How to fix a sagging slab?


Old 02-08-06, 06:33 AM
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Question How to fix a sagging slab?

I've lived in my rancher for six years, it is about 40 years old. The south end has a basement, the north end is on a slab and has always sagged a bit. It seems to be getting worse lately, with one to two inches of sag in places. Cracks are appearing in the ceiling between the drywall sheets because the interior walls are sinking with the floor.

I had a contractor come look, and they drilled underneath the floor in two spots. The concrete was in good shape, but directly below it was a mix of soft grey dirt and some small white stones / gravel. About a foot below that is the ground, which seems solid. I didn't get a great explanation, but the contractor seemed to believe the grey dirt/gravel mix was strange or uncommon. He was at a loss for options on how to fix it, and is bringing an engineer today.

Can anyone give me a better explanation of what this dirt/gravel mixture might be, and what can be done to repair the slab? I guess a drastic option would be to dig everything up and pour a new slab, but that's probably 800 square feet to redo, and sounds expensive.
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Old 02-08-06, 08:46 AM
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Should be able to pump filler in to both fill the void and raise the sagging portion. Done all the time in Florida under slabs that are settling due to sinkhole activity. My Mom had to do it 4 or 5 years ago. They refer to the filler as "grout" and it is applied by boring holes at an angle under the slab and then pumping this grout slurry in under high pressure.

More info:
Old 02-08-06, 02:52 PM
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Thanks for the links. I'm in Virginia, but I'll see about those. There aren't any "voids" that I can tell under the floor, it's not wet or washed out. It's just soft dirt.

The contractor came again today with an engineer. The plan they came up with was to dig half a dozen or so 3' deep holes to where the dirt is solid, fill them with concrete and rebar parallel to the floor, and basically create a bunch of little pillars wider than the holes in the concrete of the floor. The pillars would hold up the current floor where it is. Then they would fill in concrete on top of the floor to cover the sag and make it level and solid.

Since they would have to take out all interior walls in a 23'x33' area, it would essentially mean remodeling a third of my house, including the kitchen.

Any comments or suggestions?
Old 02-09-06, 04:11 AM
Join Date: Jul 2005
Location: Ottawa, Canada
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The concrete pillars sound good, but I don't agree with the idea of pouring concrete overtop of concrete to fill the void. From my experience it nevers bonds well.

Concretemasonry would probably have the best opinion.

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