Hi Dave:
I think I’m getting the situation. A picture is worth 1,000 words. I think we are at 750 or so.
Concrete Slab under the waterproofing under the footing makes sense. Double slabs are common. Grace makes a ‘Preprufe’ waterproofing for this application. You could also choose to pour the concrete footing directly on the Preprufe over a layer of sand over dirt (or over existing sand). It would work either way.
Respectfully,
Integrated Design Services, Inc.
Bob Freeman, AIA, EIT
(949) 387-8500