The MK5 Foundation requires a drain base as the “bottom” of the form. Heavy excavation is not required.
Remove all organics down to the first level of natural soils or hardpan and save the top-soils for backfill. Add a minimum of 8” local gravel, we use ¼ x ¼ HPB high performance bedding, no sand. Compact the base level finishing 2” above existing grade. In High static water areas finish 6 to 12” above the existing grade and in conservation areas or flood zones finish as much as 18-24” above the existing grade.
You’re not digging down into the natural soils you are building up and out of the water or rock.