I appreciate all your thoughts and ideas - this certainly provides more insight and leaves me with a better understanding of the alternatives.
The notion of having undisturbed soil is a big takeaway for me.
However, here are some challenges that I face:
- the temperature can be very cold in the Laurentiens with -25 to - 30C regularly during winter; this brings the frost lines down to a depth of 5+ feet.
- I have rocky soil, so I am not sure that an auger would work or reach 60+ inches deep (I can try to see);
- the freezing / thawing tends to grab a rough surface underground and move it up (interesting that the pylon company says that they use plastic sleeves to mitigate this).
- I have a metal pier that I built which will be at the ROR floor level (about 12” above ground), and will be mounted on this underground pier (which will stick out about 12” above ground )
As well, I just read in a construction guide (for Quebec) the following :
“Here are a few more good practices for building stable, durable concrete pillars:
Never pour concrete directly into a drilled hole; the resulting irregular shape will increase the risk of frost heave; Only use cardboard tubes with a diameter of 20 cm (8 in) or more;
Make sure each pillar projects at least 20 cm (8 in) above the ground;
Strengthen the pillars using reinforcing rods;”
Honestly, the simplest approach would have been to use a heavy duty 5-1/2” dia x 7 foot long screw-in pylon which is used for lamp posts, but I have yet to hear about any amateur astronomer that has used a pylon for the base of his pier, so it would leave me in uncharted territory.
The idea of small pylons supporting a concrete base leaves me concerned about the strong heaving action in my area.
So, unless anyone has more ideas, I believe that I am back to using a 12” sonotube with the understanding that I would use an auger if possible, or otherwise compact the soil as best I can.
Once again, I really appreciate your input which has been tremendous is helping me better understand the challenges and opportunities.