Garage floor and footer replacement help needed.

[buddio]

Hello all,

Garage description
Year built: 1953
Dimensions: 12' x 22'
Footer Construction: single poured monolithic slab/ floating slab
Slab Construction: single poured monolithic slab/ floating slab
Slab & Footer thickness: 8" thick
Slab condition: cracked, lifting and sinking
Footer condition: cracked, lifting and sinking
Garage attached to home or not: it is connected to home
Location: central Minnesota, frost depth 42".
by the book? I would like to build to code with adequate permitting and inspections.

Looking at my city's requirements for attached garages the footer must be 42" deep (for new construction)
I have concrete driveways covering all three exposed sides of my garage so all of the work will need to be done internally.

My initial thought path for making these repairs would be to remove small footer sections in every other 3' location, then dig down 42" (Minnesota's frost depth requirement) and replace footers, then once every other 3' section is complete and cured I would go back and replace the gaps until all of the footers have been replaced, then finally replacing the floor of the garage (I would be connecting all concrete pours together with rebar, footers and floor)

My biggest uncertainty is how I will be able to dig down 42" in my proposed 3' sections without ingress of exterior gravel from under my concrete driveway or soil?
My second biggest uncertainty is how will I be able to get this approved for permitting?


all of your feedback, preferred methods and suggestions would be greatly appreciated.

Thanks much,
-Jon

 

[billconner]

Tough problem. Is the building at all sagging or tilting?

I don't think you can extend deeper without undermining the surrounding flat work. If building isn't settling, I think I'd just saw it as close to sill plate as possible and pour new slab. If it is settling, I'd look into helical piers for lifting and supporting building, and then pour new floor slab. Code requirements for existing don't usually require bring it up to new.

 

[Fav Onefour]

That is a tough one.
Sounds like you are thinking about doing a DIY underpinning. It's doable, but is it worth the effort? It's also risky if you are truly set on not disturbing the surrounding pads.
It's hard to tell what type of base that you currently have under the garage, but I'm guessing it is similar under the pads unless someone came along and dug out the gunk. The ground has been moving a lot. How do the outside pads look?

I'm also in MN. If you are going to do the project, I think you already know there's a reason for the 42" spec. I'm sure the current shallow footing has not been kind to the concrete. In our area, house movers do this type of underpinning work. They charge big bucks. If you are going with pros on a garage, it might be cheaper to start over.

Couple items to add if you DIY the project. I'd cut back the pads and just plan on repouring next to the new foundation. You will probably lose too much base anyhow. Just commit to the idea and save work redoing the foundation. Also, think about bracing the walls and doing a whole side at one time. This project will be a lot of back breaking work. No need to drag it out for ages. You will gain efficiency by doing a whole wall instead of short sections. It's easier to dig and work in a trench vs. a bunch of deep holes.

 

[Kpaige]

Is this an attached garage to a house? If not and it’s a floating slab you do not need 42” frost footings the permiter 12” of concrete will be a few inches deeper with rebar through out.

There are many videos on YouTube of this being done many different ideas ending in the same good result.

Me I would use lam beams across the entire garage jack it up remove the concrete. Grade and repour.

 

[The Cobbler]

12x22 monolithic slab attached to house? that wouldn't fly here . it would need footers since it's attached .
you've got a big job, in My opinion .
move the garage off the slab, excavate , pour footings , foundation, slab then re set the building

 

[ConCretin]

It's extremely unlikely that you can successfully underpin the structure to frost depth without undermining adjacent construction unless you have very specific soil conditions. The outside face of your excavation would have to stable enough to allow a vertical cut that supports whatever is on top of it while you work. If so, you could form the inside face and place short, alternating sections of concrete underpinning to that completely fill the void.

It will be a hellacious about of grueling work and once you start, you'll need to keep at it. After that is complete, you'll need to build a proper slab base to keep the new slab from suffering the same fate as the original.

Permitting is another question entirely. Not knowing anything about your local jurisdiction, it's hard to say but I imagine they will want to see stamped structural drawings, which would likely entail a geotechnical survey. Don't be surprised if that process results in requirements for temporary shoring.

 

[billconner]

This is a difficult challenge. To avoid replacing all of the surrounding flatwork, and assuming garage is attached to house, level framing on top of what's there and pour a minimum 4" slab on top of what's there, forms against sill plate. Adjust garage door(s).

Another option would be to cut the surrounding flatwork maybe 3' from garage, demo that along with garage slab, and pour new thickened edge slab, and then new flat work around it. The new thickened edge slab will be tough not lifting building but possible.

I don't believe you have to bring existing repairs up to new code, but if it's a problem where you are, consider a frost protected shallow foundation.

Otherwise I think you might have to start from scratch, all new garage and flatwork, especially if you want to go 42" deep.

 

[Kpaige]

So here is my thought get ahold of a company that does helical piers. Two that I know of are American waterworks and DBS, if they can put helical piers in with say an I beam in place of footings to stabilize and support the structure you then just have to repour the floor. This way there is little to know soil displacement and it is a full proven system. Not cheap will be upwards of 40k for sure.

 

[The Cobbler]

another case where OP has not logged back in since writing the post... 9 days ago.

 

[billconner]

When I mentioned helical piers above, I was thinking diy. While aimed at decks, some of the manufacturers suggest buildings too. Not a small project and I'd hate to diy it in some soils. I don't know where you are nor size of garage so can't guess how many.

 

[Kpaige]

When I mentioned helical piers above, I was thinking diy. While aimed at decks, some of the manufacturers suggest buildings too. Not a small project and I'd hate to diy it in some soils. I don't know where you are nor size of garage so can't guess how many.

Helical piers for a foundation are a bigger deal then the diy etc. mine are down 40 ft till they hit very solid soil or bedrock. Then they have large steel structures that bolt onto the post and rotate under the structure and hydraulics are used to level if needed.

Here is an example there a several companies that supply they each have thier own take.

 

[billconner]

I'm familiar with them. This seems to be a small, light framed, one story, attached garage sitting on a slab on ground. The diy type is definitely not for a multi story masonry building with basement, but this garage weighs little more than a deck. Soil conditions might not allow it.

 

[Beemer]

About losing support of the exterior slabs, if it does happen, there is a technique called slab jacking that might be something to look into.
It's basically pumping soil grout into holes cored in the slab and can actually lift the slab.