Foundations

[Benny79]

Hi, First post, so please go easy, I’m planning an extension,BCO have asked for a test hole to understand existing foundations on house. So the existing foundation is 1300mm deep from ground level to top of foundation, it’s 750mm deep to bottom of foundation so 2m in all roughly, just dug by hand today. Question, can I expect that the new foundation is the same depth and thickness, or could they say go deeper?
I’m being told 2m is about the limit of most mini diggers?
Thanks in advance.

 

[^woody^]

Firstly, the foundation is the wall and concrete below ground, so it's a bit confusing to hear it described as you have. You have a concrete strip and the masonry wall built off it, and you would normally state the total depth of the trench as the foundation depth.

Foundations that deep indicate a particular problem with the ground soil, or if the ground is ok the house foundations are wrong and may not need to be replicated.

So it should be the ground that determines the foundation depth so that's what the inspector should be interested in.

2m strip foundations become uneconomical and risky, so if that depth foundation is required then piles may be a better option. Or even a raft for problem ground.

If you do go for a concrete strip, consider a mass fill up to 225mm below ground level. This saves messing about building in a trench or even trying to concrete the bottom of a deep trench.

You may need shoring for the trench sides, and if the ground is loose, be very careful of collapse.

 

[Benny79]

I think we in same page, I basically have to dig a 2m deep hole to fill the bottom 750mm up with concrete make the foundation, if he said like for like.
The ground around the new extension is flat, however a few metres away it starts to slope off which would explain the deep foundations for the house.

Also the foundation on the house has an exposed nib 300 wide, I’m guessing the walk is 300mm and the same nib in the inside, that would make the foundation on the house 900mm wide by 750mm deep, that sound right?
My width is 450mm unless I’m asked to change.

 

[^woody^]

Don't you have any plans to work to?

 

[Benny79]

Hi Woody, yes I have plans all signed off, I’m in the scenario where plans say 1m deep or as far as the BCO sais to go.

BCO Tell me to dig a test hole to determine how deep, after a discussion this morning he’s asked me to continue like for like, so a 2m deep trench with the bottom 750mm being filled with concrete to form the foundation.

of course once it’s dug he will review and could ask me to go deeper, if that’s the case I may have to dig by hand, I suppose that’s the risk anyone takes?

 

[garyo]

What were the soil conditions as you progressed down through your 2M hole? When did you hit clay or anything firm?

I wouldn't fancy standing in a 2M deep trench for any length of time...

 

[Notch7]

BCO Tell me to dig a test hole to determine how deep, after a discussion this morning he’s asked me to continue like for like, so a 2m deep trench with the bottom 750mm being filled with concrete to form the foundation.

most builders these days use the trench fill method -so they would dig down to 2.0m then fill with concrete to finish at a suitable height to allow one concrete block and maybe 1 or 2 brick courses. the cost of the concrete is set off against the labour of laying blocks in a trench. (apols Ive just noticed Woody explained mass fill)

a 1.5 ton digger will go about 2.1m, a 3t digger will go 2.7m

1.2m depth is the maximum safe depth for working in an unprotected trench. before digging see if your access can fit a bigger digger -and if necessary hire one if you need to go deeper -dont risk your life in a 2metre deep trench.

are there any trees or hedges nearby that could affect foundation depth requirements?

some BCs will require suspended floors on extensions with deep footings if the reason is shrinkable clay

 

[Benny79]

Thx All really helpful, I’ll likely have to order ply in order to shutter the sides, digging a 2m hole deep is grim, especially a small hole, I imagine diggging a big hole deep is easier!
Was thick clay the first 1.5m that’s sticks to everything, and smells like your on the beach, after that changed to A more sandy clay that was easy to dig through, will update more once I start.

 

[noseall]

digging a 2m hole deep is grim

So is trying to line up a 300mm wide wall centrally on a 450mm wide foundation.

I've only ever seen and used 600mm digging buckets (cavity wall) and 450mm for single skin.

How thick are your proposed walls?

 

[Benny79]

Good point noseall, the brickie I have is not the smallest of chaps!
The wall thickness is a standard 300 wall, inner 100mm block, 100mm insulation and 100mm brickwork.
If I can I will bring up concrete as far as I can, but that would make it very deep probably about 1.7m deep! If I have to level the foundation with the old I was thinking of laying trench blocks to make life a bit easier just to get out of the ground easier?