Existing Footing Size Verification

[BadgerPE]

I am looking at a project in which the owner is considering adding a story onto his existing one story building. Therefore, loads will be nearly doubled +/- so I will be forced to look at foundation capacity.

What non-destructive methods are available to determine existing spread footing size below an existing slab? Assuming the footing is near the bottom of the slab, would GPR or some other scanning method be possible? Would any of these methods be able to establish reinforcing as well? What about small diameter (<1") drilling to establish footing extents and depths? This would obviously not locate the reinforcing, but would at least let me determine if there is sufficient bearing area for the footing.

 

[EngineeringEric]

If it is a house, you may be able to determine what size you would need and that minimum size may be smaller than code required minimums. I would still verify the exterior as those are easy to check with a probe. The interiors... pending load may not matter.

Example:
If your wall load is 1000plf, you know you have a thickened slab (assumption on my part), then i am sure the width of said footing must be wider than 8" for 1500psf capacity. a 2ft minimum would yield 3000plf capacity.

 

[jayrod12]

I've done the drill holes at 6" intervals away from the wall and drive down a piece of rebar etc until I've reached the edge of footing. As you indicated, does not help with reinforcing but gets you an idea of size.

 

[Ron247]

I would dig on the outside first. Then you will at least see the wall location to footing relationship, some depth information and if the footing extends beyond the face of wall. I have had people tunnel under the footing to get a sense of how wide the footing is without damaging the interior. The problem sometime with boring a slab is when the footing sides cave in. The overpour width is misleading as to the true footing width.