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Pier height depth ratio

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#1 jambi99

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Posted 21 July 2020 - 08:56 AM

Hi there,

 

my concrete pier hole is  6' 4" in depth. I intend to use a 14" x 12' sonotube + 8" x 2' metal pier for a total of ~8 feet above ground high pier. My question is : is my concrete pier ratio(~1:1 with  6' in depth and 6' above ground) ok ?

 

Thanks for your help.


Edited by jambi99, 21 July 2020 - 08:58 AM.

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#2 lee14

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Posted 21 July 2020 - 09:07 AM

With a concrete pier that massive, I'm sure that depth is sufficient. It is well below the frost line. Unless the ground it sits in is particularly soft, or you're planning on mounting an unusually long or massive scope, it should be fine. The bottom of the hole should be tamped down before the concrete is poured, as well as tamping down the soil in contact with the sonotube (perhaps with crushed stone), and that the tube rigidly supported to ensure it remains plumb. 

 

Lee



#3 jfaldo

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Posted 21 July 2020 - 09:48 AM

My understanding is the general consensus is to not use sonotube below grade only above. A pour directly against the soil is much more stable.


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#4 jambi99

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Posted 21 July 2020 - 10:02 AM

My understanding is the general consensus is to not use sonotube below grade only above. A pour directly against the soil is much more stable.

?



#5 macdonjh

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Posted 21 July 2020 - 10:15 AM

My understanding is the general consensus is to not use sonotube below grade only above. A pour directly against the soil is much more stable.

+1

 

It is poor practice to have forms material (like Sonotube) below grade.  The magic of concrete is it conforms to the contours of the undisturbed soil in your hole and fills all the voids.  If you put forms in your hole they won't make contact with the undisturbed soil as well as the concrete would have and your foundation won't be as stable.  Besides, Sonotube will eventually rot, leaving a small gap between the soil and your concrete.  The soil will eventually settle/ consolidate, but that will take years more (i.e. years to rot, but then years to resettle).  Compacted soil is not the same as consolidated soil, so even placing Sonotube to the bottom of your hole and compacting soil around it isn't as good as simply pouring concrete into an empty hole and letting it fill against the undisturbed soil.

 

It is OK to have your Sonotube extend a few inches below grade to ensure you have a smooth-finished concrete surface above grade (cosmetics are important).  I extended my Sonotube 3" below grade for that reason.  But then the rest of my 6'-6" deep pier (42" above grade) was poured "naked" against the soil.

 

There is an in depth thread about all this stuff.  It was started by Cloudy Nights member speedster.  I think the title is "Pier Engineering".  There are also a couple of other member-specific threads which address the same topics.  If you search for threads speedster has participated in, you may find them.


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#6 macdonjh

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Posted 21 July 2020 - 10:18 AM

Oops, I forgot to try to answer the original question.  To be sure, we'd need to know what kind of soil your pier foundation is in, like lee14 says.  It doesn't sound like you've built your pier in sand so you'll likely be fine with what you've built, though.



#7 jambi99

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Posted 21 July 2020 - 10:23 AM

My understanding is the general consensus is to not use sonotube below grade only above. A pour directly against the soil is much more stable.

Ok, I misunderstood your comment. I thought you meant having the concrete below grade. I guess you meant the sonotube itself(concrete form) ?

 

Thanks.



#8 jambi99

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Posted 21 July 2020 - 10:28 AM

+1

 

It is poor practice to have forms material (like Sonotube) below grade.  The magic of concrete is it conforms to the contours of the undisturbed soil in your hole and fills all the voids.  If you put forms in your hole they won't make contact with the undisturbed soil as well as the concrete would have and your foundation won't be as stable.  Besides, Sonotube will eventually rot, leaving a small gap between the soil and your concrete.  The soil will eventually settle/ consolidate, but that will take years more (i.e. years to rot, but then years to resettle).  Compacted soil is not the same as consolidated soil, so even placing Sonotube to the bottom of your hole and compacting soil around it isn't as good as simply pouring concrete into an empty hole and letting it fill against the undisturbed soil.

 

It is OK to have your Sonotube extend a few inches below grade to ensure you have a smooth-finished concrete surface above grade (cosmetics are important).  I extended my Sonotube 3" below grade for that reason.  But then the rest of my 6'-6" deep pier (42" above grade) was poured "naked" against the soil.

 

There is an in depth thread about all this stuff.  It was started by Cloudy Nights member speedster.  I think the title is "Pier Engineering".  There are also a couple of other member-specific threads which address the same topics.  If you search for threads speedster has participated in, you may find them.

Ok, I could then pour the concreted directly in the hole and continue the above ground section with the sonotube?



#9 lee14

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Posted 21 July 2020 - 10:40 AM

Ok, I could then pour the concreted directly in the hole and continue the above ground section with the sonotube?

Yes. The sonotube below grade will eventually break down and effectively leave something of a void around the concrete. You can do this in two pours if you embed rebar in the lower portion, extending well up into the sonotube part. Then the sonotube must be rigidly braced to make sure it stay plumb.

 

Lee


Edited by lee14, 22 July 2020 - 09:34 AM.

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#10 t-ara-fan

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Posted 29 July 2020 - 01:26 PM

x2 on pouring the concrete in an open hole.

 

Six foot deep hole? Not sure about Ottawa but where I am 6' deep is the minimum for getting below the frost line.  I went 7' because that is how long the 18" auger was. 

 

BTW I had a 7' deep hole 18" diameter hole, plus about a 18" high 18" square form above ground as the base of my pier. The 18" square form ended up flush with my floor because the obsy is on a slight slope.

 

On top of that, I have a 56" tall 8" diameter 1/4" wall steel pier.  So my mount is pretty high up. I can see down to about 10° elevation with my scope.

 

Hi there,

 

 for a total of ~8 feet above ground high pier.

Are you going to climb a ladder with the C14 in one hand?


Edited by t-ara-fan, 29 July 2020 - 01:27 PM.

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#11 macdonjh

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Posted 29 July 2020 - 02:56 PM

Ok, I could then pour the concreted directly in the hole and continue the above ground section with the sonotube?

Yes.  When it was time to pour my pier I set my rebar cage in my hole and had my Sonotube ready.  I mixed concrete and filled the hole in the ground.  Then I set my Sonotube over the rebar and on top of the wet concrete and filled that.  I'd pack the wet concrete with a 2"x2" as I poured each sack.  Once my Sonotube was full I flattened and smoothed the top, set my pier adapter, and leveled it.  After I tore the Sonotube off the next morning I moved some dirt around to fill the 3"-6" deep hole around my pier.  That was there so I could have a bit of Sonotube below grade; that way the entire visible length of my pier would be "formed" and smooth. 

 

Yes. The sonotube below grade will eventually break down and effectively leave something of a void around the concrete. You can do this in two pours if you embed rebar in the lower portion, extending well up into the sonotube part. Then the sonotube must be rigidly braced to make sure it stay plumb.

 

Lee

This will make lee14 shudder, but I didn't even brace my Sonotube.  I checked every half hour or so while the concrete was starting to cure (for about six hours, until it "kicked").  If the Sonotube had settled out of plumb I simply moved it back until it was plumb.  Worked out fine for me.  lee14 is absolutely right, though, using some stakes and 2"x4" to brace your forms is best practice.

 

Something I didn't think about which did catch me was cutting holes in my Sonotube for my conduits to come through (I have two conduits which run up through my pier).  The exposed edges of the Sonotube absorbed water from the concrete and blew out.  Luckily that happened pretty quickly and I was able to wrap duct tape tightly around the damaged Sonotube to bring it back to a round shape.  That fixed things long enough for the concrete to kick.  Of course, I tore the Sonotube off the next morning.


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#12 sbsbbugsy

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Posted 30 July 2020 - 11:53 AM

If you use a sonotube, make sure that you have a good footer. I used a preformed one for my backyard pier and it has not shifted in over five years.



#13 jambi99

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Posted 01 August 2020 - 04:36 PM

x2 on pouring the concrete in an open hole.

 

Six foot deep hole? Not sure about Ottawa but where I am 6' deep is the minimum for getting below the frost line.  I went 7' because that is how long the 18" auger was. 

 

BTW I had a 7' deep hole 18" diameter hole, plus about a 18" high 18" square form above ground as the base of my pier. The 18" square form ended up flush with my floor because the obsy is on a slight slope.

 

On top of that, I have a 56" tall 8" diameter 1/4" wall steel pier.  So my mount is pretty high up. I can see down to about 10° elevation with my scope.

 

Are you going to climb a ladder with the C14 in one hand?

Scaffolding



#14 jambi99

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Posted 01 August 2020 - 04:40 PM

If you use a sonotube, make sure that you have a good footer. I used a preformed one for my backyard pier and it has not shifted in over five years.

I ended up pouring the concrete directly in the hole with rebar. Hole diameter was about ~16-17". The above ground section is a 6 feet high cinder block pier filled with concrete and reinforced with rebar. I will also apply a pretty thick layer of Quiekret Quickwall on the above grade section. This is to further reinforce the pier and for the look.



#15 Scott Horstman

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Posted 01 August 2020 - 05:39 PM

I ended up pouring the concrete directly in the hole with rebar. Hole diameter was about ~16-17". The above ground section is a 6 feet high cinder block pier filled with concrete and reinforced with rebar. I will also apply a pretty thick layer of Quiekret Quickwall on the above grade section. This is to further reinforce the pier and for the look.

That's how i build tall piers more or less. Can't handle the thought of the cardboard tube getting saturated and blowing out. Depending on the height I'll go 32" wide or if it's only 6 or 8 feet tall might use the 2 1" chimney block.



#16 jambi99

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Posted 13 October 2020 - 09:14 PM

Finished the observatory last weekend:

 

 

 

 

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#17 jambi99

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Posted 22 October 2020 - 05:03 PM

With the custom PZT table:

 

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