I was doing some research on using fill for house pads. Apparently, it's always best to pour foundations on undisturbed soil. In this case, I have to bring in fill. I need over 3 feet depth of fill. I read that I should compact it every 6-10". I also read that I should use something called "structural fill" which is specifically designed for this purpose. I read that coal ash (fly ash) is the best type of structural fill to use. The foundation is going to be 46'8" x 88'. What do you think of this? This is just east of Orlando, FL.