filling voids above underpinning
filling voids above underpinning
(OP)
Hello-
My office is underpinning a c.i.p. concrete column spread footing and adjacent continuous concrete wall footing with concrete, placed in phases. Comments in a previous thread on this topic were very useful but I have an additional question.
In plan, the column footing to be underpinned is about 3.5' square. The underpinning conc will be about 4.5' tall. To finish the finalt two or three inches of the underpinning, usual practice here is to use steel wedges every six inches or so and then dry pack grout to fill the remaining voids below the existing footing and above the cast-in-place underpinning. The wedges are driven first, and act to "preload" the underpinning to try and minimize any vert deflection after the existing footing is fully supported by the underpinning. Grout is then added to fill all remaining voids and provide full bearing below the existing footing.
I have noted previous comments in another thread on how light, in actuality, the load on the underpinning is.
However, I would still like to get the full bearing area under the existing footings filled solid. The relatively large plan area to be grouted makes getting the grout in a problem, though. If we pump it, it will just run everywhere. Don't see how placing the grout in dry-pack or liquid form by hand can get all the area required, either.
One possible solution might be to omit the grouting and just carry the phased concrete underpinning all the way to the underside of the existing footings in a single lift, but I would be concerned that shrinkage of the curing underpinning concrete would tend to allow deflection of the underpinned area. Even a relatively small vertical differential movement between the underpinned and non-underpinned areas may cause cracks in the existing walls.
This concern might be minimized by placing the underpinning concrete in two separate lifts, but doing this may will probably take a lot of time.
Is there a practical solution to this problem that one of you may have used successfully? Maybe I am being too fussy, I don't know.
My office is underpinning a c.i.p. concrete column spread footing and adjacent continuous concrete wall footing with concrete, placed in phases. Comments in a previous thread on this topic were very useful but I have an additional question.
In plan, the column footing to be underpinned is about 3.5' square. The underpinning conc will be about 4.5' tall. To finish the finalt two or three inches of the underpinning, usual practice here is to use steel wedges every six inches or so and then dry pack grout to fill the remaining voids below the existing footing and above the cast-in-place underpinning. The wedges are driven first, and act to "preload" the underpinning to try and minimize any vert deflection after the existing footing is fully supported by the underpinning. Grout is then added to fill all remaining voids and provide full bearing below the existing footing.
I have noted previous comments in another thread on how light, in actuality, the load on the underpinning is.
However, I would still like to get the full bearing area under the existing footings filled solid. The relatively large plan area to be grouted makes getting the grout in a problem, though. If we pump it, it will just run everywhere. Don't see how placing the grout in dry-pack or liquid form by hand can get all the area required, either.
One possible solution might be to omit the grouting and just carry the phased concrete underpinning all the way to the underside of the existing footings in a single lift, but I would be concerned that shrinkage of the curing underpinning concrete would tend to allow deflection of the underpinned area. Even a relatively small vertical differential movement between the underpinned and non-underpinned areas may cause cracks in the existing walls.
This concern might be minimized by placing the underpinning concrete in two separate lifts, but doing this may will probably take a lot of time.
Is there a practical solution to this problem that one of you may have used successfully? Maybe I am being too fussy, I don't know.






RE: filling voids above underpinning
The usual method is a drypack grout consisting of one part cement to three parts sand with only sufficient water to allow the mixture to form a cohesive ball. This is then rammed into place with a 2x4.
Most underpinning is done under walls so there is a solid surface to ram against. In your case you may need to do the grouting in stages as the footing is open on all four sides. Or you could provide a form at one or more edges as a barrier.
RE: filling voids above underpinning
The same method is used to drypack an underpinning pier along a wall.
When you pour an underpinning pier, before the concrete hardens, make sure that a 2x4 can reach all the way back to the rear of the pier so that the drypack can be properly compacted.
Refer to other underpinning threads for discussion on concrete shrinkage.
Pouring the concrete up to the bottom of the footing (without drypacking) is a last resort in my opinion. I've done it occasionally, but don't recommend doing it.
RE: filling voids above underpinning