Pier Footing Question

[glend (Glen)]

I am digging my pier footing for my ROR observatory, and need some advice on sizing. First I have read through what seems like a hundred different threads on various forums regarding pier design etc. This pier will be a full concrete one from footing to the adaptor plate. What I am struggling with is knowing when I can stop this horrible digging. My footing hole at the moment is 500 x500 x 600 deep in what has to be the hardest ground I have ever had to deal with. The soil is a mix of compacted sand and clay and small stone aggregate which seems to be in a ratio appropriate for concrete itself. The only way I can progress with the hole is to use a big breaker bar and my hammer drill with a masonary auger bit to drill holes in the bottom that I can then break up with a small mattock. It is back breaking work for an old man.
I know the footing size is all about the type of ground so I'd like some opinions on just how far I need to go? I intend to widen the hole out at the bottom before I put the concrete (and reo) in. The mount is a NEQ6, and the largest scope used will be my ultralight 10" imaging newt which weighs about 14KG with everything on it. My current tripod sits at 805mm high to the base of the NEQ6 head, so I am thinking that the pier column will be no longer than that inside the ROR Obs (which will sit on treated pine footings). The pier will comeup through the floor and not be attached in anyway to the building, so I am thinking a max length of the pier to be
something like 1200mm allowing for distance to the top of the footing below the building. The concrete for the pier will be poured into a tube form and there will be reo in the tube.

So how far to towards the centre of the earth do I really need to go?

[multiweb (Marc)]

I think you're good to go. I have very similar dimensions in clay soil. Make sure what's underground is tapered like a pyramid. Put some reo in and rods from bottom to top right through the pier itself.

[I.C.D (Ian)]

G’day Glenn,
Marc is right you are right to go with the hole as Marc said make sure the bottom is bell shape and you use reo ( if go to where they are building a new homes ,and if you ask they may have scrape reo laying around save you buying some) .How are you going to attach HEQ6 to the pier when you fill it with concert

Ian

[rally]

Glen,

As you say its all about the soil type !

You mention clay - and that is one of the worst types !
Clay absorbs water in Winter and expands and dries out in Summer and contracts.

Is your ground hard just because its Summer or is it always like that ?
If its only because its presently dried up clay, then you will need to keep digging - ideally (if at all possible) until you have broken through the clay layer.

A typical stable pier footing in ordinary soil would be between 0.5 and 1.0 cubic metre - whereas what you have described is 0.15 cubic metres.

If its in rock or stable conglomerate then I guess you could be smaller, but key to a piers design is mass, the other is stability - such that when the ground moves due to moisture your pier does not.

I assume you have tried filling your hole with water to soften up the ground ?

Rally

[xelasnave]

I went down to 5 ft hit bed rock cut into it another foot and it is rather stable.
If I could not get to bed rock I think I would frankie pile it as well...drive iron rod or water pipe into ground past your hole so ends of pipe sit in block when poured.
Just trying to worry you ...where you are should be fine...best of luck.

[glend (Glen)]

Thanks for all the advice. I decided to go down further, and eventually got past the conglomerate into the pure clay underneath, where things got a little easier.. I then excavated the sides to make the hole a trapizod shape. Final figure was, 700 x 700 x 700 deep at the bottom, I carried the 700 x 700 up the walls about 200mm and then sloped the remaining 500mm of depth to 500 x 500 at ground level. I then drove a 1.8m galvanised steel lintel into the centre of the hole so that it just reached top of the hole (that's 1.1m into the earth below the bottom of the hole). That whole space has now been filled with concrete, and my pier form on top as well. I used a couple of layers of lengths of rebar inside the pit in a tick tac toe pattern, and a few rebar verticals inserted into the footing and extending into the pier column. An exhausting day, especially with yesterday's heat.

I will let it cure for a while. What's an adequate length of time? I was thinking at least a week. Should I do anything to help it cure, like hose it down (although unlike a slab, most of it is hidden underground).

[jenchris (Jennifer)]

Goodness, I hope you don't have to remove it.
Curing time depends on the type and mix of concrete.
It will be stable within 72 hours no matter what.
Keeping it cool by hosing will stop it cracking.
I doubt a crack would be of any consequence.
I couldn't get into my ground more than about 700mm simply because it is chocolate rock/cracking clay on top of granite monolith and very hard; my pier hasn't moved as far as I can tell and it certainly isn't loose!

[louie_the_fly (Stew)]

Glen, probably a bit late to do this, but cover it with plastic film to slow the curing process, which increases the final strength.

I'm in the planning stage of doing mine. Hope to get it done within the next month. How tall and what diameter is the pier, and what did you use to cast it? Cheers.

[glend (Glen)]

Stewart It is shaded by some lock together camping foam floor matts and wet down every few hours. Its been pretty hot here so I am keepng it cool for awhile.

Re the pier metrics, which I have hesitated to discuss because I know it will touch off alot of discussion. I did read all of Dennis Persyk's fine article on Cloudy Nights about Pier Design:

http://www.cloudynights.com/page/art...amentals-r1236

However, Dennis focuses on steel pier columns with concrete bases and not a continuous pour type concrete pier/footing combination. I do agree with Dennis' design objective which is ...." if a 5-pound force is applied to the deflect the top of the pier, the star will move no more than 0.5 arc second. This will meet the needs of 90% of the amateur astronomer community."

Since I was building a total concrete solution I consulted my brother-in-law the concretor/builder and his son the structural engineer when making this decision. I know alot of people here will argue for a minimum of 8" or even 12" but in my structural engineer's view this adds mass above the footing (which should be minimised). My pier finished up at 164mm outside diametre (6.4"), with 154mm of internal concrete with rebar going full length right down into middle of the footing. The column itself is 1000mm long from the top of the footing. With my lightweight imaging setup I think it will be ok. It will certainly be better than the tripod I have been using. If I find it necessary I have a plan to further brace the pier vertical ( I already have it braced with a concrete collar extending around and up 150mm from the top of the footing). Holes were drilled in the PVC form tube to facilitate concrete flow through and bonding in this section. So only 850mm of the column is freestanding at this time.

[niko]

sounds great Glen. Can we see some photos?

[glend (Glen)]

Originally Posted by niko

sounds great Glen. Can we see some photos?

Ok Niko, not much to see really it's just another pier. Photo attached.

And before the questions, the ground mount solar panels are feeding my shed battery bank and power system - also will feed the observatory from the same 400AH bank.

[niko]

looks great - can't wait to have similar

well done

[strongmanmike (Michael)]

Originally Posted by glend

Ok Niko, not much to see really it's just another pier. Photo attached.

And before the questions, the ground mount solar panels are feeding my shed battery bank and power system - also will feed the observatory from the same 400AH bank.

That looks more than adequate for what you have described, nice job

Mike