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Old 01-08-2021, 11:40 AM
Saturn488 (Chrys)
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Telescope pier foundation

I am looking at putting a pier in my backyard but the only area I can do it is on the elevated tiled area.

There is about 400-500mm of packing sand which is the foundation for the tiled area then under that is the 'clay' based ground.

What would be the recommended depth I would need to dig into the clay ground so nothing moves or shifts? Should I dig enough into the clay and fill it up with concrete up to the packing sand and bolt the pier in? Or should I dig deep enough to put a 200mm PVC pipe in, fill it with concrete and place my adapter at the top?
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Old 01-08-2021, 11:55 AM
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You only need to get to bedrock
Read up on "frankie piling" then think how many star pickets you think you need, double that and drive them in.
Alex
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Old 01-08-2021, 02:38 PM
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Chrys, do you plan to do astrophotography or only visual observing?

If you will be doing AP then your pier foundation will need to be more solid.

I don't have the answer for you, but my thoughts are:

1. The packing sand probably wouldn't provide much support so you might have to plan for a pier that is effectively 400-500 mm taller than you actually have. In other words, the real foundation is 400-500 mm below ground level.

2. I'm pretty sure that clay soils 'move' with the season, so that might indicate more mass in the foundation, and perhaps spread out over a wider base.

3. I think it makes sense to build the foundation with a flange to which you can connect your 1200 mm pier. Changing or removing the pier in the future will then be easier.
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Old 01-08-2021, 03:48 PM
glend (Glen)
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It is possible to over do it in a big way. Some folks will swear you need a minimum of a cubic metre of concrete. Don't buy into that thinking. Think about whether you ever want to get it out, a question most ignore. It is easy to put concrete into a hole, much harder to take it out.
I do believe in the poured pier method, that is how I did mine, it is usually cheaper than a block footing where you buy a steel pier to bolt to the top. With your clay base, I agree driving in some star posts, or simple galvanised angle iron, will do alot for stabilisation. It also means less digging in heavy clay. A footing of 0.6mx0.6mx0.6m can be enough if you pour it on top of an arrangement of star posts.
My footing and pier were a continuous pour, in other words the pier is poured into its form as soon as the footing can support it, and do put reo into the pier form. Don't forget to buy J bolts from Bunnings, these are stuck into the top of the pier in a triangle arrangement to serve as the mount points for your cage.
Have fun, mixing bags of concrete in a 20L bucket will make a man out of you. Haha. I recommend a max of two bags per bucket, and get some good heavy duty rubber gloves to wear, concrete will rip your hands apart. Lazy people get a concrete mini truck delivery.
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Old 01-08-2021, 04:18 PM
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mura_gadi (Steve)
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Hello,

I think the people who pour large slabs really want to make sure of ground drift, but even with the largest of foundations you can only slow the drift. Unfortunately by disturbing the clay layers you allow water to get in and that will swell the surrounding clay. Give yourself two plumb line marks 90degree apart on the pillar for an easy check of movement over time.

If it looks like your doing a lot of bags, do yourself a favour and get a concrete mixer 2nd off gumtree. The mixer can always be resold for a $50+ lose after you have finished.

Alex's suggestion with the radials is very good at limiting drift with limited infrastructure.


Steve
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Old 01-08-2021, 05:47 PM
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Shiraz (Ray)
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hi Chrys. I had the same sort of problem with sandy loam over 20m deep clay - no chance of getting to bedrock. FWIW, I tried a fairly solid concrete slab floating on the sand (1.2x1.2mx200mm). Left the treated pine formwork in place and poured a 75mm thick concrete working platform around it. The slab has about 80kg? of eq8+tripod+stuff on it and it hasn't moved after settling down (took about a year of occasional tweaking of the PA to correct for slow drift, but it has always been stable enough for imaging). The formwork isolates the slab and I can walk around on the work area without affecting the mount. I imagine that it would be easy to pour the slab with bolts for a pier, or even to use lock bolts. Cheers Ray

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Old 01-08-2021, 06:23 PM
Saturn488 (Chrys)
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Thanks for all the feedback and help.

Yes, will be for AP, not visual.

The area of Melbourne I live in is pretty much all clay. my payload will be around the 8-10kg mark + the pier. I have seen this done in so many ways from people using wooden posts, to concrete blocks to using the same amount of concrete that was used for the foundation of a skyscraper lol. Obviously there are limits and some over engineer when there is only between 30-100kg if that of equipment.

I have updated the diagram.

First option is using a PVC pipe filled with concrete all the way in then adding the adapter at the top. Pros include more stable, cons not removable.

Second option is the same approach but stop at the top of the pavers and install a pier. Pros include pier can be removed, cons not as stable as the PVC pipe all the way through.
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Old 01-08-2021, 07:31 PM
glend (Glen)
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Quote:
Originally Posted by Saturn488 View Post
Thanks for all the feedback and help.

Yes, will be for AP, not visual.

The area of Melbourne I live in is pretty much all clay. my payload will be around the 8-10kg mark + the pier. I have seen this done in so many ways from people using wooden posts, to concrete blocks to using the same amount of concrete that was used for the foundation of a skyscraper lol. Obviously there are limits and some over engineer when there is only between 30-100kg if that of equipment.

I have updated the diagram.

First option is using a PVC pipe filled with concrete all the way in then adding the adapter at the top. Pros include more stable, cons not removable.

Second option is the same approach but stop at the top of the pavers and install a pier. Pros include pier can be removed, cons not as stable as the PVC pipe all the way through.
Actually a poured pier can usually be removed, it just requires a concrete saw (hired from Kenards). And I recommend these bolts for the top of the concrete pier, just insert them about 120 degrees apart, into the wet concrete, when doing the final pour into the pier mold. Your cage bottom plate should be drilled to suit your bolt pattern.

https://www.bunnings.com.au/whites-o...-bolt_p1060324

PS you can buy steel or thick aluminium offcuts on eBay, or check local metal supplier scrap bins for offcuts.
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Old 01-08-2021, 08:52 PM
Startrek (Martin)
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I set up my NexDome Obs with 4 x 250mm dia piers ( 3 for tripod and 1 for a future central pier mount ) they are 500mm below ground sitting on a small 100mm footing into clay and 400mm out of the ground 12mm below underside of my deck. Whilst waiting for my NexDome to arrive from Canada ( 8 months ) I sat 80kg of weight ( borrowed from my welding mate ) on the central pier for 3 months to check and sinkage etc... with heavy rain ( 100mm plus ) on 2 occasions over that period
The pier only sank 1mm if anything. So the pier is grossly over specíd and would support the Sydney harbour bridge as my current total rigs load is around 38kg and any future upgrade will be less than 80kg
I think most of us over spec our pier supports but better to be safe than sorry as itís a huge job to rectify if a problem occurs down the track
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Old 01-08-2021, 10:13 PM
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Tandum (Robin)
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Looks like you need a 2 meter pier.
2 meters is tall.

Last edited by Tandum; 01-08-2021 at 11:50 PM.
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Old 13-08-2021, 10:19 AM
Saturn488 (Chrys)
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Did a few more measurements and calculations. If I were to put the pier on the existing concrete slab I have it will need to be around 1400 - 1500mm tall.

I know standard piers are 900-1000.

Any advice/feedback/recommendations on a 1500 pier?
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Old 13-08-2021, 12:21 PM
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traveller (Bo)
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Chrys, this is the foundation I did a few years back https://www.iceinspace.com.au/forum/...d.php?t=135718
It used to hold a lightweight pier and an EQ6 with FLT 110 imaging system.
I have since replaced the pier with a much heavier one 1500mm tall and have a CGX mount and a heavier system and the foundation has held up well.
I would recommend a heavy foundation as you donít want to dig that out each time you upgrade your pier. Mine was way under 1 cubic meter but itís solid.
Cheers,
Bo
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