Footers or Slab

[BeanAnimal]

Re: Footers or Slab

Thank you for the response.

I do plan on pouring close to 8" with heavy rebar. I did not pour my walk and driveway so am not sure how well the soil was prepped or how much drainage was provided, but they certainly do frost heave every winter. My driveway heaves so much that I was unable to open the door on the previous shed, indicating at least 1" of movement. The 4 slab driveway is also drifting apart and has been ever since we moved in about 6 years ago.

I think even if it is well drained below the slab (the gravel base) the caly underneath is saturated and expands when it freezes. I am not sure there is any way around it.

Insulating such a slab is a waste of time, as the insulation will be at equillibrium with the ground in a matter of days, so I am also not sure why they did that. I can see a vapor barrier to keep the wood storage area dry...

So you put tubes on the 4 corners and tied them to the slab with rebar? How big are the footers for the tubes?

[splatgirl]

Re: Footers or Slab

I am in MN with soil like yours--heavy clay with almost no topsoil. I went with four pier footings under/tied to the lower slab and a bit heavier on the rebar. I probably also have closer to 7-8" slab thickness vs. 6". It sits on a few inches of sand and gravel. Three winters, so far, so good. I have no idea if that is at all related to the presence of the footings, but every other (4") slab I have at the house that is directly on uncorrected soil--driveway, patio and sidewalk, all frost heave in the winter, despite 2' rebar grid and being pinned into the frost walls of my house with same.
I'm not sure what is more fun--digging and moving clay or watching it destroy $20K worth of flatwork.
I agree that deciding what to do about the foundation was the worst and most stressful part. And I also agree that the possibility of a floating slab bearing load but becoming unlevel over time would be a huge bummer. Fixing that would be a project.

I know of one other pompeii in MN that was built on a floating slab. I think that one has 2" of EPS or iso under the slab, for what I'm not sure. And I haven't heard how it's weathering...

[BeanAnimal]

Re: Footers or Slab

I would go with the floating slab. A lot less work. My second choice would to go with three sonotube piers.

I had actually considered sonotube piers to support the lower slab, but the weight is simply too much for the bearing surface that clay provides. I would need 4 or 5 very substantial footers or sonotubes and that is as much trouble as just digging the entire footer.

I am rather sure that the flaoting slab will rise and fall with the frost and with enough thickness and rebar, it should not crack. The bigger concern is it shifting or becoming unlevel over time?

This is the most stressful part of this project for me... choosing the proper foundation for my ground soil and drainage conditions.

[Neil2]

Re: Footers or Slab

I would go with the floating slab. A lot less work. My second choice would to go with three sonotube piers.

[BeanAnimal]

I live in Pittsburgh, PA.

My soil is consists of a 6"-12" layer of topsoil over solid clay. While my property has a decent slope, it is poorly drained due to the clay forcing all of the water to stay in the top few inches of the yard.

I can and will provide drainage around the oven, but am going back and forth on the foundation for my 42" oven.

More information:
I built a 600sq ft deck (adjacent to the oven area) and hand dug 21 2' x 4' rectangular holes 3' deep to accomodate footers for the sonotube deck supports. I used 21 footers due to very poor load bearing capacity of clay soil. The deck has not moved (2.5 years old).

I am not sure if I should dig down below frost level (about 3') and pour a traditional foundation and build a block wall to hold the base slab, or simply excavate down 1' or so and build a well drained gravel base to pour the slab directly onto.

The floating slab would be much easier, but if I don't get the base well compacted and drained, then frost heave could be an issue. On the other hand, a foundation hole will allow water to migrate downwards toward the footer but never drain away (solid clay). The resulting water could form ice lenses that would attempt to attach to (and heave) the concrete block foundation walls. I suppose all of the cores would need to be concrete filled, or concrete foundation walls poured with smooth sides. Costly, to say the least.

When I say poorly drained, I can dig a 3' deep post hole and fill it with a garden hose and it will never drain. No kidding...

Thoughts based on my soil conditions? Any of you have similar conditions?