I have a small in-ground spa in the backyard that is not seeing any use anymore. I have decided to construct an observatory in the space. The spa has an oval shape with major and minor axis of 57" by 42" and a depth of about 42".
I am thinking of installing a concrete pier in the center. The plan was to drill through the spa's floor, around 12" diameter, about 6" deep and use 10" diameter Sonotube concrete form surrounded by about 24" inches of drain rock as backfill in entire spa area. This would hold the concrete form in place before pouring begins. Once concrete is poured in and cures in a few days fully, I plan to fill in rest of the height of the spa with another 42"-24" = 18" height of drain rock. Finally a thin layer of sand to flush with rest of the concrete patio floor.
I have a few questions/concern:
- The 10" Sonotube comes with 48" maximum height. I would like to have another 40" inches of pier height above the ground level (the top of the in-ground spa). How do I go about it? Add two tubes together? I am concerned if concrete will leak out through the gap and other kinds of issues with this approach.
- Instead of initial drain rock backfilling of 24" to support the concrete form, what if I fully finish the backfill all the way to 42"? Would that crush the concrete form (because radial pressure from the rocks around the form)?
- Do I need to be concerned about any settling time? I am guessing because all the backfill is with drain rock, there really isn't any settling time at all. This should avoid any tilting of the pier in any direction in future.
- I was also concerned with the overall approach itself. Is there any other way which is more reliable and higher changes of success?