I am starting an addition on our lake cabin. I need some advice. The addition is going to be approximately 19X28. My problem is i am not sure i can do a slab foundation. The reason is this. Corners A and B will be attached to the existin slab. Corners C and D will be the 28 ft ends. During this 28 ft, there is a decline in elevation. On C corner, it drops to 16"...On D corner it drops down to 30". The books i have read say that you can only fill up to 24" with sand or sandy loam type dirt. Less than that with gravel. The grade is iron ore. Very solid, but i am afraid of the 30" drop. My question is how do you handle this? Any help will be appreciated.