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Old 16-04-2019, 06:02 PM
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pmrid (Peter)
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How essential is a deep-sunk pier

This is a bit of a diversion from the parallel thread on backyard observatory planning. It diverges over the issue of the pier and foundations.

I am buying a small place on Bribie Island north of Brisbane. I'm planning on having a cement slab a little below grass level so I can mow over and around it and also so I can plonk my tripod down as and when I need.

I appreciate all the advantages of a fixed pier with set alignment but for the moment at least, this is not an option. I will probably sink some threaded rod into the concrete so I can - if the opportunity arises later on - put a pier on it. But for now, I plan doing a good string-line to get true south, then using the tripod at full extension, mark the position of the feet on each leg so I can at least have that much head start on alignment when I need it. I will probably set some good-sized bolts into the concrete pad so I can just slot the legs into it quickly and repeatably.

I built a couple of these pads on my farm (with the very generous help of Brundah - Dave) several years ago and visitors found the head start these markings gave shortened the alignment process considerably.

So, my question is how important people think it will be to have some massive foundation either for the slab itself or for a possible future pier. I will be imaging remotely or at least some little distance away from the slab so it will have minor (to the point of zero) vibrations. Also the FL of the scopes I am likely to use don't go above 1600mm.

Love to hear your views guys.

Peter]
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Old 17-04-2019, 08:22 AM
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doppler (Rick)
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Hi Peter, I think that it depends on the ground your are digging into. The idea of a monolith under a pier is to curb vibrations especially in soft clay type soils. My ground is hard sandstone (had to use a crowbar) so I have been able to get away with a hole 600mm deep and about 400 x 400 wide. As far as bolts go I used these concrete screws to hold the pier down you just drill a hole in the slab and screw them in, easily removed and stronger than dynabolts.
Rick
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Old 17-04-2019, 10:12 AM
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pmrid (Peter)
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Thanks Rick - for the reply and the pics. I've never questioned the conventional wisdom about deep holes, cement and so forth for anchoring a pier. I have three of these blighters in my current obs and they are mighty beasts to say the least. But the move to this new place has made me question that conventional wisdom a little and to ask whether a sufficiently thick - 100-150mm pad alone would suffice. I am dealing with sandy soil (Bribie Island). In my present plan, the slab would be triangular - an equilateral triangle with each side a bit longer than the span of the tripod legs at full extension. So it would pretty darned solid. One apex of the triangle would be aligned as near as possible to true south using a string line and the other two apexes (is that the right plural?) accordingly.

Peter
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Old 19-04-2019, 06:08 PM
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Anth10 (Anthony M)
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Peter, I’m glad you asked this question, I’ve also decided to install a pier for a potential observatory and was thinking how to secure it also. I guess Doppler makes a good suggestion- this would certainly hold the pier down no problem but I’m curious to know if this would be sufficient to prevent it from wobbling under the weight and motion of my heavy Newtonian. Tripod legs have a wide span to balance the shifting weight however the tall pier lends itself to movement? Question I’d like to hear people’s opinion on. Sorry if I have added this query on your post Peter but being topical and timely we may both gain from the experiences of others that have had success in their pier mounts.
Would you need to fill the pier with cement or sand - would that stabilise it?

Best wishes for your obs/aero pad Peter.

Anthony
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Old 19-04-2019, 07:55 PM
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Rocky and sandy substrate are the most stable foundation to build any structure on!
Clay soils are reactive so when moisture is added they swell and cause movement. (Which you don't want). Rocky areas and sand don't really get effected.
Pier isolation is a personal preference on how sensitive your equipment is.
A 600d x400w x400L deep hole is a minimum foundation but you can change this volume horizontal so it's only 400d. with the same volume as long as it's min 100mm into natural soil.
Bolt your pier in after as you can adjust it with shims if you get any movement!
Ramset have some good over the counter gear.

Hope this helps
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Old 19-04-2019, 08:10 PM
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pmrid (Peter)
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I don't have the engineering to quote the math of the flexion of a vertical pier under various conditions. I'd guess the taller and thinner, the greater the tendency to wobble about. Makes sense.

But with a tripod, with legs at full extension - say each point of the tripod being the corners of an equilateral triangle with each side about, say, 750mm and the whole being about 1200 above ground. I would have thought this would be very unlikely to produce wobbling even with a big newt on top. I would be tempted to also hang a substantial weight (like a deep cycle battery) from underneath the center bolt to further stabilize the thing - holding it down. It's these sorts of ideas that set me to wondering whether a reasonably good slab of say 100mm deep would suffice.

Peter
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Old 19-04-2019, 08:17 PM
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If it's just a tripod just a slab is more than fine!
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Old 20-04-2019, 12:03 PM
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I agree that for a tripod, being adjustable easily, a well made slab would be fine. Particularly if there is no moving load on or near the slab while in use. Bribie should be flat and sandy but there may be a lot of filled ground.

A good slab requires a minimum of 100mm of well reinforced concrete on a bed from which all organic topsoil has been removed and replaced with at least a minus 7 crushed aggregate base, preferably wet and packed smooth.

A pier however requires, well , a pier into the ground as deep as you are prepared to drill to avoid seasonal and long term movement of the upper soil . Preferably three or more drilled concrete piers and a pier cap or slab if the soil is sloping and reactive to limit rotation ( or the ability to adjust.) There are also cheap steel and screw piles which could work in soft or sandy soil.

.
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Old 20-04-2019, 08:38 PM
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Hi Anthony, in regard to your question on stability, my pier footing has no movement in the ground (still solid after two years now) and no detectable flexing of the pier itself with the 10" f5 newt and cameras on top. My pier walls are fairly thin so I filled it with sand to stop any acoustic vibrations.

Rick
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Old 21-04-2019, 01:38 PM
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Yes that 400x 400 x 600 pier base would be pretty stable in shallow natural soil down to sandstone. Love those screw in masonry anchors.

I can walk around carefully on my concrete deck while imaging as it is on top of concrete walls, beams and deep concrete piers.

With 3m deep clay soils such as we have here the soil moves seasonally up to 1 cm and about 1mm a year downhill so even a deep pier would require checking and adjustment to remain on alignment.

Bad Vibration however is a whole different problem , such being on a main road or train line. Or a neighbour likes to excavate his basement at night.
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Old 21-04-2019, 06:57 PM
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Absolutely deep, the deeper the better.
Mine was in very solid soil and it was 900 deep and 600mm x 600mm
And the pier was incorporated in it as a steel cylinder 300mm in diameter filled with cement as the whole lot was poured in one go.
Once set a Mack truck couldn't pull it out.

The things is this it is what you can afford and the skills you have.

Mine was probably over kill, but that is what i do, and it was watered in the summer months at the base due to soil drying out and shrinkage

Leon
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