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I plan to fence my property with driven metal T-posts (max cross section 1.45”). A section of it goes across a granite rock outcropping. This in the area near Stone Mountain in Georgia. I plan to buy a Bosch SDS-MAX 15amp rotary hammer for the job. I want to drill a 1.5” hole 24” into the rock and set the T-post with Sackrete . I likely have 50 holes to drill. Am I better off going with a Carbide bit (Bosch HC5092) , or a diamond one? And I don’t have a link for one. Suggestions welcome.

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    You are setting the posts in solid rock. Unless you are putting the other posts 6 feet into the dirt, 12" into solid rock should be way more than adequate, and half as much drilling. – Ecnerwal Sep 13 at 23:21
  • @Ecnerwal I live where the ground is mush, so I don’t know much about rock. However, if the hole is only 12” deep and near the edge of the “outcropping” (as the OP describes it) it wouldn’t take much for a 6’ high fence to apply enough force to break off a chunk of rock, wouldn’t it? I guess if the hole is more than 12” - 16” or so from the edge it would be okay. Doesn’t rock crack when “hammered”? – Lee Sam Sep 14 at 0:10
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    If it's near the edge it doesn't much matter how deep it is--a post like that has enough flex in it that all force is applied near the surface of the stone. Either the stone gives or the post bends over. Also, a 1.5" hole is huge for that type of drill, which is intended primarily for much smaller holes in concrete. Concrete and stone are very different things. You'll be standing there a damn long time. I suggest that you use surface-mount posts and drill for 1/2" anchor bolts. – isherwood Sep 14 at 1:37
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    Also, concrete won't work well at all for that since the aggregate will prevent it from flowing into those narrow channels. You'd use an anchoring cement, but it's probably not even necessary. Pour sand in there and call it a day. Nothing's going to work it up and out of solid stone. – isherwood Sep 14 at 1:39
  • Agree with ecnerwal, have had to drill deeper but don’t think it was needed for t posts. Mortar in the hole then the post was so strong the post were bent by mulies trying trying to get in. To tell the truth I think 6” would have been enough+ – Ed Beal Sep 14 at 2:29

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