Re: Changing polar alignment...Ground shifting?

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":

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

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