP:

About 2 years ago, a regular student landlord-client of mine started doing exactly what the OP wants to do, under the rear outrigger of a Victorian house. I advised against this because the subsoil is pure sand. Undaunted, he went ahead on a building notice and felt confident it would be OK.

During the first night after work, the side walls of the outrigger slipped in, the house was declared dangerous (road closed off etc) and the tenants had to be evacuated. The outrigger (3 floors) had to be completely rebuilt.

Which reminds me, I take it that wall you showed us a week or so ago didn't fall into the trench?!

tony1851 said:
As RR suggested, it is not out of the question to do this if the soil is reasonably firm (eg stiff clay) but even then, it would need SE input for a proper design.

Agreed, definitely needs an engineer's input.

Even moving the walls in, for peace of mind I would want to reinforce them, and integrate a toe, and construct in reasonably narrow stages. Then cast the slab on top afterwards.

tony1851 · 10 Nov 2012

RonnyRaygun";p="2570405 said:
Which reminds me, I take it that wall you showed us a week or so ago didn't fall into the trench?!

Correct, mercifully! There was a full-width cross-wall internally which I can only assume was well-bonded to the side wall and served to tie it back. I am not tempted to use this as a precedent, though!

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Lowering basement floor - sloping?

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