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Basement Floor Heaving

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Hello All:

On a few of the older homes that I have inspected the basement floor (concrete) has pushed up. When you knock on it it's as hollow as a drum.

I would guesstimate that this is due to freeze/frost conditions pushing the floor up however, I would have thought the floor was below the frost line. Also, wouldn't you need an active source of water to lift the floor. Some areas are pushed up a foot or more.



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Download Attachment: icon_photo.gif basement2.jpg

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I see this condition almost daily. Typically, a very thin layer of concrete was added later only as a soil covering. No gravel, vapor barrier or reinforcing mesh is present. As the concrete cracks and settles over soft soil, the other edge lifts. There is sometimes soil erosion from water movement and heaving as water rises under the concrete.

It can also be a result of failed waste lines washing out soil under the slab. Looking at the second photo, showing pretty ancient cast waste lines penetrating the floor, I would suspect this may be a probable cause.

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I see it daily also. City infrastructure backups can cause the old clay tiles to leak & flood the subgrade soil under the concrete. Bill described old concrete floors perfectly; it is usually just a skim of weird flinty mud over the dirt. When you flood the area, it bubbles up.

Old city sewers back up, clay tile bell & hub joints leak, & floors buckle. I always tell folks to videoscope the sewer to determine it's condition.

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There is another possibility. High water table intermittently. The water pressure cracks to concrete (stains), then goes back down and compacts the soil underneath causing the hollow sound. This is one of the rare cases where I recommend interior drains and a sump pump. The entire floor may need to be removed and replaced but after the drainage is added.



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Hi All,

Rob, that's what I thought too when I first read the query. What's the soil composition there? Any bentonite?

I've never seen one, but I've been told that in some parts of Colorado, where there is a lot of betionite, houses with basements there have crawlspaces beneath their basement floors to allow for seasonal movement of bentonite that would otherwise cause basement floors to heave without them.



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Mike: I don't know the soil comp but it is some type of clay I'm sure. The areas are near the mouths of inland river systems that dump into Boston Harbor. Both North and South of Boston ("South Showah and Nawth Showah"). I even saw it raise an under-building parking area (asphalt) so that the under-area looks like an upholstered pillow in areas. That building is modern-vintage on pilings (to bedrock)and had no problems otherwise.

If you like soil issues and buildings, check out:


Those buildings in Boston's "Back Bay" are also regularly inspected by Boston-area inspectors like myself. It helps to be knowledgeable about the history of Boston and the landfill history when you inspect in those areas.

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