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In-ground spa to Telescope Observatory

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#1 vipinpsharma

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Posted 14 October 2020 - 10:20 PM

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:

  1. 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. 
  2. 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)?
  3. 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.
  4. I was also concerned with the overall approach itself. Is there any other way which is more reliable and higher changes of success?

 

Thanks,

Clear Skies!

 

 

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#2 chubster4

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Posted 14 October 2020 - 10:41 PM

Isolate the electronics and make it a Spabservatory!  I can't think of a better way to observe on a cold winter night! To hell with "seeing"...who needs that when you're in a hot tub!  lol.gif lol.gif  lol.gif


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#3 Stevegeo

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Posted 15 October 2020 - 09:58 AM

Since it's deep anyhow and depending on how thick the concrete in the spa is you could do a couple of things.
Backfill if your concrete is at least 4 in to a sonotube tamping firmly until level lift sonotube up to almost ground height then pour with rerod inserted . This would provide stability to surrounding dirt .as it sets up then tamp solid . You can then form 2 in around that tube and pour your slab for the building and foam fill that space .
If your spa isn't 4 in ..a short 12 in tube could be poured with rerod sticking out a foot or two .backfill to that height. Tamp solid . Then a 10 in sono placed on the 12 ..pour with rerod. To your desired height.dont forget the mount bolts.
Backfill just shy of surface. Form outside the 10 in pier .pour your slab for the building. Foam the gap and build. You will have a great base regardless .as the spa which had water in it probably settled the soil to max . Your pier will support anything large without issue ..
Stevegeo
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#4 csrlice12

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Posted 15 October 2020 - 10:50 AM

On the other side, don't try filling your observatory with water.

 

Much better to find a Bottle 1 site next to a natural hot spring.....


Edited by csrlice12, 15 October 2020 - 10:51 AM.

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#5 vipinpsharma

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Posted 17 October 2020 - 12:44 PM

@Stevegeo, thanks for the options. I am thinking I will simply assume that the spa floor is less than 4" and go ahead with the option. Here are the steps I plan to follow; please let me know if this will work:

 

  1. Drill 4-5 holes in the spa floor for drainage
  2. Put couple of inches in height of crushed drain rock on the spa floor
  3. Put a Square Foot brand footer form (like this one) in the center
  4. Place a 10" diameter 8' long concrete form tube (I was able to get a 8' one) on the footer form
  5. Backfill about 3' height with crushed drain rock. This should hold the empty form tube in place before pouring begins
  6. Place about 4 8' rebars inside and along the length of the form tube
  7. Run conduit for electrical wiring before pouring
  8. Pour concrete about 2' height inside the form, tap a bit, continue pouring with another 2' height and so on until entire 8' length of the form is full
  9. Insert mounting bolts after 30 minutes of initial curing
  10. Keep an eye on the pier's plumb level as it cures and adjust as necessary
  11. 24 hours after initial curing, backfill rest of the remaining height (42" - 2" - 36" = 4") with DIRT (all backfilling before this step was with crushed drain rock) and compact as necessary.

Can you see any flaw? I am particularly concerned about the backfilling with crushed drain rock part. The pool depth is about 42" and I am backfilling 38" with crushed drain rock). Is that a problem? 

 

Thanks for all your help!

Vipin


Edited by vipinpsharma, 17 October 2020 - 12:54 PM.


#6 speedster

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Posted 18 October 2020 - 05:13 PM

Howdy!

 

Small holes in the spa shell may not be reliable in preventing hydrostatic pressure or draining water accumulated in the spa shell.  Break out a good sized chunk of the spa bottom, like 8" diameter, outside the new plastic footing form.

 

Set your anchor bolts within just a few minutes of placing the concrete.  Once curing begins, they won't go in.  Embedded items go in ASAP. 

 

Insert a half dozen 1/2" rebar dowels into the spa shell, extending about 6" up into the plastic footing form.

 

Definitely fill the entire shell with gravel before pouring.  The column of fresh concrete in the Sonotube is exerting pressure on the footing form and will easily lift it if it is not held down with ballast.  You can also help control floating the form by pouring 12", waiting 15 minutes, pouring the next 12", waiting 15 minutes, repeat. 

 

Hoping the Sonotube stays plumb without bracing can work but things can get our of control in a hurry.  Very easy to hold a 2x4 against the side of the top of the tube and drive a screw into it from the inside of the tube.  Do a second 90 degrees from the first.

 

if you cut holes in your Sonotube for conduits or whatever, seal the edges of the hole with urethane or something that the paper will absorb.  Otherwise, wet concrete can blow out the hole.



#7 Stevegeo

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Posted 18 October 2020 - 05:39 PM

Yes  large holes needed for drainage or it will become a pool again,  only with MUD,  place a few not so flat rocks over the holes, and gravel the rest especially around the bottom form.  Fill that form with concrete and rebar, and I would let it set up.

The tube next on that, with more gravel at the base to hold in place , a hole for your conduit for power, with conduit in the tube tied to a rear support. Make the hole tite to the condiut.... fill that with concrete . Let it set up, and brace it with 2by 4s in 3 directions to maintain plumb. Backfill with dirt, and tamp.  Finish pour to desired height. 

When you get your slab poured, make the form LARGER then the spa, and remove an inch or so of dirt , place wire mesh  down staked to mear level of spa itself, and pour the slab, leave a ring form  of at least 2 in around the pier when final pour this you can fill in with an expanding foam, or if you're handy cut a piece of blue or pink 2 in foam to fit over the pier, slide that down to the base,  tite to the dirt, wire mesh outside the perimeter  then pour the slab and the foam will be the form. You will need 2 pieces of foam for a 4 in pour.  Dont forget your anchor pins for the building .  

Stevegeo 



#8 vipinpsharma

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Posted 20 October 2020 - 01:05 PM

Thank you; these are excellent points. I will update the list of steps so that it is easy access for others looking for similar info:

  1. Drill 4-5 large enough holes (to combat draining water accumulation due to hydrostatic pressure) in the spa floor 
  2. Backfill couple of inches in height of crushed drain rock on the spa floor
  3. Put a Square Foot brand footer form (like this one) in the center
  4. Insert a dozen or so 1/2" rebar dowels extending up to 6" into the form
  5. Place a 10" diameter 8' long concrete form tube (I was able to get a 8' one) on the footer form
  6. Backfill another 34" height with crushed drain rock (about 6" short of ground level). This should hold the empty form tube in place before pouring begins
  7. Ensure plumb level by placing three 2x4 at 120 degrees apart extending out from the top side of the tube
  8. Place about 4 8' rebars inside and along the length of the form tube
  9. Run conduit for electrical wiring before pouring and ensure holes are sealed-up 
  10. Pour concrete about 2' height inside the form, tap a bit, continue pouring with another 2' height and so on until entire 8' length of the form is full
  11. Insert mounting bolts soon after final concrete pouring
  12. Keep an eye on the pier's plumb level as it cures and adjust as necessary
  13. 24 hours after initial curing, backfill rest of the remaining height (42" - 2" - 36" = 4") with DIRT (all backfilling before this step was with crushed drain rock) and compact as necessary.

 

As for the observatory building (a small 8-10' dome really), I am thinking of starting with these pre-formed concrete blocks around the outskirts of the spa permitter. There is already a concrete patio floor around the spa and all I need to do is to anchor the pre-formed blocks on the existing concrete floor after which I can place the pressure treated floor joists to construct the observatory floor. This is going to cause the dome floor to sit at a higher level than the concrete floor around the spa with few inches of air gap. I hope this isn't going to be a problem. 


Edited by vipinpsharma, 20 October 2020 - 01:05 PM.



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