Quote:
Originally Posted by h0ughy
For all the armchair engineers out there here is a problem for you to solve. My mate Al has his observatory built in his back yard. His pier has about a ¾ m3 about 750 deep and the soil is reactive clay soil. The concrete is surrounded by 12mm thick padded foam. With the month of January we only had 12mm rain the whole month. Since then we have had about 200mm rainfall, and hence the ground has moved again. This results in the levelling of the pier being out. He accurately polar drift aligned until nothing moved for 15 minutes (originally in the dry) and now he gets field rotation in the images.
It is obvious that there is ground water flowing and the clay has swollen.
My suggested solution is to isolate the observatory by digging a 750mm trench around the whole observatory and backfilling with coarse and fine aggregate with socked ag pipe and tailing this off further down the slope as to divert the groundwater flow from under the observatory. My question is do you think that this might help to stabilise the ground under the observatory by removing the ground flow of water directly, or would a mass pour for a new pier with 2m3 as a base suffice with the weight of the concrete displacing the localised ground water??
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Dave,
I don't know how successful your suggestion will be as I'm not a civil engineer. From my limited knowledge I would suspect it might reduce severity but it won't fix the problem.
A couple of things, though, to think about...
You say drift alignment is good for 15 minutes but still getting field rotation... Either the drift alignment is not as good as thought, or there is some other cause of the field rotation. Even if the pier/mount is on a lean, you should be able to drift align accurately.
I would advise to check the drift alignment again, and if you can get 15 minutes with no drift at zenith
and 15 minutes with no drift near the horizon,
then the alignment is close.
If still suffering field rotation, then explore other causes... (I'm not sure what though
).
It's a bugger to have an unstable pier, but even a quick drift align is still probably better than a complete setup and tear down every night...
If he really wants to fix the unstable pier problem, the best I could suggest is to go deep if that's not unfeasible. I planned to put my pier footing in a metre deep but I hit rock at 700mm, so I keyed it into the rock by 50mm and smiled
. If he can find rock, he can fix the pier movement
.
Hope this helps,
Al.