Concrete Slab

Ok, first off, i checked the archives and there is tons of useful info there. Yall have a great networking system here that i appreciate. With that said, i couldnt find the answer to my problem so i thought i would throw it out for all to ponder. It is probably a simple question, just that i am venturing off into an unknown area here. Here it is:

I have a lake house that has a 1600 sq foot cabin. We are wanting to add an addition of about 600 sq feet. It will be a one story addition with attic area. The problem i have is that the ground slopes away from the existing house.
I have read that you dont want to disturb the existing ground when pouring a slab. You just need to use fill dirt to level the ground and pack the fill dirt/sand and dig footings below grade to prevent heaving and to lock the fill dirt under the slab where it wont wash out. If i am wrong so far, please advise me of where i am venturing away from the correct path. The slab i am wanting to pour will be basically a rectangular shape about 20x28 with a couple bay window ledges also.
Here is where my problem comes in. Consider corners A and B will be tied into existing slab. Corner C will be where the ground slopes down to 16" below A and corner D will be 30" below corner B. So the ground as it goes away from the existing home slopes down and away. In my research, the books say to not fill with more than 24" of sand/gravel. The 30" drop will be too much fill right? What do i do? How do i set the forms for a slope that drastic? or am i not understanding the information right?

I appreciate any advice. Thanks in advance....

Craig

 

Check your local building code to see how deep the footers around the slab should be. At least 12 inches in most places, but below the frost line. On the footers, you could build a foundation wall behind which to fill. Lay the fill in 4 to 6 inch drifts and compact to 95% of proctor standard. This will give a suitable base for the slab. But be sure to check the code for your area.

Hope this helps.