huangyhg

filling voids above underpinning
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.
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squidale:
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.
usually, steel wedges are not used. a column footing as small as yours would probably be underpinned using two or three underpinning piers. when underpinning a column footing, you should not undermine more than about 25% of the footing at any one time (unless you have totally picked up the column and its total load in some other way). the morning after a pier is poured, you should drypack as redheadfff"> described. i usually do not use steel wedges. wedges can interfere with installation of drypack. plus, by the time you installed the wedges, you could have drypacked the underpinning pier. when you drypack, you push the drypack material back into place with the end of a flat 2x4 toward the back of the pier. then you hit the end of the 2x4 with a small sledge hammer to ram or pack the drypack tight. keep adding drypack and packing it into place with the hammer until the exposed bottom of the footing is completely drypacked. this will properly transfer the building load evenly to the underpinning pier.
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.
thank you for your comments.

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