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The old "rule of thumb" for driven shed poles was 1.2M inbedment in good ground . . . .

There's an easier way, drive a decent size (a wee bit bigger than you final post) 6-8' post first, pull it out once you get it 4' in (in really hard going pull it out after 2-3' - reduces the strain...

We are currently using a Dewalt DCD470N battery "superhog" for most of our drilled holes (otherwise I just punch in a post and pull it out with the tractor driver) and it's really good in clay up to...