observatory ground movement?

[h0ughy (David)]

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??

[kinetic (Steve)]

Originally Posted by h0ughy

.... Since then we have had about 200mm rainfall, and hence the ground has moved again.

Houghy,

In my backyard at least, it seems to be unavoidable.
My yard is the worst type of clay....I had to dig a pool through it!
I have a laser permanently mounted to my GEM base and it shines on
to an X on the wall of the house.

The island slab that my GEM sits on seems to move in a wet/dry season
cycle. This shows on the drift with the laser.
Fortunately, I have fine Azimuth and altitude adjustment of the GEM
tripod and a night of Drift Explorer in K3CCDTools has this fixed.

If you think any slab, at least the slab a typical backyarder lays,
is not going to move some amount, you are kidding yourself.

Steve

[Dennis]

Hmm, dragging the gear out every night then setting up and tearing down each night don’t seem too onerous now!

Cheers

Dennis

[h0ughy (David)]

Originally Posted by kinetic

Houghy,

In my backyard at least, it seems to be unavoidable.
My yard is the worst type of clay....I had to dig a pool through it!
I have a laser permanently mounted to my GEM base and it shines on
to an X on the wall of the house.

The island slab that my GEM sits on seems to move in a wet/dry season
cycle. This shows on the drift with the laser.
Fortunately, I have fine Azimuth and altitude adjustment of the GEM
tripod and a night of Drift Explorer in K3CCDTools has this fixed.

If you think any slab, at least the slab a typical backyarder lays,
is not going to move some amount, you are kidding yourself.

Steve

thanks good idea

Originally Posted by Dennis

Hmm, dragging the gear out every night then setting up and tearing down each night don’t seem too onerous now!

Cheers

Dennis

rub it in, note i havent set mine up yet

[rmcpb (Rob)]

That idea of a laser on a mark at a distance is a beauty for a quick realign. Lot easier than digging trenches and a couple of cubic metres of concrete.

No matter what weight is put on it the clay will expand and contract and move the footings, that is why houses are built on raft slabs. As a result that idea of Steve's with the laser mounted and aligned is fantastic.

[Quark (Trevor)]

Hi David,

Totally agree with Steve,

Far better to design the mount in the first pace with the ability to make fine adjustment in Azimuth and Altitude. It is no big deal to have to occasionally redo the star drift alignment. Surely this is easier than the major earth works you are thinking about, also a lot more cost effective.

If this mount currently, is not adjustable for polar alignment, then I would be taking steps to make it adjustable.

Regards
Trevor

[sheeny (Al)]

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??

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.

[Sharnbrook (Mike)]

Sheeny's right. The best way to prevent such movement is to bore through the reactive soil to rock, or to some other material that doesn't move with the change of moisture content of the soil. The weight of the base is of little concern, because concrete at a SG of 2.4 is very little different from wet clay at 2.2, and it will effectively "float", or anyway, sink very slowly. Ask your friend if he knows any soils engineers. If so, the soils engineer would be able to give good advice based on local knowledge, and would be unlikely to charge. (A bottle of half decent red?)

Failing that, get a bobcat with an augur, and drill a 300mm dia pier down about 150mm into non reactive soil. Fill this with concrete reinforced with 3 x 12mm dia reinforcing bars (held together with some stirrups or heavy guage wire) and you should have a pier that will go nowhere.

That's easily said from a desk I know, but if it means removing the observatory to bore the pier, that's a different matter, but it was one that you canvassed when you suggested 2 cm of mass concrete as a base.

Digging the trench is not a bad option, but consider two points.
1 Your trench must go down through the reactive soil, or it won't be isolated.
2 You will need to allow the soil within the trench boundary to reach equilibrium. At the moment it's wet, but it will dry out over time, and you must wait until it's dry before setting up again, otherwise you will have a recurrence of the problem.

Finally, ensure that any rainwater discharge from the roof of the observatory is channelled well away from the building, so that localised flooding does not occur.

Good Luck,

These are thoughts of a retired builder, but the best way to go would be ask a tame soils engineer.

[pmrid (Peter)]

I also have highly reactive blacksoil/clay at my place and my obs is sited on just that sort of soil. SO I am not unfamiliar with it. The thing about black soil/clay is that it will shift when it gets wet but it will also shift when it dries out again. If you can't get your pier deep enough into bedrock to make it stable, you have to engineer it to accommodate the grim reality that it will move wet and dry and the answer, as has been mentioned here, is to ensure you have a reasonably accessible adjustment plate on your pier.
Peter

[sheeny (Al)]

Originally Posted by sheeny

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.

To save time, use the drift alignment tool in K3ccdtools both near zenith and horizon. It will show up any drift quicker than you'll see it by eye on the reticle, so you won't spend so long getting the mount accurate.

OK, if you aren't familiar with the K3 drift tool you'll have a learning curve, but if it ends up that the movement just has to be lived with, this learning curve will be worthwhile!

Al.

[bert (Brett)]

My 2c, go deep with the pier if you can.


Sticking 10sqm of concrete 700mm deep will still give you the same movement with hydraulically reactive soil. If you go a lot deeper with the pier the ground/water ratio is a lot more stable further down (its water retention is a lot more stable than on the surface or a little way down), and you have an averaging effect over the length of the concrete pier below ground.

Get a bobcat to drill a hole as deep asyou can go, use some large rio for reinforcing to stop the the concrete pier footing from snapping under uneven hydraulic load and it will be a lot more stable. Putting a large mass of concrete near the surface I believe will have very little difference from what you have now.

Brett