Removal of elevated basement slab??


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Old 07-16-14, 09:14 AM
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Removal of elevated basement slab??

I bought my house ( built in 1920) in Sept 2013 with intentions of finishing the basement. I'm finally getting to it and to get more space I wanted to see if i could remove this elevated slab down there. It's 2' high, 6' deep and 15' wide. I dug against two sides of the interior wall on top of the slab to see how far down the foundation/footing goes, only to find that there is no foundation footing. About an inch under the top layer, the walls just end with only dirt underneath. I am planning on installing a french drain, so i started digging on the other side of the basement where the ceiling hieght is 7'6". The wall goes down about 6" but then no foundation footing there either. Under the floor are a lot of what look like river rocks. The previous owner added an addition to the back of the house in 1992, which buts up against where the rasied slab ends(the orginal part of the home). Through the open window in the picture is where the addition is. Is this how homes were built back then, with no footings? Or am i just not seeing it? And would it be safe to remove this slab? I'm assuming no since if i do, the back of the house will be supported by wet mud. There are some very small hairline cracks in the poured concrete basement walls, but no toher signs of the house shifting. And my property does slope towards the front slighty. Any and all comments are apreciated. Thanks.
 
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Old 07-16-14, 10:04 AM
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It is not surprising that there are no footings in a house of that age. Footings just support the long term load of the home framing. The foundation probably weighs more the the house will ever weigh plus the people rattling around inside. Footing requirements are in codes to give a minimum starting point for code standards with minimal soils (bearing strength). - That fact that the rigid foundation walls have minimal cracks that are probably due to long term shrinkage and to low quality of the original concrete means it is serviceable.

Why would you want to remove the slab? - Too increase the lower square footage? or to get access to the area under the 1992 addition that may have been built according to some code.

I suggest you decide what you want to do and contact a local engineer for assistance to see if it can be done with what you have. I good engineer will at least do some probing around the addition to determine the real conditions an what can be done. Considering there no footings under the walls in the elevated slab the walls adjacent could move in if you have what you describe as

The biggest factor with basements is the lateral load from the soil (saturated).

That "elevated slab" you are considering removing provides lateral support for the walls it connects to or abut. Since you describe your soil as "wet mud", you could have problems unless the drainage around the home is improved by surface improvements and drain tile.

Dick
 
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Old 07-16-14, 12:11 PM
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Thanks for the info. I was just hoping to get more square footage in the basement by removing the slab, but now that i know whats under it, i wont be removing it. Since there is no footing, would it be a bad idea to install a french drain around the perimeter of the basement? The foundation walls go down about 6 inches so i would install the french drain as close to the wall and surface as possible.
 
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Old 07-16-14, 07:28 PM
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If you have a water problem, you're looking at the wrong solutions. Leave the slap alone. If anything, dig outside & seal the foundation with a membrane.
 
 

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