They will till down to about 10 inches. To clear grass and weeds from any area, surround it with 4 ft chicken wire and add chickens, then sprinkle oiler sunflower seed over the area, and the hens will dig up everything in the area to get at every seed...plus eat all grubs and beetles. They tend not to like earthworms, oddly enough, which is good. Then they fertilize the area for you. Then you move the fence to the next area and repeat.
Thanks, I will post this on the weekly garden thread that you could join (ping list).