I just burned a couple piles last weekend. One was about the size of a single car garage, the other twice as large. They each had a fair amount of dirt in them from root balls and were expecially hard to start. I had the local volunteer fire dept. out as they had the potential to be a problem given their size and the proximity of one to a tree line. I used a mixture of gasoline and diesel fuel to get them started and used my FEL to bank them in and keep them burning well.
After about four hours the fire volunteers left satisfied with how things were going and I started mowing then. I'd mow for a while, go bank the fires, mow for a while, go bank the fires, etc.
I left four hours after the fire dept. when I was done mowing and the fires were burning well but dramatically down in size. It rained that night and I checked them the next morning and found each still smoldering. By pointing my bucket teeth down I was able to rake together some of the larger, unburnt pieces and was rather surprised that I had flames going again.
A couple days later I used a hand rake to go through the ashes and found not only some smaller unburnt pieces but also a bed of hot embers. Once again, I had a fire going.
It takes time and tending, but you should be able to get everything burnt to ash even with a fair amount of dirt in there.
Good luck with your project. I hope this helps. /forums/images/graemlins/smile.gif