foundations extension - join 1m deep to existing older shallow.

DaveSpud47 · 5 Dec 2021

Can anyone advise on how extension foundation trench should be dug to butt up against our existing house foundations?

House is old 1800's, old footings for two story seem to be some compacted red brick/rubble, quite shallow, soil has a lot of clay.

New foundation is to be 1 meter deep, what is the best way to do this to butt up with existing old shallower foundations - I suspect not to dig straight down as it could collapse?

Do I dig out foundation at 45 degrees slope away from house foundation to bottom of trench to avoid collapse of old foundation? - but then new concrete footing in this section will be not as deep as 45 degree wedge of mud underneath?

I cant see a way to get all the earth out without risk of collapse?

cheers

stuart45 · 5 Dec 2021

Like this.

DaveSpud47 · 5 Dec 2021

Thanks for photo, its a bit pixelated, could you confirm - it looks like the end of trench is sloped down 45 degrees to bottom? ...is it under the existing old foundations?

stuart45 · 5 Dec 2021

It's cut down straight, not at 45%. It goes just under the existing foundation and the new concrete goes just above the bottom of the old concrete.
You shouldn't slope the ground at 45% as you will have a weak spot at that point.

RonnyRaygun · 5 Dec 2021

You should also cut the existing foundation flush with the masonry, and add a slip membrane alongside the existing wall/foundation to allow for any potential differential settlement of the new foundation

cdbe · 5 Dec 2021

If you're worried about the existing foundation, or the soil below, "falling out" and into the new trench then buy some line pins ( think they're actually called fencing pins) and hammer them in, in a row, to retain it.

^woody^ · 5 Dec 2021

Vertical trench, butt up. Simple.

^woody^ · 5 Dec 2021

RonnyRaygun said:
You should also cut the existing foundation flush with the masonry, and add a slip membrane alongside the existing wall/foundation to allow for any potential differential settlement of the new foundation

Lol. What happens if you leave out this membrane on about 200 extensions and find that the new foundation has not slipped? Is that a good thing anyway?

Asking for a forgetful friend.

RonnyRaygun · 6 Dec 2021

^woody^ said:
Lol. What happens if you leave out this membrane on about 200 extensions and find that the new foundation has not slipped? Is that a good thing anyway?

Asking for a forgetful friend.

Is your, erm, “forgetful”, friend the same one who says, “I’ve done hundreds of extensions just like this and I’ve never / always, blah blah blah”?

I had a “friend” once who used to paint his beam splices before bolting them together. I’ve no idea if they ever slipped either

^woody^ · 6 Dec 2021

This is bombshell news. I'll need to ask my other friend who inspects these trenches and his friend who does a few specifications, and they will need to ask their friends and friends of friends.

In the meantime, what's the actual point of this slip plane at this location? Do I need to get some shares in a polythene company in case this revelation becomes a thing?

Diver Fred · 6 Dec 2021

So, when you dig the foundations out dig out to the existing. When BC come to inspect tell them that you will dig out under the existing foundations to the full width of existing and depth of the new on the day of the delivery of concrete. In the morning of the delivery dig out the remaining material. The concrete must support the old foundations so order sufficient to fill to at least the top of the existing foundations.

RonnyRaygun · 6 Dec 2021

^woody^ said:
This is bombshell news. I'll need to ask my other friend who inspects these trenches and his friend who does a few specifications, and they will need to ask their friends and friends of friends.

In the meantime, what's the actual point of this slip plane at this location? Do I need to get some shares in a polythene company in case this revelation becomes a thing?

It's already a thing in most of the places I've worked, and certainly among most party wall surveyors I've worked with recently.

The reason is that an allowable soil bearing pressure allows for a reasonable amount of settlement to take place (say, 10mm), usually over a number of years. If a building has stood for many years, the chances are the soil under the foundations will have settled whereas the nearby soil won't have. When new foundations are laid (and a building constructed on top of them) we should expect some settlement to take place over the coming years. This can be allowed for by using a slip membrane between the existing and new foundations (two layers of Correx is appropriate), which prevents the existing and new concrete bonding together and potentially a sudden movement in the future. A suitable wall starter is also required.
It might never be needed, or it might help to prevent a future issue. Either way, no harm in installing it at a miniscule cost.

Happy to help ^Woody^

^woody^ · 6 Dec 2021

No help at all.

Any new foundation may settle regardless of any slip plane, and if it does it will do so whether a strip of polythene/slip plane is used or not. Complete waste of time.

And this "sudden movement"

you mention ,well if that does occur, or even gradual movement, that is going to be a significant problem unrelated to whether a slip plane is formed or not and a slip plane is not going to have any material influence in that - wont stop it, and lack of such a slip plane wont affect the existing building or existing foundations.

DaveSpud47 · 6 Dec 2021

wgt52 - thanks for comment I suspected this might be the best option.

Also...
Photo shows old foundations, red line is to be height of foundation concrete - below the old shallow foundations there is a stack of paving slab style rock at depth -about 1/2 meter below surface , I presume original builders probably put some of the natural sheet rocks I have been finding in place to help aid foundations, I can see the start of the stack at around 3ft out from the old house into the trench, I don't think I should break these out flush with the wall, can I dig out the mud on the day and leave rocks in place just concrete over?

RonnyRaygun · 6 Dec 2021

^woody^ said:
Any new foundation may settle regardless of any slip plane, and if it does it will do so whether a strip of polythene/slip plane is used or not.

True, but a slip membrane will allow settlement to take place over the whole footprint of the new extension without any bonding between the footings taking place and potentially holding up the natural settlement process locally, which may lead to non-linear cracks opening up.

And this "sudden movement" you mention ,well if that does occur, or even gradual movement, that is going to be a significant problem unrelated to whether a slip plane is formed or not and a slip plane is not going to have any material influence in that - wont stop it, and lack of such a slip plane wont affect the existing building or existing foundations.

Settlement that is designed in shouldn't be a significant problem as making good can be carried out during the process or once the settlement has stopped. And I just think that if you're spending thousands on concrete, a few quid on some plastic sheet isn't going to break the bank.

But anyway, you keep doing what you're doing

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foundations extension - join 1m deep to existing older shallow.

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