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Astronut1982

How deep does my pier need to go into the ground?

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Hi,

I'm currently digging out the hole for my pier to go into for concreting & I'm wondering how deep I need to go. The hole is 2 foot x 2 foot in width & was originally planning having it 4 foot deep. The problem is, as I get deeper it's getting more & more difficult so I was wondering if 4 foot is a bit overkill & could i get away with just 3 feet?

I'm putting in steel reinforcing bars to steady it & 3 feet is pretty deep (Its more than half my height!) so I'm thinking that should be sufficient but if anyone could let me know if that's enough or not, my back & I would be very grateful! :grin:

Cheers,

Jeff

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I would say so :) Mine isn't quite that deep but a bit wider - with it's integral concrete pier it's rock solid :)

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However deep you dig, don't forget that you'll have to dig another hole the same depth to bury the spoil from the pier's hole.

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That to me sounds more than enough, the footings on my house extension are not that deep, what your looking to avoid is frost heave

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Thanks for the advice guys, 3 foot it is then!

I'm off to get a trench spade in the morning as well to make life easier. I've stupidly been digging it with a standard garden spade & my back is aching now!

Jeff

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I would suggest a hole one meter square, one meter deep , the pier should sit in a metric tonne of concrete if your doing astrophotography

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It's funny you should mention that Martin as we did find some broken up blue & white pottery. It was a bit dissapointing though as there was a Roman settlement where I live so I was hoping for gold coins or something else equally valuable!

I was hoping my 2ft by 2ft by 3ft would've been enough as well. :embarassed: I'll have to make another trip to the builders merchant then.

Tinker, thats one of the items I was was thinking of buying to finish digging with, either that or the long trench shovel. I'm going to have a look at both in the morning & see which would be best for what I'm doing.

Jeff

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Make your hole wider rather than deeper

That's good advice IMHO...

My pier in Taunton was on a 1.5ftx1.5ftx1.5ft concrete block - I had the hole dug and the concete prepared by my neighbour who lays tennis courts/ embeds basket ball hoops etc for a living. The ground where I was was pretty firm. Two severe winters and there was absolutely no movement.

Where I am now the hole needed to be about 2ft by 2ft as the ground is a little softer. If you've run into harder ground then you can probably stop digging :smiley:

James

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My pier is 1m deep with a 40cm x 40cm sized hole all filled with concrete. It's certainly very very solid.

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Hi. I dug the hole for my pier 3 x 3 x 3 feet. I also made the pier itself from concrete. The whole thing was done in one pour so the base pad and pier are all one piece - no joints - and rebar goes right down through the pier and into the underground 'cube'. It's very rigid indeed.

That (very permanent!) one-piece construction may not suit everyone but it does eliminate the need for base-plate bolt fixtures. As well as a getting mass into the base, for astro imaging it's important to get the whole structure as rigid as possible. Watch out for weak points of potential flexure at the pier base and at the top mounting plates (think short fat bolts for an adjustable top plate rather than tall thin 'stilts').

Btw, I was advised to avoid putting the rebar rods right into the ground at the bottom of the pit (where they can start rusting), but to ensure that they stopped short of the bottom of the concrete block so that the rods were fully encased and no part of a rod was closer than 2 inches from the outside edge of the concrete.

Good luck with the build. I guarantee that you will never regret putting your scope on a permanent pier!

Adrian

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For the pier, I have PVC piping & I'm planning on concreting in one go as well. Does the piping go all the way to the bottom or should it stop a little from the bottom of the hole & the rebar stick through to the concrete in the hole to strengthen it?

Jeff

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I'd certainly run the rebar from near (but not right AT) the top of the pier all the way down to within a couple of inches of the bottom of the hole. This can only further strengthen your one-piece approach.

Regarding the PVC pipe, though ...... I haven't made one this way, but personally I would not extend the pipe into the below-ground concrete. In other words, regard the pipe as just a mould for the above-ground part of your one-piece pier. My thinking is that the strength of this design is the continuous solid reinforced concrete mass, and that the PVC pipe adds no strength. In fact, maybe it weakens the structure if it goes deeply into the base block and thereby creates a break in the solid concrete. However I'm not absolutely sure about the mechanics of that and others may like to comment.

My own preference actually would be to raise the PVC pipe several inches above ground level and support it on some wood formwork - a square box a little wider than the PVC pipe. When you pour the concrete, this would form a low concrete plinth above ground level that the cylindrical pier sits on. Just feels better to me to go from the wide base block to the relatively narrower cylinder in two gradual steps. But that's just me and I'd be interested to hear other opinions on whether or not to (or how deep to) sink the PVC pipe.

Adrian

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BTW, you probably already know it, but it's important to agitate the concrete as you go, and to tap the sides of the pier to help get air bubbles to the surface and avoid voids (?) in the setting concrete.

Adrian

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I used large plastic plant pots (less bottoms) to enlarge my 7.25" pier to over 12" at the bottom. The bottom of the bottom pot was held in a piece of chipboard fastened onto the top shuttering around the base to stop the concrete leaking out as the pier was poured. I too thought a sharp change in size was a bad idea - I have a little grounding in structural engineering and know that sharp changes in reinforced concrete can cause problems in load bearing structures. Of course we are not building a great big bridge or anything of the sort but I think the principles still apply. You certainly don't want the pier plastic tube going into the base part.

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If you DO have to have the plastic pipe go into the base for some reason, drill lots of big-ish holes in it so that the concrete of the base can "connect" with the concrete inside the tube.

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There's no actual reason for me to have the pipe go into the concrete so I won't bother, I just thought that was what I was meant to do. I have 4 1.50m lengths of 12mm rebar so I'll just thread those into the base & just have the piping above ground.

Jeff

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Hi,

I'm currently digging out the hole for my pier to go into for concreting & I'm wondering how deep I need to go. The hole is 2 foot x 2 foot in width & was originally planning having it 4 foot deep. The problem is, as I get deeper it's getting more & more difficult so I was wondering if 4 foot is a bit overkill & could i get away with just 3 feet?

I'm putting in steel reinforcing bars to steady it & 3 feet is pretty deep (Its more than half my height!) so I'm thinking that should be sufficient but if anyone could let me know if that's enough or not, my back & I would be very grateful! :grin:

Cheers,

Jeff

Sounds deep enough to me. If the ground is as firm as you say it is, then 3 feet is plenty. Knocking the re-bars in is also a good idea.

I hit bedrock after 2 feet so couldn't easily dig much further anyway. Still - it's nice and solid though!

Img_7319.jpg

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I would suggest a hole one meter square, one meter deep , the pier should sit in a metric tonne of concrete if your doing astrophotography

A cubic metre of concrete weighs approximatley 2.3 tonnes not 1 tonne, should hold a scope :laugh:

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