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?

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

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: "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!