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I got a bobcat in and he drilled me a 600mm hole down 1.25 mtrs deep.
I went down 1.25mtrs as I was going to form the base about 400mm above the ground as my observatory floor is raised above the ground to let storm water pass underneath.
If you are going to stay at ground level then 800mm to 1mtr would be enough and 500mm would be not.
I then made a steel cage from some old gal fencing and placed into hole.
Do use some sort of reinforcing, you don't want to take shortcuts now and have future issues.
I then formed the pier base with form ply and made it with a top 500mm x500mm with the cage coming up through the hole as my pier base was going to be raised 400mm above the ground.
I then fitted a stainless threaded rods welded into a cage type assembly to keep them square with a ply template ontop that was the same as my future steel pier to bolt ontop.
We then filled the whole lot with 25mpa concrete
I have catergory H clay soil and this has proven rock solid.
Regards Matt.
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