Garage floor repour. Sub-Base prep. questions

jrz126

Hi all,

I'm in the process of repouring my garage floor. It wasnt properly backfilled when it was installed in 1970.

Here's a pic of it right when I bought the house:


Here's a pic after the old stuff was removed:


The top of the concrete slab started at the bottom of the first course of bricks. There was an air gap of 12-14" around the edges. Luckliy they had it tied into the block with some rebar.

My original plan was to rent a plate compactor and run it over the dirt. Then bring in a triaxle of the crushed limestone and backfill/compact it up to grade.

The ground is still really soft though. I can pound my prybar (3/4" diameter solid) down 4 feet with a sledge hammer in only 5-6 swings. I only weigh 155 lbs too so its not because I'm Mr. muscle. Thats in the center of the floor too. Its even easier at the edges.

What should I do? Do I need to worry about compacting all of it? I'm planning on rebaring the crap out of the new slab as well as installing some PEX tubing for radiant heat.

Other info:
House is in Erie, PA so there is freeze/thaw.
There is a full basement on the house.

jrz126

Anyone?

rustyjames

There was a 14" void under the slab? That's some serious settling. The slab shouldn't be tied into the block. I would run over what you've got with a vibratory plate or maybe even a rammer. Add about 10" of suitable fill, well tamped, and then, I'd lay out some geogrid and backfill on top of that with clean 3/4-1" stone (about 4-6" depending on slab thickness). And don't forget the vapor barrier.

jomama45

Everything Rusty said above, although I'm not sure I'd necessarily use the grid unless the soils are very unstable. Make sure to compact any and all gravel in about 4" high "lifts" if using a small, walk behind compactor. The machines are often times advertized at compacting 12" max., but won't in real life.


As for the soft material, it could very well be sand from the explanation & pic. It was very common here in that era to use sand backfill. It generally faired well IF it was flooded with water before the floor was poured.

I've seen voids of a few inches before, but I can't recall ever seeing that much!