Pier Movement help please

[dennisjames1]

Hi everyone


Have a steel pier in concrete and it is moving in the ground and that is transmitting a vibration through to the scope, any ideas for a fix?




BACKGROUND


Put the pier in first as above and build roll off OB around it, also the pier is surrounded by a very nice floor, so would need to pull that up for a big fix which i want to avoid.


I was thinking of running water around the earth near the concrete hoping that might help impact the soil.


Very conditions here for ages.



cheers


Dennis

[Wilso]

That's a shame it's become unstable.
Not large enough concrete base or deep enough into natural soil perhaps?
Anyways I think your on the right track.
I'm not sure what your soil type is there but some clay soils are highly reactive. It has been a very dry summer here also.
So you could try a dripper system around the pier to expand the soil, with a little luck might be an easy fix.
Good luck

[dennisjames1]

Hi Wilso


Thank you yes i am sure you're right not enough base width i think, i will try the soaking and fingers crossed.


cheers

[bojan]

How about soaking with "concrete milk" first?
This is mix of cement and water, without agregate... sometimes called "laitance"

[leon]

Definitely a dry ground situation, I used to water my pier on a regular basis, especially in the hot summer, it dose make a difference

Leon

[xelasnave]

Old houses get cracks because the clay dries out, often a good water sees the cracks close.

If not drive down some pipes (see franky piling) around whats there and pour cement to link pipe and pier stand.
Water should fix it however.
Good luck.
Alex

[bojan]

It may be a good idea to dig around a bit (0.5 m around and deep), nail couple of thick 1m galvanised steel rods (L or Y profiles, Bunnings has them) into the clay and pour the new concrete into this.

[dennisjames1]

Thank you all for the suggestions, will try the watering, luckily i have it OB raised a little to get a hose underneath, i do like the other fixes but would need to cut holes in the flooring etc.Then i would probably redo the job in that case with experience a good teacher.


So just a trickling hose around the pier area for what 30 minutes a few times and a few days apart, see what goes, is that the way to do it?


Thanks again


Dennis

[rally]

Dennis,

How deep and what size approximately is the pier footing ?
Do you know if the footing is on original compacted soil or is it on backfill or disturbed soil
You mention vibration - is this vibration from you moving in the observatory or around it or maybe wind against the observatory being transmitted to the ground and back into the pier, or is it from vibration elsewhere - eg buses, trains and large traffic can be a problem, but even motors and pumps, air conditioners, fans or other machinery might be generatating some vibration.

How heavy is your pier, what are its dimensions - length, diameter, cross section, material etc ?

What sort of scope and mount is in use.

That information might help diagnose the problem

Vibration is really a different animal to movement caused by reactive soils, but of course a footing that has become "loose" due to shrinkage will be more susceptible to whatever vibration you have.

Rally

[dennisjames1]

Hi Rally


I really can't give you all those exact details as an old builder friend did it for me and i was the lackie.

It's around 600mm down and is square steel 100mm channeling, and it's adjustable inside using the three threaded rod and nut combo on base plates which are welded, that all is very secure.I can see the the cement move a little in the soil not the post in the cement.

There is a gap in the pier as it comes through the floor into the Observatory no vibrations transmitted there.

When you touch the pier there is that quiver at the eyepiece, painful and with more force you can see that in the pier, so all pier related. Not enough width of cement for sure adding to the issue.


i HAVE a 10inch Meade LX200 on a Skywatcher AZ/EQ6 AND a ED 80mm piggy backed on the SCT. At this stage 3x5kg counter weights, will need another for cameras etc.


cheers


Denns

[troypiggo (Troy)]

You've got pretty reactive clays up there in Toowoomba, which means they'll shrink/swell with seasonal moisture changes. At only 600 deep, sounds like your footing is right in the zone that will see that effect, as evidenced by your description of gap between surrounding ground and concrete footing, and that's where the movement is.

So sounds like your footing is undersized and should be either deeper or larger in plan area to combat this movement.

Wetting the surrounding ground may cause it to swell as it takes in the moisture, closing that gap to the footing. However if/when it dries out again, it will shrink away leaving the gap again. So you'll need to maintain the moisture content.

Filling the gap with a reasonably wet and fine mix of sand/cement/water like what Bojan suggested above may take up the gap at its current moisture content without you having to constantly wet it. No need for reinforcement.

[dennisjames1]

Hi Troy
Thanks for your input and suggestions.There is actually no gap in the soil and the cement at the pier base, movement yes, The gap i referred to is when the pier is coming through the floor and into the Observatory, a space around the pier and the raised floor i have.

I should have been more clear.


The soil is right up to the cement,but as you say not put down correctly deep and wide enough.


cheers


Dennis

[troypiggo (Troy)]

G'day Dennis,

To get the best advice/help, need some more info. Some detail photos of the pier would help as well.

When you say above that the pier is "square steel 100mm channelling", do you mean it's 100PFC (which is a channel shape like this: http://sable.net.au/product/channel-steel/100-pfc/ ) or is it 100SHS (square hollow section like this: http://www.steelweb.info/100x100x6.htm )

[Imme (Jon)]

Would I be right in assuming the shrinkage/swelling would also not be uniform so the 'trueness' of the pier will change?

Will make alignment an ongoing issue. If it were me I'd bite the bullet and redo the footing

[troypiggo (Troy)]

The shrink/swell amount is a function of the reactive soil layer thickness, the soil properties, and moisture variation. Over the width of a pier footing, you wouldn't expect any of those to change greatly or suddenly. It is a consideration, in reality probably negligible.
But would certainly pay to do a check (drift align?) a couple of times a year. I'd do that anyway, but that's just me.

[dennisjames1]

Hi Troy


It's SHS with some concrete down the centre, will try the soaker a few times and see if there is an improvement an then go from there.


cheers


Dennis