Pier height vs depth

[D4N]

I just finished digging out a hole for my pier foundation, I was hoping to get to 2m but I hit bedrock at 1.3m so that's as deep as it's going.

The question is how high is sensible with this depth?

I have a 150mm diameter 4.3m long steel pipe. Wall thickness is 5mm.

I was thinking 1.5m above ground would be a good height for access, horizon and stability. So I would cut to 2.8m to achieve this.

I plan to fill the hole and the pipe with concrete.

Does anyone have experience of insufficient stability in the pier because they couldn't get deep enough?

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[Garethr]

D4N

At 1.5m deep and 1.5m above groung filled with concrete and 5mm thick walls I would have thought you would need to hit it with a sledge hammer to produce any vibrations at all. Probably more likely to suffer from after shocks from earthquakes anchored to bedrock.

That sounds like a good solid base and pier.

Gareth.

[RichM63]

Clearly I don't know how tall you are, what mount head you will be using, what OTA, or what raised floor height you will acheive, but 1.5m above ground is pretty tall for a 'naked' pier.

I hope to be reusing an existing concrete EDF pylon that rather than remove they have agreed to cut down to my specifications, which I have set at 1.2m (and I think that this will be too tall) as my existing EQ5 'naked' tripod is set at 0.85m before adding the mount.

I do have access to a petrol disc cutter in case I decide to further trim and have no risk of disturbing 3m of concrete below ground. but I'd check your complete height combinations as best as you can before cutting and pouring.

Best of luck,

Rich

[Mr Spock]

I think that amount of concrete would survive a 9.5 on the Richter scale!

[D4N]

Ok that all sounds reassuring.

I wasn't actually planning to look through the scope so it could be 10m up so long as I can get to it to take the lens cap off

If it turns out too tall I can always lop a bit off the top!

I imagine that's the first time you have been happy about having a pylon there, not many of those around here, the power cables are underground. In fact there is one running kinda close to where my pier will go.

Cheers

/Dan

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[RichM63]

Sounds good Dan, I hope that all goes to plan.

(yup my Avatar pic was taken with my back to the pylon, fingers crossed)

Best,

Rich

[laser_jock99]

Sounds like your biggest problem will be removing the pier when your finished with astronomy or move house!

For this reason both my piers are of the bolt down variety.

I have 6x 1m long M18 studding holding my pier down.

[ollypenrice]

All mine bolt onto the surface of the concrete as well. In fact they are all piers with flat tripod extensions which stand on the floor and are then bolted down.

I'm not sure that concrete makes the best 'fill' for a steel pipe. The pipe won't flex so it's ringing vibration you might want to dampen. For this purpose I've heard that a mixture of old engine oil and sand is a good idea and it stops corrosion in the pipe, too. Concrete, being rigid, might not dampen high frequency vibration very well.

Olly

[101nut]

Don't put old engine oil into the pipe as it will contaminate the ground around the pipe (which is illegal in the UK). I'd agree dry silver sand is good for dampening though. If you're worried about corrosion I'd paint it very well (there are lots of good products) or get it galvanised.

AndyG

[D4N]

Bolt down is nice but my welding isn't so I'm just gonna stick the whole pipe in the cement

I'm not too concerned about corrosion, there is enough wall thickness to take quite a lot and still be fine. I have descaled and painted the outside anyway for aesthetics and the inside will have very little oxygen so shouldn't corrode much.

I was planning on having the adaptor plate held in place by 3 x M12 threaded bars that will facilitate levelling the mount, these will be set into the cement inside the pipe.

/Dan

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[D4N]

Well I finally got a forecast with 48hrs above 0C so I just poured half a tonne of concrete into my hole and lowered my pipe into it.

It is all wrapped up in insulation now to give it a chance to set... Concreting is really a lot easier in the summer!

Once that is stable I will pour some sand into the pipe then concrete at the top to fit my adaptor plate.

Getting exciting now, I might be able to use it next week

/Dan

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