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Old 28-11-2020, 08:29 AM
RugbyRene (Rene)
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Pier footing depth - how deep?

Hi all,

Iíve been given the go ahead by the aesthetic committee to install a pier in my front yard. I have the pier in mind but I was wondering how deep I would need to go for the concrete base. Iíll be using it for astrophotography and it will hold a SW AZEQ6 mount, and a SW Esprit 100 (with all the bits attached).

The problem I face is my house on Sydneys north shore sits on a heap of sandstone. Thereís sandstone all over the place. In fact you donít have to go that far into the ground to strike a huge block of it. Iím not that keen to be drilling into sandstone.

So I was wondering what is the minimum depth I could get away with so the pier is stable enough for astrophotography.

Rene
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Old 28-11-2020, 08:46 AM
glend (Glen)
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Well if you have sandstone below the surface, you can look at using that as part of the footing. Dig down as far as you can, and then use a hammer drill to bore some holes in the sandstone. Get some short rebar lengths from Bunnings, using an angle grinder cut it down to maybe 200 or 300mm in length, put some good construction epoxy on one end and drive it into the sandstone. Don't use the cheap 5min epoxy, that stuff is hydroscopic and will break down if constantly wet. Now you can pour your footing over the sandstone porcupine, and it will be fine.
Typically footing dimensions can vary from 0.7m x 0.7m x 0.7m to a full cubic metre depending on your subsurface and rock etc. There was a thread on the Cloud Nights forum about a guy building on rock, and he just put the footing right on the rock. My footing is on clay and conglomerate, and I just hammered in some galvanised angle iron, once my back said enough of this digging. You will likely get all sorts of opinions on this topic, but there are no absolutes.

Last edited by glend; 28-11-2020 at 09:23 AM.
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Old 28-11-2020, 09:35 AM
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leon
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Hi Rene, I think Glen has nailed it, so to speak, that will work a treat, at least you know your sand stone block is well established and sturdy.

Leon
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Old 28-11-2020, 10:18 AM
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The_bluester (Paul)
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I went for a full meter square and meter deep. I am in an area which is fairly rocky but it is mostly floaters and the rest is fairly heavy clay, so I wanted a good solid lump of weight under there. I ran in to a rock right at the bottom so I pinned it to that as Glen suggested above. Might as well make use of the mass if it is there.

In your own case if you run in to sandstone you probably need to work out if it is a big solid lump of it or something smaller to decide if the best bet is to break it out and keep going, or pin to it and make it part of the block. When I was doing that, I have a big hammer drill so I plunged the hammer bit in to it's full depth, I figured if it broke out of the other side I should break it up but as it was still in rock at full depth for the bit I assumed it is a reasonable size and pinned to it.
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Old 30-11-2020, 01:07 PM
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doppler (Rick)
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Hi, it really depends on your ground. My yard is hard compacted sandy loam with decomposed granite. My pier base is 350mm x 350mm and about 600mm deep, the pier is 1200mm tall and carries a NEQ6 and 10"f4.8 newt. Its been 2 years and no movement yet. A pier is a lot more stable than a tripod so really you cant go wrong whatever you do.

A word of caution regarding driving steel into the ground then pouring concrete on top, when the steel eventually rusts it will expand and crack the concrete (google concrete cancer), exposed steel is a big no no in professional concreting.

Rick
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Old 30-11-2020, 01:47 PM
Startrek (Martin)
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Iím setting 4 piers in my NexDome , 3 for the tripod and 1 central pier for a pier mount later on
Piers are 250mm in diameter and 800mm high which sit into a concrete footing 450x 450 x 500deep
My ground is as follows - 200mm of top soil then clay underneath that to 500mm then I assume rock below that.
Iím allowing at least 2 months to settle in before my Dome arrives , hoping some good rain comes as well to aid the process.
I donít think itís going anywhere !!
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Old 30-11-2020, 02:37 PM
jahnpahwa (JP)
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Quote:
Originally Posted by doppler View Post
Hi, it really depends on your ground. My yard is hard compacted sandy loam with decomposed granite. My pier base is 350mm x 350mm and about 600mm deep, the pier is 1200mm tall and carries a NEQ6 and 10"f4.8 newt. Its been 2 years and no movement yet. A pier is a lot more stable than a tripod so really you cant go wrong whatever you do.

A word of caution regarding driving steel into the ground then pouring concrete on top, when the steel eventually rusts it will expand and crack the concrete (google concrete cancer), exposed steel is a big no no in professional concreting.

Rick
Good lookin dog, Rick.
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Old 30-11-2020, 09:59 PM
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muletopia (Chris)
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Rene,
I have a 1.3*1.3*1 metre lump of concrete under my pier. Absolutely still during imaging,PHD2 guide trace shows no sign of me walking around in my ob.
BUT the annual wetting and drying of the material in which it is placed means that polar alignment is required every few months.
So you probably have to be prepared for this, even following the advice in previous replies.



Chris
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Old 01-12-2020, 12:43 PM
RugbyRene (Rene)
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Thanks for all the info. I think a cubic metre of concrete under my front yard would not go down well with the wife. I don't think need much as the pier will be in the front yard and I'll be in the house. The only source of vibration would be traffic but as I live on a fairly quiet residential street and it will be night time, I don't see that being much of an issue.

Rene

Last edited by RugbyRene; 01-12-2020 at 12:58 PM.
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Old 01-12-2020, 12:53 PM
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h0ughy (David)
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Quote:
Originally Posted by glend View Post
Well if you have sandstone below the surface, you can look at using that as part of the footing. Dig down as far as you can, and then use a hammer drill to bore some holes in the sandstone. Get some short rebar lengths from Bunnings, using an angle grinder cut it down to maybe 200 or 300mm in length, put some good construction epoxy on one end and drive it into the sandstone. Don't use the cheap 5min epoxy, that stuff is hydroscopic and will break down if constantly wet. Now you can pour your footing over the sandstone porcupine, and it will be fine.
Typically footing dimensions can vary from 0.7m x 0.7m x 0.7m to a full cubic metre depending on your subsurface and rock etc. There was a thread on the Cloud Nights forum about a guy building on rock, and he just put the footing right on the rock. My footing is on clay and conglomerate, and I just hammered in some galvanised angle iron, once my back said enough of this digging. You will likely get all sorts of opinions on this topic, but there are no absolutes.
Absolutely better using the existing rock
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Old 01-12-2020, 03:17 PM
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The_bluester (Paul)
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I will add, I put in a cubic meter (And a bit) but the main reason was because I can. Even some pavers well set into the dirt for your tripod feet will improve the situation from just setting up on the dirt, in terms of easier and more repeatable setup.

The problem is that mine is about to breed, I need at least another two and possibly three blocks. One for a pier for my new mount which is on the tripod in this photo, one for a friends pier and a fourth to have somewhere flat, level and hard for a dob to get put down.


Then an observatory over the top!
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Old 04-12-2020, 04:10 PM
appiice (Ed)
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Hello Rene

Bit late to the party but here is my 2 cents worth, I think Glen was right about using what you have, sandstone has a strength of around 77Mpa, concrete on the other hand in a house slab will be about 30, higher in other applications, but 77 would be rare.
Sandstone would be easy enough to drill into with a decent drill - not a consumer " hammer " drill look for a SDS drill, it will be like a hot knife through butter, construction epoxy ( https://ramset.com.au/Product/Detail/4/ChemSet-101-Plus - it is available in regular calking gun size ) and your pier won't be going anywhere.
If you move cut off the bolts at ground level and you won't have a large chunk of concrete in the ground.
Hope this helps
Ed
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Old 08-12-2020, 01:57 PM
RugbyRene (Rene)
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Quote:
Originally Posted by appiice View Post
Hello Rene

Bit late to the party but here is my 2 cents worth, I think Glen was right about using what you have, sandstone has a strength of around 77Mpa, concrete on the other hand in a house slab will be about 30, higher in other applications, but 77 would be rare.
Sandstone would be easy enough to drill into with a decent drill - not a consumer " hammer " drill look for a SDS drill, it will be like a hot knife through butter, construction epoxy ( https://ramset.com.au/Product/Detail/4/ChemSet-101-Plus - it is available in regular calking gun size ) and your pier won't be going anywhere.
If you move cut off the bolts at ground level and you won't have a large chunk of concrete in the ground.
Hope this helps
Ed
That's great advice. Thanks for that. I'm at the point where I'm ramming some spikes into different locations to figure out how deep my sandstone/bush rock is. Once I decide where to put the pier I'll start excavating.

Wish me luck!

Rene
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Old 08-12-2020, 04:29 PM
morls (Stephen)
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Hi Rene, thought I'd chip in here, although this might not be relevant to your situation...
I've gone a different route with my pier - used a plate and micropiles, and it's worked a treat. Very stable, and I've been able to move the pier by myself to a better location in the yard, leaving behind no trace.

http://www.iceinspace.com.au/forum/s...light=surefoot

I cut down the micro-piles to about 700mm in length, and it's very stable
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Old 09-12-2020, 10:00 AM
RugbyRene (Rene)
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Quote:
Originally Posted by morls View Post
Hi Rene, thought I'd chip in here, although this might not be relevant to your situation...
I've gone a different route with my pier - used a plate and micropiles, and it's worked a treat. Very stable, and I've been able to move the pier by myself to a better location in the yard, leaving behind no trace.

http://www.iceinspace.com.au/forum/s...light=surefoot

I cut down the micro-piles to about 700mm in length, and it's very stable
Hi Stephen,

Thanks for that. I had seen a post about this (possibly yours). This looks very interesting but I might run into the same problem as with a concrete footing, i.e. shallow soil. I'm not sure I'd have 700mm of soil to drive into before I hit sandstone.

Rene
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