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Basement Next to Slab

Posted by bigmike440 (My Page) on
Fri, Oct 12, 07 at 12:51

My house is 10 yrs old and built on a slab with block walls. I want to build an addition but with a basement instead of a slab or crawl space.

The problem is that everyone has a different opinion on how to place a basement beside a slab. The structural engineer has come up with some complicated plan that is very expensive and the architect agrees that it is overkill.

I thought the general rule was for every foot you dig under the original foundation you need to come away from the foundation one foot. For instance, I have to dig about 4 feet deeper than the original foundation so my basement would effectively be about 4 feet more narrow than the first floor. The soil is very hard and sturdy according to the soil engineer.

Does this seen right?

Thanks, Mike

Follow-Up Postings:

RE: Basement Next to Slab

Where are you located? Are the walls set on a separate footer, or is the slab structural (i.e., the slab is the footing for the wall)? If on a separate footer, how far below the top of the slab is the footer?

RE: Basement Next to Slab

I am located in Southwest Ohio.

The existing block wall is on a 10"x 16" footer 56" (7 Blocks) below the slab.

The engineer is worried the block might not be reinforced. He wants to install controlfort walls every 8'. Another engineer is going to look at it tomorrow.

Thanks, Mike

RE: Basement Next to Slab

Mike - what did you come up with? I think people here, and future people, would be interested.
I moved here to the Dayton area 5 months ago and am finding that many if not most of the ranch style housing outside of subdevelopments are on slabs. Quite disappointing.

RE: Basement Next to Slab

Re: "I thought the general rule was for every foot you dig under the original foundation you need to come away from the foundation one foot." - You got this right. What you've said is the civil engineering rule of thumb for worst-case foundation conditions, that is, loads are transmitted down and outward at a 45 degree angle. So, one foot down and one foot out. It typically is better than this, but this is the worst case possible. Most construction is based on this to be safe. Just be sure to not disturb the soil that is supporting the existing foundation and pour a concrete retaining wall against the existing soil wall after excavating to hold everything in place. I'd probably place a slab on top of the remaining soil ledge and try to work some sort of storage or mechanical equipment into the space. Good luck!

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