Bad Existing Footing Condition
Bad Existing Footing Condition
(OP)
I have an existing building where the owner wants to make some relatively minor modifications to an existing CMU wall. During the site survey I went into the basement to look at the condition of the CMU in the basement. I encountered a condition that made me very nervous and has become the focus of my effort on this project.
The attached sketch shows the existing condition (in red) and my current thinking on how to address it (in blue). Basically, at some point someone dug part of the basement lower to pour a slab on grade and, in the process, left the footing exposed on one side with soil below the footing sloughed down. My current thinking is to pour a mass of concrete around the exposed soil to confine it and drill into the slab on grade to restrain it from sliding.
The building has been this way for at least 30 years, but now that I'm involved in the project I feel it's necessary to address this condition in its entirety. Has anyone encountered this type of condition before and how have you handled it?
The attached sketch shows the existing condition (in red) and my current thinking on how to address it (in blue). Basically, at some point someone dug part of the basement lower to pour a slab on grade and, in the process, left the footing exposed on one side with soil below the footing sloughed down. My current thinking is to pour a mass of concrete around the exposed soil to confine it and drill into the slab on grade to restrain it from sliding.
The building has been this way for at least 30 years, but now that I'm involved in the project I feel it's necessary to address this condition in its entirety. Has anyone encountered this type of condition before and how have you handled it?






RE: Bad Existing Footing Condition
I have seen this type of "new" footing before, and though I dislike it intensely, it is sometimes all you can do. I wonder if you need to staple into the existing footing, or at least roughen significantly... Otherwise I don't see the detail working other than based on gravity and the (very) minor tension capacity you can get from the new chemical anchoring of your vertical rebar.
FYI: Something similar is common residentially around Toronto for deepening old basements. It is even included as an "approved" detail in the TACBOC Standard Details book [see B01c].
RE: Bad Existing Footing Condition
Sometimes referred to as a Dutch or Bench footings. Yes, City of Toronto has a standard detail for something similar. Like CEL, this is not my preferred method to address this situation. I would look at phased underpin of the footing, also contained in the TACBOC detail book.
RE: Bad Existing Footing Condition
I've done this at least two times that I can recall at age 86 and a concrete pump works fine. You don't need a grout pump. Just be very careful and immediately stop the pump if any problem shows up. I found it works to lift a sunken slab also, if that is present.
RE: Bad Existing Footing Condition
I like the idea of the pvc and pumping concrete in to compact the soil, but I'm not sure how I would convey the criteria at which they should stop pumping. I won't be performing the work, and I can't put a note on the drawings to "be careful and stop pumping if any problem shows up".
RE: Bad Existing Footing Condition
Or you might specify that the earth there should be compacted from the side immediately before placing concrete in the new work, progressively along the existing wall.
RE: Bad Existing Footing Condition
Were there any limiting backfill, earth slope, foundation drains, water table elevation, or height above slab dimensions to the "approved" detail in Toronto? Without these, and similar, specific limitations, I would never use such a detail, approved or otherwise.
Mike McCann, PE, SE (WA)
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Thanks for all the input, guys.
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