How big is your pier footing?

[codemonkey]

Now that I have a pier (thanks Phil!), I need to start thinking about the footing. I've heard that you need at least 1 cubic metre, and then I've heard that you don't need anymore than 100mm deep. It's hard to know which advice to listen to, especially when what works for one might not work for another due to other variables.

So... how big is your pier footing? How would you describe the soil in which it sits (clay, sandy etc)? How often do you need to re-align your mount?

My soil is fairly sandy. I was more or less thinking of digging down until it got hard and using that as a gauge.

[Paul Haese]

I used 1 cubic metre each on all of my pier supports. In sand you will need to go fairly deep. Maybe go down to 700mm and see what is there.

I have not had to realign my first mount ever in 4 years since it was installed.

[Manav]

Depends on payload and size of mount, general rule of thumb is around one cubic meter. I found that too be a bit of a overkill for my setup and settled on 700mm^3.

[codemonkey]

Great, thanks Paul :-)



Thanks Yugant :-)

I'll be using an either an RC8 or an ED80 NEQ6Pro and I'm unlikely to be getting any bigger scopes, though I suppose I may upgrade the mount at some point. Not really sure what that weighs in total.

If my math is correct, I think 1m^3 will require roughly 1080kg of gravel, 720kg of sand and 360kg of cement, not to mention water. Man, that's a lot of hard work right there. My planned site isn't readily accessible to large vehicles so that would have do be done by hand (with a mixer preferably).

I might need to reconsider my planned site because I'm allergic to hard work ;-)

[CJ]

Can you get a small truck in there? Google "small concrete trucks".
You can get small pumps too.

Cheers

[jenchris]

I am using only about .25 cu m.
But my soil is non existent and my sub base is granite covered by chocolate rock.
In two years it has moved not at all and it has no resonance even if I jump up and down on the ground near it.
It really is a matter of how well your bond is with the surrounding substrate.
Driving star pickets at angles into the hole base helps to increase integration

[codemonkey]

Thanks Chris :-)

Possibly, I'm not entirely sure. To be honest I didn't think we'd get the ute up there but the wife is confident we will and I'm a bit spatially retarded so maybe.

I didn't know about those small pumps, that could be a winner if we can get the ute up there with a trailer.

Might consider a site closer to the house that would be more accessible, but I might have to cut down some trees then.



Thanks mate :-)

[codemonkey]

I think I might try something a little bit left field. I might try building the footing out of besser blocks and hebel. Should greatly reduce labour and be fairly cheap. If worst comes to worst and it proves ineffective I'll just cover it up and do another one nearby.

And of course now that I have everything dismantled, we have the first clear night (while I'm actually home) we've had in donkeys.

[louie_the_fly]

Do you have a soil report for your block? If yay then use that as a guide. Otherwise consult your local council and enquire about soil types in your area. have a look at this site. http://www.build.com.au/building-reactive-soil-sites

My site is only slightly reactive, and top soil is shallow. My footing will only be 600 x 600 x 700 deep. because it can be.

[glend]

It's all about the mass, and block footings for a pier are not going to have the density of a solid block of concrete. Hebel blocks are aerated concrete.
Your going to have to dig the hole anyway, so why not use concrete. The big labour is in digging the hole. You can get concrete delivered and save on the mixing labour.

[codemonkey]

I don't, but that's a very good point. Thanks very much :-)



All good points Glen, but the problem is I don't think I can get that delivered due to limitations in site access, as described above. Nearest water will also be 100m away (the dam, with no pump available to draw water to the site either), so there's a number of complications there.

I've seen this done with besser in the US, apparently without problems. I think there' more too it than just mass, there's going to be a lot of variables here. Inertia is important but only if there's forces acting on object. If the soil isn't very reactive I can probably get away with less. Additionally, who's to say that we actually need these massive 2000kg footings? Maybe similarly sized footings of 500kg is enough. I suspect we generally over-engineer things.

I'm going to give it a shot... if it proves ineffective you can tell me "I told you so!" :-)

[glend]

Well I won't tell you that because my pier and footing are under spec according to the experts - good luck and try to use some star pickets in the construction if you can.

[codemonkey]

Thanks mate :-) It might not work but it'll be cheap and relatively easy to try so I don't think there's a lot to lose.

That's one thing I meant to clarify above but forgot about. How is it best to integrate star pickets into the construction?

[Capricorn1(Tom)]

Minimum 1 cubic metre for astrophotography......visual......?

Cheers Tom

[codemonkey]

Sorry I'm not sure I understand exactly. Are you suggesting that minimum 1m^3 for astrophotography but visual you're not sure? Or are you asking whether I'll be doing AP or vis? If the latter I'll be strictly AP, with both an RC8 and an ED80.

Thanks :-)

[Dealy]

I reckon you're pretty right about that. Unless you have a scope the size of a mule I'm sure something the size you're talking about will work fine.

[Neil]

:D:DMine is in clay, so I went 1 cubic metre, over two years now It hasn't moved, and by the way the hard work is definitely digging the bloody hole!
Clear skies.:rofl:

[codemonkey]

Thanks :-)



Spot on there, digging the hole certainly wasn't fun! I've only gone 1100 square by 500 deep for the hole. Footing will finish about 100 above the ground, giving a total size of 1100L x 1100W x 600D.

Top 200-300 was nice, soft, sandy stuff, then we hit a layer with a fair amount of rock, below which was clay. Hopefully this'll do the trick, because even that size was hard enough yakka for an office slob like me ;-)

I have a local guy coming in to do the concreting tomorrow morning. He was going to get a truck in but after seeing the site he confirmed my suspicion that it wasn't possible and will have to be done by hand.

Turns out since starting this thread my mount has changed again and it'll now have an EQ8 sitting on top of it. For the moment I'm literally going to set the pier tripod it came with on top.

[codemonkey]

No going back now!

[jenchris]

If you need 1cu m for a footing, how come a tripod works?

[codemonkey]

Well the main reason I had this built was to try and reduce set up time and the need for frequent polar alignment, which a tripod doesn't provide me with. Ideally this will mean I never have to do it barring the initial alignment, but I'd be happy with infrequent, small adjustments.

[el_draco]

I'm in the process of digging the hole and casually glanced at my compost bin the other day. I hate the idea of having to build the form work in the hole and this may provide the perfect solution. The bugger holds 750l which would equate to about 1.5 ton of concrete and its got a bit of a taper on it with a round hole at the top.

Anyone ever tried this?

[sheeny]

Mine is 350 x 350 x 750 deep. It would've been 1000 deep but I hit rock at 700mm so chipped into it about 50mm to key the footing to the bed rock and you can't get any better.

Depth is what you want in a pier footing rather than mass. The pressure holding the footing from the sides increases with the square of the depth, and you can have active soil layers above the bottom of the footing which can expand and shrink all they want, but the footing won't move. Same principle as foundation piles.

Even a huge massive footing on top of an active layer will move.

Al.

[sheeny]

Formwork in the hole? Nah! Pour direct. Too much risk of inadequate compaction and subsequent subsidence around the pier, or worse... uneven compaction causing the footing to lean over - ever so slightly, but over a period of time, and you'll be chasing polar alignments all the while.

Al.

[Meru]

Hi Lee,

I only used 0.4m^3 (against general consensus) and had a similar setup, RC8 and NEQ6. I didn't just dig a square hole, it was a wide slab and directly over the pier i dug deeper:

http://www.iceinspace.com.au/forum/showthread.php?t=114718 (see page 2)

Never had any issues since and works fine, you really dont need a tonne of concrete (excuse the pun :P) to make this work. I live in Vic and used a company called Vicmix, very cheap and they come to your door. Maybe call them and see if they have contacts in QLD?

On a side note, my mount is screwed onto a plate, and then this plate is screwed onto the pier. This way i can take my mount inside by unscrewing the plate and when i put it on next night its still polar aligned. In Melbourne where it rains and is cloudy for weeks at a time, this is really helpful in prolonging my mount's life (and not worrying at night if rain or frost is on my mount ;)).

Just decide a way forward and stick with it. Goodluck!

[codemonkey]

Thanks everyone! :-)

I ended up going about 1100x1100x600D, about 100 of that is above ground though. I'm actually glad I did, because we found some documentation from the previous owners of the property when they had the bore put in. Seems the clay that we hit about 30cm down (well, clay mixed with first sized chunks of granite) continued down for 3m. There was no way we were going to dig down until we came into something better, so I'm happy enough. Remains to be seen if I ever need to realign, and of so, what the frequency might be... still haven't got it sorted out yet.