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We're getting ready to build a 65×30 metal addition, 11' in height. I'm about to start building the grade up but I need 75% of it to rise 24". I know I'm going to need roughly 18 trucks of road base, I know to build 6" and compact until where I need to be.

Where I'm struggling, is I'm assuming the footing will be 24"x24" all the way around the perimeter(haven't had concrete contractor verify this yet) It seems dumb to build up the entire pad and then dig my rock back out, is here a way to build the grade up around a 24"x24" cavity? Bags of some sort maybe? Any ideas?

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  • Usually the grade is done after building. Any reason not to?
    – crip659
    Dec 4, 2022 at 22:41
  • I had a similar size building done, but it was on a floating foundation. The grade for the front end needed building up first,to be level with the back, before the foundation was laid on top. Maybe explain your question a bit better.
    – crip659
    Dec 4, 2022 at 23:07
  • Yeah, I know it's not a stellar question, doing my best with the details. It's going to be slab on grade foundation. Building the grade up for it is my question. The footing around the perimeter being 24"x24" and 4" slab. So if I build the grade up I would have to pad significantly wider then dig a 24x24 trench around the perimeter. Seems dumb to build up 24" of rock then dig out 24" of rock
    – Mlrs
    Dec 5, 2022 at 15:54

2 Answers 2

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Assuming you'll use forms for the side of the footings, rather than "dump concrete in a trench" you might bring in the bulk of the interior fill and compact it, while leaving a wide trench where the footing is with a stable/safe slope to the top of the new material, then pour your footings, and compact material in around them after the forms come off. That's pretty much how mine was done, but with a footing quite a bit deeper and a frostwall on top of that.

Don't break the new concrete by abusing it before it's had time to cure and get some strength.

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  • Ok that is making some sense.... so the concrete contractor might be able to do the 24×24 perimeter, then after that cures up, I'd just bring rock in to build up grade, then he'd pour the slab even with his perimeter?
    – Mlrs
    Dec 5, 2022 at 15:58
  • Pretty much like that, yes. Does seem like a huge footing. I'd throw some steel in there, it's cheap .vs. the overall cost of the job with considerable benefits, and I'd be sure to stub up for an Ufer ground, (concrete encased electrode) because it's trivial when you pour (tie the steel well, and stub up a bar that is clearly identified NOT TO BE CUT by following contractors) and impossible to add later.
    – Ecnerwal
    Dec 5, 2022 at 16:06
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24x24? The standard thing is 24" wide, 8" tall, and then an 8" concrete wall on top. This depends on the bearing class of your soil. I typically go for a monolithic pour.

Normally you'd form, pour concrete, strip the forms and then put your drain rock inside your foundation for your slab base.

18 trucks of road base sounds insane. I'd do something like a 6" max layer of gravel, 4" foam, vapor barrier, pour a 4" concrete slab and then just put 2x8s (7.5") on top of 2x4 pt sleepers (3.5"), then your layer of 5/8ply and your finished floor material. That would be ~25" lift from your current elevation.

Unless you are driving mining trucks on top of this new extension slab?

Unclear how many, if any, of those trucks are for backfill against the outside of the foundation.

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  • The problem is that most of the current ground is 24" lower than where the top of the foundation needs to sit
    – Mlrs
    Dec 5, 2022 at 15:48
  • For my Quonset hut of similar size the footing/foundation was about the same size(24x24), but it sat on top of the grade(floating).
    – crip659
    Dec 5, 2022 at 16:00
  • So you have a 24" foundation wall, backfill the outside and build up the inside with a floor assembly. You get a better warmer floor with a floor assembly anyway. 18 trucks of road base is probably $9,000 without installation labor to compact and distribute. Dec 5, 2022 at 16:41

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