I need to drill 50+ holes in some very rocky soil. The soil has is a gravelly silt but there are lots of rocks fist sized up to volkswagen sized. The rocks range from badly weathered granite to granite that is very hard. The holes need to be at least 4 inch diameter and 4-1/2 ft deep. Would a skidsteer auger, like the bobcat 30C, and a rock bit be appropriate for this? My main concern is that the typical rock bit might break if a it hit a rock that moved a little and tried to wedge the auger bit. My other concern would be that unweathered granite might take forever to bore through. If anyone has tried an auger in similar soil I would very much like to hear how it went. thanks