Pouring concrete under existing foundations

[John Williamson]

I'm building an extension on a sloped site which requires some underpinning. Just wondering if anyone has the best method to pour underneath the existing foundations as shown in this picture after the face shuttering goes on. Had various thoughts like creating a wedged gap to pour the concrete into and even core drilling a hole to pour the concrete through from above.

Any ideas or pointers very gratefully recieved

 

[noseall]

I'm building an extension on a sloped site which requires some underpinning. Just wondering if anyone has the best method to pour underneath the existing foundations as shown in this picture after the face shuttering goes on. Had various thoughts like creating a wedged gap to pour the concrete into and even core drilling a hole to pour the concrete through from above.

Any ideas or pointers very gratefully recieved

View attachment 342840

At what height will the new pour finish? If it's lower than the underside of the concrete catch area, then you need to build a stepped shutter, in order to be able to build up the concrete levels under the existing. Either that, or pour the concrete allowing it to set lower, then build a retainer and pour a later concrete catcher.
As long as the conc' is wet enough and you have the right tools, you can pour through a small gap.

 

[John Williamson]

At what height will the new pour finish? If it's lower than the underside of the concrete catch area, then you need to build a stepped shutter, in order to be able to build up the concrete levels under the existing. Either that, or pour the concrete allowing it to set lower, then build a retainer and pour a later concrete catcher.
As long as the conc' is wet enough and you have the right tools, you can pour through a small gap.

Great point, sorry I should have mentioned. So the pour will finish 300mm below the existing. So probably best to finish the pour and then tackle the "wall" with a solid and flat surface beneath it?

That would make sense, and the rebar will run through both to keep them tied together

 

[Ivor Windybottom]

You could make formwork to include a pouring spout alongside the existing. Fill it all up then a couple of days later remove the formwork and cut or chip away the cast of the spout shape, leaving a vertical joint between the old and new. This would become a movement joint, you could put sleeved rebar or movement ties through the formwork before casting.

Or if you want a rigid joint then could you just create a raised step in your new foundation alongside the old and fill the whole lot in one go?

 

[John Williamson]

You could make formwork to include a pouring spout alongside the existing. Fill it all up then a couple of days later remove the formwork and cut or chip away the cast of the spout shape, leaving a vertical joint between the old and new. This would become a movement joint, you could put sleeved rebar or movement ties through the formwork before casting.

Or if you want a rigid joint then could you just create a raised step in your new foundation alongside the old and fill the whole lot in one go?

Do you mean like a birdsmouth as below? I was thinking of doing all in one go, but this will be my first pour and the thought of collapse of the wall formwork would be just one terror too many for me.

 

[Ivor Windybottom]

Why not? Just prop it up well.

But decide whether you want that joint between old and new to move or not and tie as needed. If a movement joint then you want a vertical break with movement ties across it.

 

[John Williamson]

Why not? Just prop it up well.

But decide whether you want that joint between old and new to move or not and tie as needed. If a movement joint then you want a vertical break with movement ties across it.

You're right of course, only my nerves holding me back. I'll see how solid I can make the form and can either fill in one go or fill in parts

 

[John Williamson]

Following your advice @Ivor Windybottom Concrete will arrive on Friday so I'll update with any disaster or success story.

 

[George_engineer]

Pour up to 75mm below the underpinned foundation. This leaves a gap. Then 24hr later fill the 75mm with dry pack. Wet pour concrete will shrink a bit so doing it this way minimises the risk of it not properly supporting the existing foundation.