Auger in rocky soil

[nirias]

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

 

[Tazza]

a 4" hole isn't too bad, but the rock you want to cut through doesn't inspire me, i know granite is really hard. Even with a rock auger, i see it being deflected off if it can't grind through.
I had a hard time drilling 1m deep holes in clay/shale with my 743, i got there, but i sure worked for it. The more down pressure the better, so ideally a mini excavator may do a better job.
Hopefully someone with experience drilling in this sort of dirt can give you some advise too.

 

[walio123]

a 4" hole isn't too bad, but the rock you want to cut through doesn't inspire me, i know granite is really hard. Even with a rock auger, i see it being deflected off if it can't grind through.
I had a hard time drilling 1m deep holes in clay/shale with my 743, i got there, but i sure worked for it. The more down pressure the better, so ideally a mini excavator may do a better job.
Hopefully someone with experience drilling in this sort of dirt can give you some advise too.

I've tried this with a rock auger drill and if you hit something solid like granite, it does not work. The only thing you can do is damage the screw. Because the holes are not deep, there are rock boreholes of such a diameter, I think they will be able to do this job. They work with a compressor/like impact hammer/ and the hole / ground / must be dry so they can blow the dust. However, if there are so-called floating stones, it will be almost impossible for this probe too. If the holes do not have to be exactly positioned you can try with rock auger and if you hit some hard just change plase for hole.