How deep to dig for pier base!

[D4N]

Ground movement is much less of a problem in the UK, indeed modern houses are often built on floating foundations.

It would just fall down in the spring if you did that in Sweden of course. The insulation is key there.

Do you have an adjustable mounting surface so the flange can be levelled if the pier does move?

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[NickK]

Frost level in the UK is 450mm (reading around).

The fun is down to clay and movement due to water-induced movement. Supposed recommendation is 1m for the worst case scenario on that..

The base for mine will be 600x600x800 mmm so 0.3^3m volume..  then additional for the pier itself. In total about 735 Kg of material..

[pete_l]

Frost level in the UK is 450mm (reading around).

The fun is down to clay and movement due to water-induced movement. Supposed recommendation is 1m for the worst case scenario on that..

But even if the base was subject to a little "heave" every few years - so what?

All you have to do is tweak the polar alignment. It's not as if you're building a house and can allow zero subsidence under any circumstances for a 100 years or more.

For a telescope, all you need is a sufficient mass that it won't bounce around when you walk around while observing. If you are imaging, then there's no need to be in the vicinity of the telescope / concrete / pier once the session starts, so the requirements are even less.

[NickK]

But even if the base was subject to a little "heave" every few years - so what?

All you have to do is tweak the polar alignment. It's not as if you're building a house and can allow zero subsidence under any circumstances for a 100 years or more.

True, I'll also be adding a large scope at a later date (hopefully) and will be walking around on the grass for visual.

Mine will only be 800mm to prevent it's use as a small table during summer months causing too much movement.

I'm also tempted to put a long tube in the middle to hold the garden umbrella.. (you can see, now I have some compromises!)

[Earl]

When I dug a pier it was as deep as my reach with a trowel, lying down the get that final bit

[Gina]

I did much the same   Quite adequate IMO.

[NickK]

I think that may be a bit relational sizing! 6'4" here... S measured - I have 78cm (31inch) long arms (to finger tips)..which is basically is the same size as my 80cm hole depth I have planned

[Tinker1947]

I used these hole diggers, went down around a metre soft silty soil 10 minutes if its a pre built pier i wouldn't make it to wide you only really need 6" space all round i used pea shingle so it can be removed if needed, hoover the pea shingle out and left out the pier its solid and nice drainage through the shingle.....

[iapa]

On 16 October 2014 at 10:52, NickK said:

Frost level in the UK is 450mm (reading around).

The fun is down to clay and movement due to water-induced movement. Supposed recommendation is 1m for the worst case scenario on that..

The base for mine will be 600x600x800 mmm so 0.3^3m volume..  then additional for the pier itself. In total about 735 Kg of material..

Hi @Tinker1947

Can you advise where you got the figure for frost level for the UK.

I am somewhat further north of yourself, and some winters extended period of <-10C are not that uncommon for day time temperatures,

No, I'm not in the Gaeltachd, only Central Scotland )

[Tinker1947]

Frost levels, why would i need them the base is in a Obby and shelter from frost but i don't think we have had much in the way of frost the last few years....the ground my pier is in used to be the North Sea, its silty and bone dry heavy rain one day bone dry the next.....

[Pete D]

I would go a bit deeper just to be sure.

 

[NickK]

10 hours ago, iapa said:

Hi @Tinker1947

Can you advise where you got the figure for frost level for the UK.

I am somewhat further north of yourself, and some winters extended period of <-10C are not that uncommon for day time temperatures,

No, I'm not in the Gaeltachd, only Central Scotland )

 

I was reading around the internet - one of the building (house foundations) sites indicate the levels.

For example: http://www.heritagetesting.co.uk/factsheets/soil-heave.html "The depth of frost is usually about 450 mm in Southeast England."

There are others that also align with that figure. So I would take that as a minimum - if you're more northerly.. if you look at Minnesota in the US the southern areas are 1.1m and the northern areas are 1.5m. Considering Minnesota gets from -8 to -16degC as an historic average with lower during exceptional times - it's likely that the most we in the UK are going to get is that range as an average exception. I'll leave you to draw your own extrapolation/thinking from that - but I get your point! Tis cold on the wall

 

In the end it took me about an hour with a post hole digger to dig out the entire hole (24x24" and about 90cm deep). Far faster than I was expecting. A very worthwhile purchase! And with all the compromise creating too much complexity - I simply made it a solid base-pier with a single pour. So I have a very solid pier.