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I live in an old (1920) Baltimore rowhouse. The concrete basement floor was—as far as I can tell from having drilled in a few spots—poured over a pretty variable, haphazard substrate, probably just the preexisting dirt. At some point, someone raised the whole rear section of the basement up about a foot, with concrete poured over dirt/rubble and bounded by a wooden form from from the stone wall on one side to the other, something like this:

Cutaway diagram of original state of raised pad

By the time I moved in, the wood had mostly rotted away, and I removed what little remained. In the time since, the dirt and rocks have sloughed outward quite a bit: Photo of current condition

Or, in corresponding diagram form: Cutaway diagram of current condition

Obviously this is not ideal, either structurally or for keeping things remotely tidy. The raised section is handy to have (it raises the floor to about ground level) and would be a lot of mass to remove, so I'd prefer to keep it and fix / cover / reinforce the side somehow.

What I'm currently envisioning doing is pushing the loose rubble in some if I can, removing what's left out front, putting up a new wood form, and pouring a concrete wall that will hopefully fill in the oddly shaped void. (I'd remove the form afterward, unlike the original.)

Proposed solution

Is this viable? If so, what's a rule of thumb for the required wall thickness, and will I need concrete adhesive to securely adhere the new pour to the upper and lower existing concrete? Is there any good way to securely push as much of the loose rubble up into the void as possible (without it falling back out before the new concrete is in place)? Any likely pitfalls I'm not considering, or a better approach?

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    You can't mechanically compact uniformly sized sand. If you're stuck building on the stuff, there's one method of compacting where you mix it with Portland cement. Not nearly as much cement as concrete uses, so it's weak, but compared to soil it has great strength. You might try mixing a bag of concrete really, really wet and then adding your loose rubble until it's stiff enough to mold (like moist beach sand). Put this mixture back in that cavity, where rather than drying out and sliding out again like beach sand would, it will instead harden into the molded shape.
    – popham
    Oct 18, 2023 at 23:59
  • Do you want/need the raised portion to be raised? I'd lower it for a flat floor all the way across, instead. If you do raise it, clear out as much loose material as possible, don't try to sweep it back in. Fill all the space with concrete, not loose "stuff"...
    – FreeMan
    Oct 19, 2023 at 1:15
  • @FreeMan Well, yes, it's raised up to about ground level (well, one step below), which is really convenient for bringing my bicycle in, and because I use the space as a workshop. (I've built a ramp down to the lower level.) Beyond that, it would be a lot of mass to break up and haul off if I were to remove it all. Oct 19, 2023 at 1:33
  • That makes sense! May want to mention it in the question as others may wonder the same thing. Again, don't try to put any of that junk back in - you want a solid foundation for your new concrete to last.
    – FreeMan
    Oct 19, 2023 at 1:36
  • Good point, thanks. Edited. Oct 19, 2023 at 15:11

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I like your proposed solution with a few changes.

I would dig out slightly more rubble toward the back under the slab. (as shown in the yellow outline.)

Also add #3 rebar (as shown with the blue line) to help reinforce the pour.

If you drill into the upper and lower slabs about 3 inches and add the rebar the area will be extra strong.

enter image description here

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    Yep, you want the concrete to be at least 4" thick in all places. And at this point you might as well just bust off the lip of concrete at the top so you don't have to worry about pouring concrete sideways underneath it.
    – Huesmann
    Oct 19, 2023 at 14:46
  • Sounds like a good idea, though the bottom floor may or may not be 3" deep there. I've tried drilling in some other areas to attach anchors and found that in some spots it's only about an inch thick. The upper, newer (probably still quite old) slab is significantly thicker. Oct 19, 2023 at 21:32

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