Replacing Foundations - advice...

15 Jan 2018
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United Kingdom
Hi guys,

Any advice or comments on the following would be greatly appreciated,

I have knocked down a single story small extension, it will be replaced with 2 story extension.

The existing foundations cannot be reused, they are not deep enough. :(

Please refer to photo for the next part...

The foundation wall with RED lines either side needs to be dug out, the new concrete foundations need to be 60cm wide.

I would like to keep the old concrete floor intact as the width of extension stays the same as the original single story.

I need to dig trench 15cm into the edge of the existing concrete floor (refer to green line).

How would you go about removing 15cm off the edge off the concrete floor slab to avoid having to completely replace it?

I suspect if I just cut the end off completely I damage the blue dpm and the whole floor will have to be replaced to avoid any damp? ..or just cut it from concrete from inside and try to avoid damaging dpm, then trim dpm after?

Also - I suspect the row of bricks which can be seen in the photo under the edge of the floor slab are holding in the sand/gravel , If I cut 15 cm edge off of the floor sand will escape and cause floor to crack/move over time?

Thanks guys!!

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Surely the slab will need to be replaced with an insulated slab anyway.
Any advice or comments

My advice would be to make sure you remove enough concrete to be able to work. It can be successfully repaired later on. The last thing you want is to smash your knuckles whilst digging or accidentally lift the slab whilst exiting the trench with a digger bucket.

Quit trying to be too precise at this stage as it can make life difficult and slow. You won't save money long term.
I suspect if I just cut the end off completely I damage the blue dpm
Cut part way through the concrete (80%) and remove the bricks to the side. Drive a bolster into the cut, the concrete should peel away without damaging the dpm. As noseall says, give yourself some room. I'd cut it back 250 or 300 and just refill with concrete afterwards.
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Dig it all out and start again. If it's a small extension a couple of cubes of new concrete isn't going to be much, and as others have said you can insulate properly.
Hi thanks for comments, I think I will hack off edge for now just to get foundations in - and re assess later.

The extension goes a further 4 meters outside of this photo, the new joining slab will be insulated, however the existing slab in the photo extends further inside the house - approx another 3 x 4 meters, I could hack the end out in the photo but not sure if its worth it if there is further 3 x 4 meter length un-insulated inside?
If you have to insulate the outside bit, but not the inside bit, won't you have to step up to get into your extension?
good call, I was hoping to put the insulation under the new concrete slab.

I read online that you can : "Insulation goes on top of the DPM and around the edges. Next, the concrete is poured. "

Under the existing old slab is DPM > sand/rubble > Mud.

If I dig out the new slab floor deeper to accomodate celotex sheet on top of dpm - just means DPM sides will be deeper to wrap around top of the new concrete slab - I don't know if this would be acceptable? can anyone foresee any issues of doing it this way?

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