Can you use ground anchors for a pier instead of a concrete block?

[old_eyes]

For those of you not following my observatory build thread, A thought about fixing piers occured to me.

Most of the people setting up a permanent pier end up with a big block of concrete. We debate how big it needs to be, but generally it is a big block.

It is cheap in materials, but a lot of hard work unless you have a friendly neighbour with a digger and a concrete mixer.

It also means that if you move, or heaven forbid give up the hobby, you have a large and potentially unsightly trip hazard in your garden. I have often wondered if there is a better way, and read somewhere of an observatory in Australia (I think) that uses ground anchors based on driving three scaffolding poles at angles to each other into the ground using a vibrating hammer. I think the main application was supporting mobile phone masts and things like that.

However, I have just installed some big ground anchors from Spirafix to hold my observatory firmly in place.

These are their medium size anchors - 50mm x 1050mm

In the right kind of soil these are rated to resist forces of over one tonne (1400 kg).

You just hammer them into the ground and they screw themselves in.

I have put  four in today and the most difficult one took five minutes with a normal club hammer to hammer in to the lower edge of my frame.

Even better, you can just unscrew them with a wrench.

I think with three or four of these you could create a stable platform for a pier. If it is a commercial pier the challenge will be to accurately line up the threaded hole in the top with the holes in the pier, but an intermediate plate might help.

I don't know if it would work, but something tells me that one of these screws driven a metre deep would be pretty firmly fixed and if driven down to ground level would have little scope for flexing or vibrating.

The do longer 75mm diameter anchors as well.

It would not be as cheap as concrete, but it would be a hell of a lot faster, and you can undo it and move it if you wish.

Just a  thought!

 

[saac]

In terms of securing the pier to a stable base then I see no reason why it would not work; the idea in principle is similar to steel piles used to stabilise and provide foundations in civil engineering projects. Like you say perhaps say 4 of those anchors driven into the ground and then topped with a metal plate (bolted or welded onto the heads of the anchors) would then form a secure  base for your pier.  You would of course want to think about  isolation if you intend to image so you don't induce vibrations in the setup as you walk around the pier. If you are remote imaging or visual then the isolation of the pier should not be such a concern.  Nice idea but is it an easier (less expensive, quicker, less demanding) solution than a conventional concrete pad?  I guess it would certainly be quicker.  I think  you would also want to be convinced that the ground is not prone to movement (heave or subsidence ).  Be interesting if you go ahead with it to hear how you get on  

 

Jim 

Edited August 21, 2019 by saac

[JamesF]

You'd have thought that it has to be worth a try.  Three or four of those on the bottom of a pier surely aren't going to shift around too much.  And completely removable too, so you'd only be buying once.

James

[Chriske]

Good idea...!
And indeed interconnecting them is an absolute must.

[old_eyes]

On 21/08/2019 at 22:03, saac said:

  ...You would of course want to think about  isolation if you intend to image so you don't induce vibrations in the setup as you walk around the pier. If you are remote imaging or visual then the isolation of the pier should not be such a concern...

Jim 

That is an interesting question and one I have puzzled about. Surely a concrete block is also strongly coupled to the surrounding soil. Yes it has greater mass, but the resonant frequencies would still be high compared to the vibrations caused by footfall. I would guess that induced vibrations in the pier from walking nearby would be no worse for the ground anchors than for a concrete block.

Ground movement would be a problem for both anchors and block, but may affect the block more due to a greater surface in contact with the soil, particularly at the base.

i am sure there are some civil engineers around who could put us right, but at the moment I still have a nagging feeling it would work.

Unfortunately, I was already committed to a standard concrete block before I bought the ground anchors and saw how easy they were to install. If I ever put up a second pier, I will certainly give it a try.

[Gina]

The problem here would be the pieces of flint from insignificant to huge that are in the clay that is about a foot under the topsoil.  I had "fun" with a crowbar removing large lumps of rock when I dug the hole for my pier block and even for the 6 smaller holes for the observatory foundations.  I did try hammering lengths of iron water pipe into the ground but couldn't get them in very far!  It all depends on your ground.

[old_eyes]

2 hours ago, Gina said:

The problem here would be the pieces of flint from insignificant to huge that are in the clay that is about a foot under the topsoil.  I had "fun" with a crowbar removing large lumps of rock when I dug the hole for my pier block and even for the 6 smaller holes for the observatory foundations.  I did try hammering lengths of iron water pipe into the ground but couldn't get them in very far!  It all depends on your ground.

Agreed, but I was impressed by these little beauties. We are on boulder clay. It if full of stones from insignificant to actual boulders. Digging with pick axe and shovel is soul destroying, and you certainly can’t drive pipes or steel bars very far in. Somehow, the screw thread on these anchors works. Luckily I did not hit a large rock dead centre otherwise I am sure I would have stopped dead. 

Of course many have an even worse substrate to work with. However, I did call the company to explain my soil type and get their opinion. They said it is rare to be unable to drive them in unless you hit bedrock. In the worst case you have to unscrew them and find a new location. That was fine for my obsy as I had the length of all four sides to play with. Harder if you are trying to create a pier base.

Edited August 23, 2019 by old_eyes

[saac]

11 hours ago, old_eyes said:

That is an interesting question and one I have puzzled about. Surely a concrete block is also strongly coupled to the surrounding soil. Yes it has greater mass, but the resonant frequencies would still be high compared to the vibrations caused by footfall. I would guess that induced vibrations in the pier from walking nearby would be no worse for the ground anchors than for a concrete block.

Ground movement would be a problem for both anchors and block, but may affect the block more due to a greater surface in contact with the soil, particularly at the base.

i am sure there are some civil engineers around who could put us right, but at the moment I still have a nagging feeling it would work.

Unfortunately, I was already committed to a standard concrete block before I bought the ground anchors and saw how easy they were to install. If I ever put up a second pier, I will certainly give it a try.

I agree it would work. When using a concrete block however the normal practice is to isolate the concrete block from the surrounding by digging an oversized hole then infilling the gap with some non homogeneous material (sand, pea shingle foam etc).  I was wondering if when using the ground anchors something similar may be needed particularly if the setup is for imaging while attended.  Without question the frequency response from the anchors would be different from that of a concrete block. How different that response would be , better or worse , I have no idea. It would however be a simple matter to isolate the ring of ground anchors by a similar method to that used when using a concert block.  In any respect if the setup is non attended  (remote imaging) the concern over vibrations is somewhat reduced. I personally think your ground anchor suggestion is a serious alternative to the concrete foundation and it would be great to see somebody trialing it.  

 

Jim