Hi everyone, I have a question outside my realm of expertise, arising from the work of a contractor at my house.
Situation: we are pouring a concrete slab (15'x~8', thickness ~6") for a swim spa. The space is surrounded by deck posts holding the raised deck above. The contractor put his forms against the deck posts and said they would have to stay in after the pour (he used pressure treated wood for this reason). At this point he's made his base of crushed stone (he called it "recycled concrete"). A lot of the stone (5" of it in places) is held by the wood forms (as opposed to the forms being on top of it). The spa that will go on top of this weighs 13386 lbs full (with nobody inside). I'd say it can accommodate up to 10 people max, for another 2000 lbs. I'm attaching photos showing the overall setup and how the stone is held by the forms, which are held by the deck posts.
My concern: I'm worried this is a dangerous setup for the stability of the deck posts. My main concern is that the combined pressure of the concrete slab and the full spa will create a significant lateral pressure on the remaining forms and the posts, which could result eventually in posts moving and the deck becoming unstable. I'm also concerned that movement in the slab due to heaving could pull the posts up (we are in Ontario, Canada, and the ground underneath is clay). We don't expect the ground under the slab to freeze as long as the spa is functioning, but it could be out of service at some point.
Photos:
Situation: we are pouring a concrete slab (15'x~8', thickness ~6") for a swim spa. The space is surrounded by deck posts holding the raised deck above. The contractor put his forms against the deck posts and said they would have to stay in after the pour (he used pressure treated wood for this reason). At this point he's made his base of crushed stone (he called it "recycled concrete"). A lot of the stone (5" of it in places) is held by the wood forms (as opposed to the forms being on top of it). The spa that will go on top of this weighs 13386 lbs full (with nobody inside). I'd say it can accommodate up to 10 people max, for another 2000 lbs. I'm attaching photos showing the overall setup and how the stone is held by the forms, which are held by the deck posts.
My concern: I'm worried this is a dangerous setup for the stability of the deck posts. My main concern is that the combined pressure of the concrete slab and the full spa will create a significant lateral pressure on the remaining forms and the posts, which could result eventually in posts moving and the deck becoming unstable. I'm also concerned that movement in the slab due to heaving could pull the posts up (we are in Ontario, Canada, and the ground underneath is clay). We don't expect the ground under the slab to freeze as long as the spa is functioning, but it could be out of service at some point.
Photos:
![[cheers]](/data/assets/smilies/cheers.gif "[cheers] [cheers]")
