To pour or buy pre-made block of cement for base?

[FrancoRodriguez]

Hi there, fellow astro dudes!
Hopefully a very simple question that won't take up much of people's time...

As the title says, is there anything wrong with simply buying a big cement block, like an old retainer wall, around 1m^3 and chuck it in a hole, and drilling holes for screws to put a pier on? I'm guessing they don't have rebar inside. Is that an issue?
The other option would be more costly (and I don't know what I'm doing so would need to get a concreter): To fill up a 0.8-1m^3 hole with cement slurry +- rebar (is that worth it?). There's also the option then to put J pegs in the slurry (also, is that worth it? It sure would suck if they inserted them wonky).
It'll be the DDM85 mount I mentioned here the other day. The total weight would probably be around 100-110kg all included, pier and all (a wild guess).
TIA!

[glend]

I poured my footing, hand mixed from Bunnings concrete bags, complete with rebar. You need to work quick, wet on wet. And my pier, is also concrete, and part of the contiguous pouring into the footing just as soon as it would support the form tube weight, used a big plastic pipe for the form, with reo down into the footing and galvanised L bolts placed in the top.
The problem I see with buying a big cement block is how the heck do you transport it and get in into a hole. Bags of concrete are easy to transport and move.
Which ever way be prepared to be sore for days, unless you hire someone to do it.
I also dug (by hand) my observatory footings.

[Sunfish]

A mass precast concrete footing with chemset bolts would be fine if it was sitting on compacted hardcore so it did not move and then back filled with hardcore. But you still need to get it there and dig the big hole and compact the bottom. Wet concrete in a fresh hole is easier. Hand digging a big hole is ok if you are fit and have the time.

I can not see why, if you have the access for a small track mounted machine you could not drill 3 , 450mm diameter holes 2m deep and fill them up with concrete , just like any building . Or even one would do. A contractor will drill and fill, with a barrow if required . With reinforcing in the top part you can then join by hand forming a small slab on top for the pier base, or chemset straight on. You can add a few extra piers for your future shed while you are there.

[FrancoRodriguez]

Cool, and it doesn't hurt to live next door to a professional fencer! He's got a massive auger and a bobcat. Well then...wet cement it is! I was just thinking because I have a bunch of massive cement blocks on my property...but maybe they were put there for a reason (retainer wall and such) Thanks :)

[leon]

Defiantly a pour, blokes will eventually find different levels and will fail.

Leon

[Wavytone]

Guys please bear in mind the question of what happens when the time comes to remove this. Down the track it could even become your own problem, or your partners...

[The_bluester]

I am strongly in the "Pour" camp (Hoping to pour one in the near future actually)

If you are going to use a block of concrete to mount a pier it needs to be poured into the hole, if you crane in a precast block then it is inevitably going to settle and move around over time a lot more than a cast in-situ block will. And if you don't need mechanical assistance to lift it in to the hole then you are probably not using a big and heavy enough block.

Nick does have a good point though. I have lived where I am for 18 years and don't have any plans to move for decades yet, but even for me I am looking forward. The piers would need at least partial demolition but the obs plan is for one that is big enough to simply use as a functional shed by just locking the roof down should astro use come to an end.

[Sunfish]

A good argument for drilling a big 450 hole to ground level , filling with concrete and bolting on a removable steel pier.

[bojan]

Have a look here:
http://www.iceinspace.com.au/forum/showpost.php?p=1447616&postcount=25

[I.C.D]

Hi If you have not poured
your pier you mite have a look at this type http://www.surefootfootings.com.au/
Ian

[FrancoRodriguez]

Hi, I'm the OP. I only just saw your post. I haven't made the foundations as yet. What a great idea--a concrete free footing. Have you got any experience with these? I wonder if they would be good for a pier or if they only got their increased strength when a few were laid down and connected by beams, as per their intended purpose? I do like the idea of being able to remove it if needed (although it would be highly unlikely that this would need to be done)

[DarkArts]

They're fine for a pier. I'd recommend the S500 model. There's a YouTube video of somebody setting one up at a dark site - it's pretty easy.

Edit: Here it is ... https://www.youtube.com/watch?v=GSuO1Sruktg

[Wavytone]

By you, probably not. Next property owner - and anyone looking to buy - almost certainly.

As an engineer I can say most people do not think about how to dispose of what they have created. But the time comes, for everything.

[Sunfish]

Screw piles are a fine alternative to drilled and filled concrete piers in soft ground which is their intended use. Really an alternative to hammered piles. Not too good in rocky soil or with big floaters and no cheaper than concrete as cost is not their advantage. There are many types of small scale angled pile footings that can be placed using a jackhammer which are not really screw piles but affective if access is a problem. Not necessarily better or cheaper or as long lasting as concrete but that may be fine for a telescope mount, (or a granny flat in soft soil with no services nearby)

[StarChildDazee]

Yes, saw this type of footing being applied to a small shed somewhere on Youtube recently.

[Troy]

So do you have to buy a gadget to vibrate/knock the tubes into the base plate?

[appiice]

I think the OP should go with the " buy a block option " if he does, can he post some video of him lifting a 2.5 ton mass into the hole, best to wait until the end of the month though.

[AstroApprentice]

The poles go through sleeves welded onto the base plates at different angles. A jack hammer is used with a special adapter that fits into the poles. I only had to hire a jack hammer for half a day as I found it amazingly quick to bang them into sandy soil. The adapter has to be bought from Surefoot.

[jamespierce]

Another vote to just pour, a cubic meter of concrete isn't very expensive. The crane truck you will need to move a block like that will be real money, and you still have to dig a very big hole!



It cost me about the same to get the digger in to drop in the hole and footings for the deck. Pouring concrete isn't hard either, you just need to be prepared. We had a big column drilled with a 600mm auger bit, set in a bolt cage and then have a steel pier above that.

[StarChildDazee]

James Pierce, Absolutely, I just recently poured a slab for my spa and spa deck as well as footings for my Viewing deck. 5 guys and a couple of hours later it’s all done.
1 1/2 cu meters of sand/gravel mix and some concrete used.
Now my cruise has been cancelled, I can now get on with both deck constructions and stay home and just star gaze.
All pending clear skies lol