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I understand the principles and practice of putting in a basement perimeter drain... but I've got a slightly irrational paranoia about the concept of cutting the basement floor free that way. Intuitively, it feels like I'd be removing part of the support for the walls... and these aren't even the fielstone and brick walls at the other end of the house; this is poured concrete, probably with rebar, so I'm sure I'm worrying unnecessarily.

Well, pretty sure.

I'd just feel better with a "yes, the floor is just a floor; stop worrying" consensus... thanks.

For what it's worth: The leakage isn't so much a general problem, but two specific sources -- somewhere in the bulkhead stairs enclosure or how it joins the foundation (coming in past the door), and from somewhere between the new basement of the 1945 extension and the bulkhead that stabilizes what was originally the back wall of the 1890 brick-over-fieldstone foundation. Of course the small sump pit is on the middle of the wall opposite both of these, so on the relatively rare occasions when I do get some water, it travels across the floor. I'd rather find a way to direct it to the sump and pump without making a mess. If the door wasn't involved I'd consider just trying to channel it, but.... This is starting to turn into another question, isn't it.

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Well, a (concrete slab) floor is just a floor, supported by its substrate. You may have some subsidence afterwards, so I'd advise my tips here, especially the one about undermining it a little to lock it all in place once you re-pour. Rebar is a must.

Done right*, I have no concern for the foundation. I'd be worried that there's a pocket under the slab somewhere and a section might drop a little. But that's not a show stopper, it's just more work.

*AFAIK, done right means you don't dig below the bottom level of the foundation.

If the slab is keeping your foundation from collapsing, your house has some big problems.

0

It is very unusual (and usually obvious that it's NOT the case by inspection around the edges) for the floor to be in any way supporting the walls.

Normally, it's footings, then walls, THEN floor; and the floor is not in any way tied to the walls.

There are a few buildings with "bathtub" foundations, but they tend to be major projects floated (somewhat literally) on dubious soils, not houses - and if you need French drains, there's proof you don't have one of those.

As for rebar in your walls, you should be so lucky - IME your average foundation guy seems to put a bare minimum in the footing and none elsewhere unless forced to - profit (tiny increment) over strength improvement (large.)

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