Foundation underneth existing doorway

[Emtoucan]

Hi,

I'm currently having a supporting wall removed, which will need an RSJ. This RSJ will rest on a newly built wall which is currently a doorway.

The problem was that the foundations did not continue under the doorway, which means I've had to dig this out myself.

The foundation size is 45cm width, 70cm length and 60cm depth.

I've had to dig through about 10cm concrete site base and then another 50cm down in the earth. Can I use this 10cm concrete as part of the 60cm or would it have to be 60cm into the earth?

Also from what I've read the concrete mix should be C15P - 1:2:6. Is this correct? Also can I use a ballast mix instead of buying the sand and aggregate seperately, or would get them seperately be better.

Thanks for the advise.

 

[alpcon]

What does the engineer specify for the new footings in the notes submitted to building control

 

[noseall]

it is likely that you will have to dig down at least 600-800mm or so, as this is a recognised minimum. they (b.c.) could make you dig down to 1m.

if the ground is not virgin earth, then who knows.

use ballast, which is sand and gravel mixed. mix the concrete 6 ballast to 1 cement.

 

[Emtoucan]

The plans state that the new foundation should be 450 wide by 600 deep, which is what i've been working to.

I don't know how they think I could get any deeper as I can hardly reach the bottom now, let alone get another 400mm out

I'm just not sure if the 600mm depth can be measured from the top of the existing concrete or from the start of the earth underneath.

When you say virgin earth, I'm assuming you mean it not mixed with rubble etc. The earth has a few stones and is very dark. Looks good to me.

Thanks for the advise, I'll just get ballast and cement, although from the calculations I've seen, to fill a 0.19 cubiuc metre hole requires a lot of concrete. I worked it out to be about 12 bags of ballast and 2 of cement. Would this sound about right?

thanks guys

 

[noseall]

'dark earth' usually means fertile soil, not good.

sandier or undisturbed ground that is not part of the growing matter is what you want.

your quantities sound ok to me. get another bag of cement just in case.

 

[Emtoucan]

OK thanks.

Last thing, is it OK to add the concrete a bot at a time? I don't need to have it all ready in one go do I?

Is there a maximum that I shouldn't wait in between layers etc.

 

[noseall]

the concrete should be completed in a single pour and definitely not 'laminated'.

 

[Emtoucan]

Thanks for all your advise.

The building inspector came round yesterday and approved the depth. He didn't even bother to measure it! I could have saved time and my back and no bothered to dig the final 5cm.

I couldn't believe how much concrete it took to fill it either. Had to make it in batches in the end, but only left it about 10 minutes between each fill.

Well the wall comes down on Saturday, so I suppose that's when the fun start.

Thanks again.