Drilled holes in hard clay for chain link fence posts. Perfect size for posts. Drill a hole, look inside the pole! You just can't rush - even worked a few roots. Used my cordless drill at low speed. You can't just drill all the way down. Drill a few inches, then pull to dislodge loose dirt from the hole, then drill a few more inches, etc. It's definitely better than hand-hammering the posts (I've tried a few times and given up ). It would be easy to bend the worm shaft under excessive speed and/or force. (It says "garden driller" here, not hole digger!)
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