I own an 85 year old house situated on a sandy hill. I want to replace the basement slab, which is in poor condition. I would like to drop the new slab down 1 & 1/2 feet, since I only have 6 feet of headroom right now. The problem is that my concrete walls only go as deep as the present slab, and they have no footings! I have thought about coming out 2 feet from the walls, then building a "retaining wall" down 1 & 1/2 feet, then pouring my new slab. However, this causes a lot of wasted space. I have also thought about pouring a new foundation below the walls to the depth I need. However I am worried about the walls shifting on me during the process (very sandy soil all around). Perhaps I could make the walls more stable by pouring some piling beneath them first?