Turned Down Slab

We’re working on putting new equipment on an existing foundation. Here is a picture of the existing foundation:

When evaluating bearing pressure on this type of foundation, what area can you use for the bearing area? We assumed it’s just the turned down area (in this case the 1’-6" length) but wasn’t sure if the diagonal area (2’-6" in length) from the thickened edge to the center 12" slab could be counted as well.


Since the soil within the truncated pyramid of the foundation is so confined and well compacted, for bearing purposes it will act as a block as if it were all concrete…so the bearing area is the same as the top area of the foundation.

Above is a snippet.

@Ron Excellent advice!

Depending on the length of the foundation perpendicular to the page, getting well compacted fill in the trapezoid under the slab could be a problem. It could not readily be placed before placing the concrete; it would have to be packed in from each end.

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