How big is your pier footing?

[codemonkey (Lee)]

Originally Posted by jenchris

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

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 (Rom)]

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 (Al)]

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 (Al)]

Originally Posted by 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?

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 (Michael)]

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/s...d.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 ) 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 (Lee)]

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.