Years ago I helped build a couple miles of fence on just about pure rock. We rented a gas powered jackhammer and drilled holes for the T-posts about 6” deep at least, hopefully more. If we could leave the fins on, we did; if not, we removed them. Gravel and sand kicked down into the holes while wiggling the posts settled them pretty well at the time, and later on they got rock solid. We used oilfield pipe for corners, digging as deep as we could with the jackhammer and concreted them in. I don’t think we ever got one in over a foot deep. We then came out in line with the pull some 6’ and drilled a 6” hole at a 45 degree angle sloped back toward the corner post. If the hole sloughed off or caved in (rare), we went out a little farther. We put a piece of 3” pipe into this hole running up and back to the corner. This was cut off flush and welded. After we were finished it seemed like it would hold up OK, but I quit soon after and have never been back, so I can’t guarantee the long term results.