void under garage floor

I'm investigating the feasibility of modifying my garage roof to increase the headroom in order to allow the installation of a 4 post lift to store a classic car. It's a double garage about 30 years old of single skin brick construction with a trussed roof.

I'm in discussions with a structural engineer regarding the roof modifications that will give the greater headroom required. I'm also looking to confirm there won't be any issues with the floor. As I have no information on the garage floor construction the engineer suggested using an SDS drill to ascertain the floor's thickness. I've done this and it's 20cm. The colour of the powdered concrete was pretty uniform and the resistance to the drill seemed pretty constant all the way down. No indication that I hit any rebar - perhaps I was (un)lucky? But there's then a 19cm void below this before I hit mother earth. I've used a borescope to confirm these distances.

I was not expecting such a void. Any ideas what sort of garage floor construction this might be?

It could be a raft construction and I would drill at least two more holes a few metres apart to give you a better idea of the underlying ground conditions. It may well be that you have just found a pocket that has slumped but you are limiting your self to getting the full image with just the single hole.

So have drilled one more hole and then attempted to widen it a bit. The top surface is tough with small rocks/pebbles within the concrete. Didn't go very deep yet.

I was able to drill all the way through with a small diameter (8mm?) SDS drill but when I tried to enlarge the hole with a 13mm bit I got about 16cm in and couldn't go any further. At one point the bit even jammed.

Using the borescope it looks like either a rock or a piece of rebar - can't really tell.

Any further thoughts?

Forgot to to add - the concrete is hard as nails; it doesn’t come away in chunks but more like chips.

The hole was drilled about 2 meters away from the first hole and the void appears to be 22cm in depth below the 20cm of concrete.

I tried to use a long round file to ascertain if what I can see is rebar - result was inconclusive.

Test for rebar using a small magnet on length of string.

bernardgreen said:

Test for rebar using a small magnet on length of string.

Click to expand...

Gold star to you; you're clearly more than just a pretty face.

Found a small bar magnet and not only does it stick to the obstruction it also stops 2 or 3 times on the way down which suggests there are multiple layers of rebar. Hard to draw any firm conclusions though.