Excited (and kind of scared) for the journey ahead! Starting with a question

[malas7]

Hello from Southern Ontario, Canada
Tomorrow we are breaking ground to what will be the biggest DIY project I've taken on in my life. Outdoor kitchen, 40'' diameter oven, grill, sinks, the works. Although scary, I am actually looking forward to it.

A question to the group (I've seen a few things mentioned about this with a variety of answers).
Tomorrow we will use my neighbours mini-ex to prep the ground for the slab. He also has the auger attachment. Would it be recommended to drill a few holes and place sonotubes? I was thinking 2 under the oven part and 1 on each end of the kitchen length (15ft). 4 in total. Then 5 inches of Gravel on the slab, compact that. Rebar on top of gravel and rebar inside the sonotubes. Tie rebar together. Finally a monolithic pour of 5.5 inches that will tie everything together.
What do the experts think? Would something like that work for the cold climate of Canada?
If that works, does it only make sense to go below frost level (4ft) with tube? Would going 3 feet be useless or still better than none at all?

[Giovanni Rossi]

malas7 , I struggled with this decision for months during COVID isolation when I was doing my planning. I was trying to decide if I could save some digging by dropping 4 Sonotubes below the frost line on each corner. Ultimately I decided on a foundation for the slab on grade but hit bedrock in some places before I made it that deep.

I'm certainly not an expert in foundation construction, but I think your plan has some challenges in a freeze-thaw environment. The fool-proof plan would be to drop a full perimeter foundation extending below the frost line, but that's a ton of expense. I get your thinking with the tubes, but anywhere the slab sits on the ground will be susceptible to frost heave. Sonotubes are used to support the full project load at just those locations (like decks). So, those are the only points that need to address frost heave.

Regarding the depth of the sonotubes, if you're not going below the frost line, I wouldn't bother. I would just use that concrete to increase the thickness of the slab. On that line of thinking, since you're probably not going to do a full foundation, and you have equipment, why not make the slab little thicker? I assume you're getting a redi-mix delivery for a project that size, so the incremental cost wouldn't be dramatic.

Depending on how much weight you have sitting on the slab and where, there will be the potential for stress cracks in the slab. Ex. there will be a differential in the downward force where the oven sits vs. the rest of the slab. With the tubes on the end, the stress will probably be near the midpoint between there and the oven. Without the end tubes, the slab will be stressed closer to oven.

No matter your final call, I think the best thing you can do is make sure the soil under the site will drain well. If your soil has a high clay content, and the site topography is supportive of drainage, it may be prudent to excavate a bit and mix in some sand then backfill and compact.

Good luck. If I've missed the mark with any of this, hopefully others will respond.