Changing polar alignment...Ground shifting?


Tom Blahovici
 

Hi
I thought I would ask the experts here about my shifting polar alignment.
5 years ago I rented a backhoe, and dug a hole 7 feet deep in my backyard.  I then poured a concrete pillar in a form 3'x3' by 6 feet deep.  On this I have a paramount pier.
All the earth was filled in and I have used this mostly over the winter. Last year all was fine.  This year I had about 1 minute of arc drift in each axis using pempro.
Recently I started noticing that my stars appeared trailed during long exposures and autoguiding was acting up.  PHD 2 told me my polar alignment was off by 4 degrees. . So today I checked my polar alignment with pempro and sure enough, 4-6 degrees off in drift.  I have put it back to within 1 minute.
I thought I was golden with such a pier. Recently we had a lot of rain and then the temperatures have been going between + 12C and -12C.  Could this be a factor?  How often do you check your alignment?
Thanks


deonb
 

4-6 degrees of drift? (Not minutes?).  

Was there an earthquake in your area by chance? We had a few over the last week, and my polar alignment has been all over the place.


Mike Dodd
 

On 12/29/2020 10:10 PM, deonb wrote:
4-6 */degrees/* of drift? (Not minutes?).
That's what I was going to ask. If it's off by that much, you should be able to put a level up against the pier (or just look at it) to check if it is still plumb.

It seems unlikely that rain could have shifted a 3x3x6 concrete block that much. That's 8,100 pounds of concrete -- a huge mass!

Was there an earthquake in your area by chance?
That would be the only thing that would move such a mass. Where are you located?

I've built two concrete piers without any super-duper footers. The most recent is 12" in diameter, rising about 66" above the ground, and sunk about 40" in the ground with a 24" "blob" of concrete at the bottom for a footer. I used a power earth auger to dig the hole, then hollowed-out the bottom for the "blob" footer. Definitely nothing special about this pier, but it hasn't moved in four years. It DOES have an observatory around it, so it's sheltered from rain.

Tell us more, please!

--- Mike


M Hambrick
 

Do you live in an area where the water table is close to the surface, or where the bedrock is far below (e.g. Texas Gulf coast) ?


deonb
 

I think he might be from Anacortes (if he's the same Tom as from Astromart).

If so, Tom, you had an earthquake the day before yesterday:
https://pnsn.org/event/61707166#overview


Woody Schlom
 

Tom,

 

There are places where the ground swells and shrinks over the year depending on moisture content and/or freezing.

 

Freezing ground with lots of moisture in it causes swelling.  I think it’s called frost swell – or something like that.

 

Another ground condition that causes seasonal rising and falling is a high clay content.  Apparently clay swells when it’s wet and shrinks when it dries.

 

Huston, TX is famous for seasonal ground rising and falling due to moisture content.  I understand the ground in Huston can rise and then fall at least a FOOT depending on moisture content.  And this goes for buildings, parking lots, and single-dwelling houses.  And weight means nothing.  If the ground wants to swell and rise – 100,000 lbs. of concrete is nothing.

 

And here in Southern California we have earth quakes – which moves the ground up, down, and even sideways.  After a really big quake, some of the guys in my club with permanent piers have to re-adjust P.A.

 

So there are several possible causes of PA changing.

Woody


Dale Ghent
 

Ouch. If you really want to know if it's your pier then you can get a digital inclinometer and periodically measure the pier on two axes to get an idea as to what's going on.

I'm going to auger out and pour a pier footer in the upcoming spring and I've been taking heavy notes from a good thread on the CN observatory forum called "Pier Engineering":

https://www.cloudynights.com/topic/652025-pier-engineering/

The basic takeaways are from the author's experiences are:

1. Auger, don't dig out, the hole for the pier, and remove all loose material from the side wall and bottom
2. Pour the footer into the hole, using the hole itself as the form. You can use a sonotube form or whatever for the top several inches to give it a finished aesthetic above ground, but for the majority of it you want direct contact with the surrounding compacted soil.
3. Mind your frostline
4. A bunch of other things that should be considered aside from basic hole digging and concrete pouring

The reasoning is that you'll have concrete directly in contact with the existing undisturbed and compacted soil instead of loose fill. There is also no sonotube that will ultimately decay and leave voids between the footer and the surrounding soil. These voids invite shifting, and the lack of loose fill surrounding the pier means that it will be better supported and more stable.

I'm no soil engineer but it makes sense and, in my case, I really have to nail it on the first try because the place where I want to put a pier is the only location in my yard that I get the most sky... so a do-over would mean a less ideal location, even if it's just a few feet to the side.

Hope the cause of your tilting has an easy solution.
/dale

On Dec 29, 2020, at 22:04, Tom Blahovici <tom.va2fsq@videotron.ca> wrote:

Hi
I thought I would ask the experts here about my shifting polar alignment.
5 years ago I rented a backhoe, and dug a hole 7 feet deep in my backyard. I then poured a concrete pillar in a form 3'x3' by 6 feet deep. On this I have a paramount pier.
All the earth was filled in and I have used this mostly over the winter. Last year all was fine. This year I had about 1 minute of arc drift in each axis using pempro.
Recently I started noticing that my stars appeared trailed during long exposures and autoguiding was acting up. PHD 2 told me my polar alignment was off by 4 degrees. . So today I checked my polar alignment with pempro and sure enough, 4-6 degrees off in drift. I have put it back to within 1 minute.
I thought I was golden with such a pier. Recently we had a lot of rain and then the temperatures have been going between + 12C and -12C. Could this be a factor? How often do you check your alignment?
Thanks


Phillip H Coker <pcoker36@...>
 

I drilled three 1" holes 15 Inches into the base spaced to match the slots in the flange at the bottom of the 10" diameter steel pier. I epoxied three threaded 1" x 18" steel bars into the holes which left about three inches protruding above the base. With nuts and washers on the bars beneath and above the flange, it was very easy to adjust the pier to be perfectly vertical and it never moved in the five years I used it. The base was not concrete however. I lived on the side of Cheyenne Mountain in Colorado Springs at the time and had my observatory sitting over a megaton boulder.
Phil

On Dec 30, 2020, at 00:30, Dale Ghent <daleg@elemental.org> wrote:


Ouch. If you really want to know if it's your pier then you can get a digital inclinometer and periodically measure the pier on two axes to get an idea as to what's going on.

I'm going to auger out and pour a pier footer in the upcoming spring and I've been taking heavy notes from a good thread on the CN observatory forum called "Pier Engineering":

https://www.cloudynights.com/topic/652025-pier-engineering/

The basic takeaways are from the author's experiences are:

1. Auger, don't dig out, the hole for the pier, and remove all loose material from the side wall and bottom
2. Pour the footer into the hole, using the hole itself as the form. You can use a sonotube form or whatever for the top several inches to give it a finished aesthetic above ground, but for the majority of it you want direct contact with the surrounding compacted soil.
3. Mind your frostline
4. A bunch of other things that should be considered aside from basic hole digging and concrete pouring

The reasoning is that you'll have concrete directly in contact with the existing undisturbed and compacted soil instead of loose fill. There is also no sonotube that will ultimately decay and leave voids between the footer and the surrounding soil. These voids invite shifting, and the lack of loose fill surrounding the pier means that it will be better supported and more stable.

I'm no soil engineer but it makes sense and, in my case, I really have to nail it on the first try because the place where I want to put a pier is the only location in my yard that I get the most sky... so a do-over would mean a less ideal location, even if it's just a few feet to the side.

Hope the cause of your tilting has an easy solution.
/dale

On Dec 29, 2020, at 22:04, Tom Blahovici <tom.va2fsq@videotron.ca> wrote:

Hi
I thought I would ask the experts here about my shifting polar alignment.
5 years ago I rented a backhoe, and dug a hole 7 feet deep in my backyard. I then poured a concrete pillar in a form 3'x3' by 6 feet deep. On this I have a paramount pier.
All the earth was filled in and I have used this mostly over the winter. Last year all was fine. This year I had about 1 minute of arc drift in each axis using pempro.
Recently I started noticing that my stars appeared trailed during long exposures and autoguiding was acting up. PHD 2 told me my polar alignment was off by 4 degrees. . So today I checked my polar alignment with pempro and sure enough, 4-6 degrees off in drift. I have put it back to within 1 minute.
I thought I was golden with such a pier. Recently we had a lot of rain and then the temperatures have been going between + 12C and -12C. Could this be a factor? How often do you check your alignment?
Thanks


deonb
 

+1 for the advice on Pier Engineering. It worked out well, I can kick/pound my pier as hard as I can and at 2800mm FL have no visible vibrations transfer onto the scope.

I don't even have that big of a pier - I wanted to make sure it's not too heavy to lift back out again with my tractor. It's 18" wide below ground (3.5' deep)  and 8" wide above ground so comes in just around 1070lbs (920 lbs below ground, 150 lbs above). But because the concrete pour has been allowed to follow the contour of the compacted soil (Pier Engineering thread), it's held firmly in place. It's a good principle.

I built it in 2 stages with the below ground stage poured first and rebar sticking up out of it. I then embedded a piece of wooden "dovetail" across the top of the wet concrete, waited 2 weeks, chiseled out the wooden dovetail again, then poured the narrow sonotube part around the rebar and into the dovetail slot. Just make sure your rebar placement doesn't interfere with where you want the L-bolts of the pier plate to go (North aligned generally).

It's been rock solid since I poured it, until this week. It would hold polar alignment perfectly for weeks, and then suddenly the next day it's off by 15'. Fix it, then again, next day off by 15' again. Couldn't figure it out until my wife mentioned that the news was reporting earthquakes this week, and the days lined up perfectly. Was concerned there since the timing also aligned with when I put on the Mach2GTO, so I was quite thankful to hear about the quakes!

Oh well, that's why it's important to have a bolted design like Dan's Pier Plates that you can just re-level. Couple of turns with the wrench and the top plate is back to level.


Tom Blahovici
 

Hi
I'm in Montreal. No earthquakes here lately. Occasionally get some 4ish quakes one every 5 to 10 years.
Could well be the rain though. We had like 2 inches of rain lately and then -12c weather.
I also just remembered that at one point before the poor tracking showed up, that my rotator came loose. I then tightened it quite snug.
Maybe that was the culprit?
In any case, all seems well now. 
Tom


Mike Dodd
 

On 12/30/2020 11:16 AM, Tom Blahovici wrote:
I also just remembered that at one point before the poor tracking showed
up, that my rotator came loose. I then tightened it quite snug.
Maybe that was the culprit?
I absolutely would suspect rotator (and other equipment) attachment to the OTA before I would suspect rain or ground movement shifting the pier 4-6 degrees.

For peace of mind, I recommend pressing a level against the pier on two sides to determine if it is plumb or not.
--- Mike


dan kowall
 

"...I'm going to auger out and pour a pier footer in the upcoming spring and I've been taking heavy notes from a good thread on the CN observatory forum called "Pier Engineering":

https://www.cloudynights.com/topic/652025-pier-engineering/

The basic takeaways are from the author's experiences are:

1. Auger, don't dig out, the hole for the pier, and remove all loose material from the side wall and bottom
2. Pour the footer into the hole, using the hole itself as the form. You can use a sonotube form or whatever for the top several inches to give it a finished aesthetic above ground, but for the majority of it you want direct contact with the surrounding compacted soil.
3. Mind your frostline
4. A bunch of other things that should be considered aside from basic hole digging and concrete pouring

The reasoning is that you'll have concrete directly in contact with the existing undisturbed and compacted soil instead of loose fill. There is also no sonotube that will ultimately decay and leave voids between the footer and the surrounding soil. These voids invite shifting, and the lack of loose fill surrounding the pier means that it will be better supported and more stable.

I'm no soil engineer but it makes sense and, in my case, I really have to nail it on the first try because the place where I want to put a pier is the only location in my yard that I get the most sky... so a do-over would mean a less ideal location, even if it's just a few feet to the side...."
________

You might want to re-examine this advice with a more critical eye.
His basic argument is that a pier poured into an unlined, augered hole is more stable than a pier with a wide base because the augered pier is poured against 'undisturbed' soil.
First of all, consider the basic physics of each type. Ignoring the soil contribution for a minute, which design is more stable?



The base of the augered pier on the right is much smaller than the pier base on the left and would tip over with much less force than would be required to tip over the pier on the left.
Now let's consider the contribution of soil on each of these designs. The cited author maintains that 'undisturbed soil' is supremely strong and will resist movement of the augered pier. He cites stability figures to three significant figures without ever showing his work or specifying the type of soil. Does not the type of soil matter? Sand, loam, clay-Do they all generate the exact same three significant figure of stability?
What is the contribution of soil on the pier with the wide base? Firstly, the weight of the soil on the base (as shown by the red arrows) further adds to its stability because any sideways tipping force exerted on the pier must lift the soil over the base. That is not an inconsequential amount.
Now let's consider whether you want your pier sides below the soil level to be smooth, as from a sonotube, or rough, as poured into an augered hole. If you have soil that freezes in the winter it will grab your pier below ground. Simple physics shows that it's easier to grip a rough surface than a smooth surface. The augered pier has little to resist uplift from a frost heave. The pier with the wide base does resist uplift better as any uplifting force has to lift the pier base and any of the soil above the base.
For some practical matters when pouring concrete, the concrete achieves its maximum strength when properly mixed and worked. This strength is maintained all the way to the inner edge of the sonotube pier. Concrete loses strength when it is adulterated with things like loam or clay, things that will easily be incorporated when pouring into an augered hole and working the concrete.
'Undisturbed soil'? Is there such a thing? All soil above a frost line moves in the winter. Is your pier near your house? I can guarantee that most soils around a house get disturbed during construction. How many years have to pass before it can be reclassified as 'undisturbed'?
If you want a pier with a wide base and the protection of 'undisturbed soil' it's easy enough to compact the replacement soil in an excavated hole. Just tamp it down every few inches and flood it with water.

dan kowall


Christopher Erickson
 

Virtually all soils, locations and environments are unique.

Earthquakes, frost heaves, settling, moisture saturation, slope erosion, water table level, etc. all happen. Here in Hawaii, the younger volcanoes inflate and deflate.

Easiest thing to do anywhere is to either tune up your polar alignment twice-or-so a year as part of your periodic maintenance schedule, or just simply tune it up when you notice a change.

"My advice is always free and worth every penny!"

-Christopher Erickson
Observatory Engineer
Summit Kinetics
Waikoloa, Hawaii


On Wed, Dec 30, 2020 at 8:08 AM <daniel.a@...> wrote:
"...I'm going to auger out and pour a pier footer in the upcoming spring and I've been taking heavy notes from a good thread on the CN observatory forum called "Pier Engineering":

https://www.cloudynights.com/topic/652025-pier-engineering/

The basic takeaways are from the author's experiences are:

1. Auger, don't dig out, the hole for the pier, and remove all loose material from the side wall and bottom
2. Pour the footer into the hole, using the hole itself as the form. You can use a sonotube form or whatever for the top several inches to give it a finished aesthetic above ground, but for the majority of it you want direct contact with the surrounding compacted soil.
3. Mind your frostline
4. A bunch of other things that should be considered aside from basic hole digging and concrete pouring

The reasoning is that you'll have concrete directly in contact with the existing undisturbed and compacted soil instead of loose fill. There is also no sonotube that will ultimately decay and leave voids between the footer and the surrounding soil. These voids invite shifting, and the lack of loose fill surrounding the pier means that it will be better supported and more stable.

I'm no soil engineer but it makes sense and, in my case, I really have to nail it on the first try because the place where I want to put a pier is the only location in my yard that I get the most sky... so a do-over would mean a less ideal location, even if it's just a few feet to the side...."
________

You might want to re-examine this advice with a more critical eye.
His basic argument is that a pier poured into an unlined, augered hole is more stable than a pier with a wide base because the augered pier is poured against 'undisturbed' soil.
First of all, consider the basic physics of each type. Ignoring the soil contribution for a minute, which design is more stable?



The base of the augered pier on the right is much smaller than the pier base on the left and would tip over with much less force than would be required to tip over the pier on the left.
Now let's consider the contribution of soil on each of these designs. The cited author maintains that 'undisturbed soil' is supremely strong and will resist movement of the augered pier. He cites stability figures to three significant figures without ever showing his work or specifying the type of soil. Does not the type of soil matter? Sand, loam, clay-Do they all generate the exact same three significant figure of stability?
What is the contribution of soil on the pier with the wide base? Firstly, the weight of the soil on the base (as shown by the red arrows) further adds to its stability because any sideways tipping force exerted on the pier must lift the soil over the base. That is not an inconsequential amount.
Now let's consider whether you want your pier sides below the soil level to be smooth, as from a sonotube, or rough, as poured into an augered hole. If you have soil that freezes in the winter it will grab your pier below ground. Simple physics shows that it's easier to grip a rough surface than a smooth surface. The augered pier has little to resist uplift from a frost heave. The pier with the wide base does resist uplift better as any uplifting force has to lift the pier base and any of the soil above the base.
For some practical matters when pouring concrete, the concrete achieves its maximum strength when properly mixed and worked. This strength is maintained all the way to the inner edge of the sonotube pier. Concrete loses strength when it is adulterated with things like loam or clay, things that will easily be incorporated when pouring into an augered hole and working the concrete.
'Undisturbed soil'? Is there such a thing? All soil above a frost line moves in the winter. Is your pier near your house? I can guarantee that most soils around a house get disturbed during construction. How many years have to pass before it can be reclassified as 'undisturbed'?
If you want a pier with a wide base and the protection of 'undisturbed soil' it's easy enough to compact the replacement soil in an excavated hole. Just tamp it down every few inches and flood it with water.

dan kowall


Mike Dodd
 

On 12/30/2020 1:03 PM, daniel.a@cox.net wrote:

/The basic takeaways are from the author's experiences are:/
[...]
/4. A bunch of other things that should be considered aside from basic
hole digging and concrete pouring/
Including embedding rebar in the footer so it extends upward to near the top of whatever projects above ground level.

Remember the rebar!

--- Mike


deonb
 

Dan, those same downward forces can be had by just building the pier below ground larger than above ground. You don't need to build a cantilever at the bottom:



It also seemed logical to me that you'd want the majority of the weight of the pier + scope below the frost line. I built it wider to achieve that, but you can build it just deeper as well.

As far as disturbed soil, we have fairly loamish soil here and I've done a 10 ft backfill 3 years ago in an area, compacting and wetting it often. I can still easily shovel multiple feet down into it years later. Right next to it is undisturbed soil that I can't dig into more than 6 inches without using a pickaxe. It seems obvious to me which soil I wanted around the pier. I know not everybody has the luxury of undisturbed soil, but if you do, it seems a shame not to put it to good use.

If you're concerned about ice grabbing onto rough edges, you can always pour concrete into undisturbed soil at the bottom, and then start the smooth sonotube just below the frost line.