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#1 Tori


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Posted 05 April 2013 - 06:10 AM

I'm just about to begin construction on my observatory and I dug a hole where I'd like the pier to go and hit ledge about 2' down. In fact, I've hit ledge about 1-2' down everywhere around the observatory site. I do live in New England, so I'm not exactly surprised.

I'm concerned about how to proceed. The concrete pier is going to be about 2' in diameter and 8' (AGL) tall with a 12" wide 3' tall aluminum pier on top of it and hold 600-700 pounds of mount/telescope/etc; I'd hoped to go down well below the frost line and pour a big base and have a 2' diameter pier up from there.

I could drill holes in the ledge and epoxy in threaded rods or rebar to make the ledge part of the pier, but what about the rest of the building?

The guy who is helping build it suggested removing all the earth under the building and back-filling in with 1 1/2' of stone and then pouring the concrete slab on top of that for the foundation of the observatory.

I have no idea what the right thing to do is.

Any suggestions?

#2 Kaizu



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Posted 05 April 2013 - 07:11 AM

Hello Tori

Do you know if that ledge is the bedrock or separate big boulders.
Our (CSSF) cosmologigal observatory and the mount are both bolted direct to bedrock.

The Cosmologigal Observatory

An other observatory that I have planned, stays on a big rock that is not bedrock (diameter ca 100m). It vibrates a little because of passing big trucks. This we have mounted as your frend supposed but so that the observatory floor and the pier are isolated from each others.


These both work and are in use.


#3 Raginar



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Posted 05 April 2013 - 07:18 AM

That big rock will be better than concrete in soil by a long shot. Drill and epoxy.


#4 stmguy



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Posted 05 April 2013 - 07:25 AM

If it is truly ledge (and it probably is) and not a big boulder I would think drilling holes in the ledge, epoxy rebar , clean the ledge good , pour concrete and I don't see how it can go anywhere. We ran into the same issue at Stellafane with the McGregor Observatory and I'm pretty sure that is what we did there. Unless you already have the aluminum pier perhaps shorten the concrete pier to keep the mass down closer to the ground and make the aluminum pier longer. You will probably have to start out with a very wide base , I don't see any good photos of the base of the telescope in the McGregor but here is one here . It still doesn't show how wide it is under the floor.

I'm also in NH and I had the opposite situation, pure sand and the holes kept filling in as we were digging them for the corner piers of the building (I didn't want to dig huge diameter holes )

It might sense for you to take a look at what we did there in the McGregor as you truly don't want to make any mistakes with a project that big. You can contact me if need be and I can arrange for you to go on site and meet with our site manager that was in charge of the project or just get an idea of what we did.


#5 Mr_T



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Posted 05 April 2013 - 07:54 AM

We hit Granite at our local Obs so we drilled in holes for re bar then filled with concrete leaving a space around it to Isolate from Floor Pad it worked out fine with a 12" Round Pier and it stands over 4 ft high


#6 Greyhaven



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Posted 05 April 2013 - 07:55 AM

I used PT 4"X4"'s in the construction of my 8'X10' ror I carefully checked for the best looking end grain all centered well and I still had one of my 4"X4" develop a twist over half its' length. The post carried my rollers and hangers and had changed my stem clearance on the rollers from 1.5" to less than .5". Over this winter it got to be so close I was concerned about the roof geometry becoming so bad I could not count on its' opening completely. I was able to bring things back with a couple of monstrous C-clamps and some 1/2" carriage bolts now it slides open and closed with ease. Because of this issue I would not recommend PT wood for applications where stability is required unless you bolt and brace it with angle iron. PT is so wet you need to take precautions in construction. I was lucky because I was able to return things to normal without having to rebuild the roof assembly. It's so much fun being owned by an observatory.
Be Well

#7 roscoe



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Posted 05 April 2013 - 08:11 PM

I'd also suggest drilling and anchoring rebar, both for your pier and the building support posts. You are very lucky, as the earth's crust in central New Hampshire is not going anywhere for a looong time, so you will have a very stable mount. The idea of digging down to the ledge and refilling it with rock or gravel is a good one, and if you have any way to install a drainage pipe to keep water from pooling under the building, that'd be best. Your pier base could certainly flare some - if you are planning to get premix in a truck, a box 3' x 3' or more cut to fit the ledge and filled with concrete would be nice, and I assume your builder knows this, but you'll want several sturdy rebar - at least 3/4", 1" would be better, under the pier base, because if there is any way at all for water to ooze in between the ledge and concrete, it'll freeze and try to lift it. Angling your rebar instead of standing it up straight makes it way more resistant to pull-out also. For the building corner-posts, epoxying in thick threaded rod tall enough to anchor the building's sills is the way to go. Instead of sonotubes, you might consider tapered plastic concrete forms, nice thing about them is that you can backfill around them before the pour, which makes them much easier to fill, and trimming the tops to your desired concrete height is easy when they are stable in the ground. Being tapered, and smooth on the outside, they are pretty frost-proof. Even though it's the same hunk of ledge, I'd still suggest leaving a space around the pier to minimize floor vibration issues.


#8 Tori


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Posted 06 April 2013 - 07:52 AM

Thanks everyone,

I don't know for sure if its really ledge or a huge bolder, more digging today should tell me. One thing that concerns me right now is that the rock is flaking, I know I need to get that 100% cleaned up before pouring.

I can definitely install a drain pipe, that's a great idea too.

Thanks again all for your thoughts!


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