Hi Lee,
your pier sounds very much like the one I finished a few months back for my gm-8. Its only 950mm high made from 150mm x 5mm with 10mm plates top and bottom and also has some ribs out of 10mm steel. I find it plenty strong enough for the newt and gm-8. It to sits on 1/2" leveling bolts set in concrete 450mm x 450mm. If I give it a good rap I can only see a few secs of vibration with high magnification...
With the gm-11 a 600mm block would be pretty stable. To save cement just get yourself a bunch of old bricks to throw in the hole as you shovel in your cement..theres 4 bricks in my block..
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Like you it would be easy to unbolt and move.. The same with the whole observatory. Just unbolt it of the stirrups and chuck it on a truck!!
Also what might be a good idea is weld 2 pieces of flat bar near the top to hang your gemini/control box from as well. Thats what I have done also..
I reckon just the normal cement mix would be fine, just let it cure over a few weeks keeping it damp, a old bit of carpet kept wet over the top would work well after it has set well.
cheers