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Voids Under Basement Slabs

Posted by GreenDog194 (My Page) on
Tue, May 22, 12 at 2:40

We are remodeling our 1950s two level house. Preparing for adding a bathroom in the basement and ripping the wall board off the wall revealed issues with the foundation. After they cut open the slab for the new plumbing they could see 6 inch deep voids in parts under the slab. We had a tech come out and radar (or is it sonar) the slab. Apparently, 50% has voids. The next step is to core drill holes in the slab, especially near footers and posts, to see what kind of trouble we have going on here (and hopefully not causing a collapse during the investigation). I am hearing that an option might be to pressure inject cement-like slurry into the cored holes to fill the voids. Another option would be to just re-do the entire slab 1500 sq ft (might as well get that addition in and radiant heat in the slab while we are at it -- so much for the budget -- argh!) The problem might have been caused by our downspouts and their failed tight lines to the sewer. There might be water coming from the city lines in the street too (there is a depression in the street in front of our house that keeps needing fill). The basement has always been dry.

Any wisdom or condolences to share?

Follow-Up Postings:

RE: Voids Under Basement Slabs

If the problem is running water, the settlement will continue. But if six inch voids in 50 years is representative, I would stabilize it with slurry, fix my downspouts and forget it. I've put in radiant heating and find it much overrated.

I once came across a nice 80-year old home where the owners had decided to lower the basement floor for a rental apartment. Shortly after they jackhammered the concrete away and started digging they hit an underground stream. Before they could find sump pumps, the water rose five feet, leaving a "high-tide" mark on the walls. Now, two pumps operate 24-hours a day and will continue to do so until Lake Ontario runs dry.

RE: Voids Under Basement Slabs

If it is from runoff it may be a problem, but if it is from natural settling it is not an issue and nothing really needs to be done (especially if the slab has not cracked).

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