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Pier depth requirement?


bluesilver
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Hi,  sorry for the odd tittle there.

I am looking at installing one of the Nexdome observatory  kits on a wooden deck.

I am going to be mounting a iOptron CEM120 in there,  but at the moment i am just going to be using the 360 tripod from iOptron and letter on look at a solid pier.

So,  for the main center pier, i was looking at making it 300mm in diameter  ( around 12 inches in diameter )  and then also put in three other 250mm diameter piers for the tripod feet.

I am not 100% sure if i should be looking at going a tad larger in diameter with the main pier or not,  the plan was to have all the piers sit just below the deck of the observatory,  just below the floor boars.

looking at iOptron site for a permanent pier, it looks like it's base is 240mm x 240mm square

But the main question is in regards to the main pier and how deep i should possibly looking at going,  the ground around here is pretty much clay based ground.

Any advice would be greatly appreciated.

Thanks.

 

 

 

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The pier footing should be below the frost line so that your alignment will not be disturbed by the ground "heaving".   At my west Texas observing site I had to go down about 15" to get to the frost line when I installed my Todmorden pier.

 

Todmorden-TX-3.JPG

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18 minutes ago, CCD-Freak said:

The pier footing should be below the frost line so that your alignment will not be disturbed by the ground "heaving".   At my west Texas observing site I had to go down about 15" to get to the frost line when I installed my Todmorden pier.

Frost is not the only cause of ground heave. Rainfall on clay can be enough. Chopping down trees can create the same effect as the volume of water transpired is changed. It's all about local conditions and how they change.

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2 hours ago, UKDiver said:

Frost is not the only cause of ground heave. Rainfall on clay can be enough. Chopping down trees can create the same effect as the volume of water transpired is changed. It's all about local conditions and how they change.

Good point.  if you are on clay that will be a concern that will require some research.  I am pretty lucky since my site is fairly dry and drains well.  The soil is quite firm even after a heavy rain which doesn't happen often.  Last month we got .06".  Digging the hole was a bit of a chore.  I was having to soak the ground with water to soften it so I could dig.   

One thing I know to do is to "bell shape" the hole at the bottom before pouring the concrete. 

Todmorden-TX-8-sm.JPG

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I am on clay and had no idea what was the 'right' depth when digging in 2007.

But if you skimp and have to start again, there is a big lump of concrete to break out - probably after removing the observatory.
Much more work than just digging a bit deeper in the first place.

So I decided concrete is cheap. A bigger/deeper hole is only a bit more effort.
I went for almost a metre into the ground. From memory, the hole was about 600mm round, like a bell pit mine, maybe a bit bigger.
No doubt more than it needed to be. But it has not moved with the seasons and weather.

To loosen the really tough clay (worms in my garden have muscles like Arnie Schwarzthingy) I used a 6" post hole borer.
This being much easier than working with a spade in a deep hole.

I chose to continuously pour concrete above ground level, into a 300mm diameter sleeve.
The sleeve being galvanised steel, spiral ventilation duct.
Again more than necessary. But if you kick the pier you swear and the scope doesn't budge.

In my case I was paying someone with a cement mixer to do the shovelling and pouring. I was helping/supervising.
If you do the concrete yourself, even if only below ground, hire a mixer.

HTH, David.

 

 

Edited by Carbon Brush
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48 minutes ago, CCD-Freak said:

Good point.  if you are on clay that will be a concern that will require some research.  I am pretty lucky since my site is fairly dry and drains well.  The soil is quite firm even after a heavy rain which doesn't happen often.  Last month we got .06".  Digging the hole was a bit of a chore.  I was having to soak the ground with water to soften it so I could dig.   

One thing I know to do is to "bell shape" the hole at the bottom before pouring the concrete. 

0.06"! That's half an hour's worth here. The reason Devon is so green. 😄

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57 minutes ago, UKDiver said:

0.06"! That's half an hour's worth here. The reason Devon is so green. 😄

The average annual rainfall at SRO is 21.68" most of which comes in a "couple three" thunderstorms in the spring and early summer.  We had a thunderstorm there back last spring that dropped right at 3" in just over a half hour and I was able to drive on the ground without getting stuck just 2 days later.  It does not stay green there for very long. 😜

SRO scope area.JPG

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9 hours ago, CCD-Freak said:

The average annual rainfall at SRO is 21.68" most of which comes in a "couple three" thunderstorms in the spring and early summer.  We had a thunderstorm there back last spring that dropped right at 3" in just over a half hour and I was able to drive on the ground without getting stuck just 2 days later.  It does not stay green there for very long. 😜

That's about 2/3 of ours, although I don't have to travel far up to the moors (wet hills) to get double the rainfall. Shows how important the rainfall spread is.

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When I built my observatory in 2011, I followed the old tried and trusted 1m cube method to mount the pier.

pic12.jpg.5a03b9e299caf3cf01c1e7c9a1dc3751.jpg

The bottom of the hole was filled to a depth of 200mm (8") with ballast and then a strong mix of concrete added

pier2.jpg.0b04fa26f04d8c4a38792a883516472b.jpg

Now naturally climate is different between Texas and Hertfordshire, and in my case the tube was later filled with reinforced concrete rather than having a commercially made pier bolted to the base, but I've not experienced any issues with frost or dampness on the pier over the years

 

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This is interesting, particularly the clay bit as I'm planning a build this spring. My address includes a reference to "clay" so you can probably guess what it's like round here.....

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