Is it possible to "foot" a floating slab building?

[jimgerken]

Can a building that was originally built with the floating slab, be later footed? I can imagine digging all around it and pouring a footing down 4-5 feet, then setting block up to nearly under the slab (thickened) edge, then shimming it to finish, etc, something like that. BUT I can't figure out how the excavated area can be back filled properly again. I can't excavate the exact volume that the footing and block need, I am sure that sand and dirt will tumble out from several feet under the slab edge, and it then becomes somewhat unsupported.
The goal is to minimize or eliminate the freezing cold Minnesota winter from getting under the slab and freezing the ground under the building wall, thus lifting it upward. If this condition is eliminated, I could then add my new house onto the existing garage properly. If the garage can't be footed somehow, the house will have to be built near but not attaching to the garage.
A few ideas have come up in our discussions. One is interesting me a bit. Imagine installing a 2 inch extruded foam board, outside the slab edge, vertically into the ground four feet (no footing in this scheme). The cold cannot penetrate thru the edge of the slab or thru the dirt under it for several feet down. The cold will not penetrate the dirt thru the floor inside the building anyway, because it will be closed up and heated to about 40 degrees air temp all winter.
Another idea we had was to build the footing and block wall, insluation board applied to the inside or outside of the bloock, then mud jack into the excavated unsupported space inside the wall thru core-drilled holes in the floor, a couple feet inside the wall.
Another idea was pumping the space full of isonene foam or similar.
A few other ideas got REALLY crazy.
Anyone with experience in this direction? What is commonly done? Thanks.

 

[Duffster]

I would excavate out about 4' wide and a foot or so deep and lay 2" foam horizontally and then vertically along the edge of the slab to the top then back fill.

As far as the footing if you poured a concrete wall you could pour up against the dirt bank. If you want to use block you could pump flowable fill behind the block wall.

 

[blueriver]

I would pour a concrete footing/wall from the desired depth all the way up to the concrete floor ... this would eliminate backfilling under the slab behind the footing wall.

It could be formed with 4x8 sheets to the top of your exsisiting slab ... stay out about 3 to 4 inches to run the concrete in the form and then after you pull the forms put your styrofoam insulation against the wall and back fill.

Clear as mud??

 

[Egon]

what is the purpose of the footer?

 

[Duffster]

what is the purpose of the footer?

Hold up the wall.

 

[wedge40]

I'm asking not telling.
Couldn't he dig the footer a few feet outside the slab perimetere, and then pour in the difference after the block is put up? Seem like he could splice some rebar into the slap to hold the new floor to the old floor.

Wedge

 

[ustmd]

Here in Texas and other areas of the Blackland Prarrie, we use a similar process to deal with post construction foundation issues due to the high level of soil movement we have. Instead of pouring a footer all around the edge of the slab, we either use a (50K ton) hydraulic jack to force (until point of refusal) solid concrete blocks into the soil under the grade beams at key points. On all of the blocks are in, the structure is shimed to level using steel shims. Alternatively, holes are dug and concrete piers are poured (my neighbor has piers going down 30ft) and then the structure is leveled. In either case, the entire structure is sitting on the concrete piers and is isolated from ground movement.

 

[gwjd]

why not build house and a breeze way with an exspansion joint like you might see in a comical building..... like a over pass to road joint then if the garage does move it does not really matter on the out side you could just use trim as the cover ......

also good fire break ....

 

[slowzuki]

Is the building heat? A footing or frost wall does nothing to stop heaving or slab movement if you don't have a heat source in the building.

Several states produce frost protected slab design guidelines, heres one, not the one I was looking for but its a start.

DESIGN GUIDE FOR FROST-PROTECTED SHALLOW FOUNDATIONS

 

[jimgerken]

Wow, already some good input! Thanks.

Yes, the building is heated, although to a fairly low temp, like 40 degrees F.

The expansion joint idea is kind of the default plan if nothing else seems workable. for the joint, I was thinking building the house with atttached breezeway pretty close to the garage, like a foot away, and around the doorway there would be an extension that could screw to the footed breezeway side, like a large extension jamb, and a layer of foam on the side where the garage door would be.

Glad I don't live in Texas, with soil conditions like that.

It still seems that the only method of filling under the slab, inside the thickened edge, is to pump it in there, pouring it will not make it flow uphill underneith.

 

[not2old]

If the garage is separate from the house have you considered raising it, removing the existing floor/slab and constructing a foundation under it as deep into the ground as necessary?

 

[jimgerken]

Not2old: yea, that would be the best way to do it I suppose. That also gives the opportunity to relocate it if desired.

 

[TonyF]

I would try this method before digging a new footer. Maybe you can get the insulation under the existing slab.

http://www.countryplans.com/Downloads/shallowfound.pdf

 

[arsenix2001]

Your basically going to be under pinning, proper way is to dig to frost next to the edge of slab and dig back at least 12 inches, rebar and form the outsides and pump a peagravel mix right to the top, and small joints left can be packed with a portland sand mixture.

 

[inveresk]

There are a number of ways to do this but depends on the wall material and whether the slab edge is carrying a lot of weight. Does the slab edge support framed walls or brick? How much weight is added to the wall or slab edge by the roof? How much reinforcement do you have in the slab? If you excavate along a full side, is there a risk of subsidence? Could the load on the slab from roof and walls risk it cracking if 12" or so is cantilevered over the excavation during the works? If either of the last two, you have to undertake excavation on a 'hit and miss' basis - excavate a length of about 4 feet at a time and leave 4 feet undisturbed. Corners, too, have to be excavated one side at a time, not in a L shape.

Other option is digging down locally and use sona tubes at, say, 4 foot centres to provide concrete piles. This wouldn't give you a continuous foundation but it might be enough to do the job. And it would be a lot less work. Depends, again, on thickness of slab, load on it and how much reinforcement it contains.

 

[pat32rf]

Normally the footings support the structure. In your case the floating slab is designed to do this. All you really need to do is insulate the slab edges, down a foot or two, then a sloping skirt of insulation that extends out from the footing and carrys away any ground (rain)water to a weeping tile. If you keep the soil dry a bit of frost won't heave things. Its the expanding moisture (ice) that causes problems.. Blue styrofoam is wonderful, but seal the seams...

 

[dstig1]

If the garage is separate from the house have you considered raising it, removing the existing floor/slab and constructing a foundation under it as deep into the ground as necessary?

Bingo! Just what I was going to suggest. House lifting/moving isn't too expensive, really.

And then if you want to turn it into a shallow frost-protected footing instead of full 4' depth, our own U of Minn has this to say:
Tomorrow

 

[boxygen]

In this case, I would do nothing at all or I would jack the house up and get rid of the existing slab completely. You will spend much more time and effort retrofitting the slab to be frost free than the original slab would cost to replace. If you want your house on a frost wall or full basement I would design the connector between the two to be a shallow frost protected slab. You would form and pour the basement/frostwall then backfill and prep and form the shallow frost protected slab connector between the house and garage. This is all without seeing your site conditions and what you want for a building to connect to the garage, which can dictate a lot.

 

[not2old]

Not2old: yea, that would be the best way to do it I suppose. That also gives the opportunity to relocate it if desired.

If you want to move the garage, it isn't that big of a deal either. 40 years ago, my uncle raised his garage onto blocks and then lowered it onto a bunch of old telephone poles lined up in the direction he wanted to go. He then towed it into place with The Big D. (late 1960's Datsun pick-up)

 

[CurlyDave]

...The expansion joint idea is kind of the default plan if nothing else seems workable. for the joint, I was thinking building the house with atttached breezeway pretty close to the garage, like a foot away, and around the doorway there would be an extension that could screw to the footed breezeway side, like a large extension jamb, and a layer of foam on the side where the garage door would be.

I would use the breezeway idea, but make it much longer than a foot. At least 5 or 6 feet and maybe as much as 8'.

That way, if the garage rises and settles with the season, the angular adjustment needed in the breezeway is much less than if it is only a foot long. You should be able to enclose, heat and insulate the breezeway, which makes the distance seem much easier to live with.

Secondly, if the breezeway idea doesn't work out and you eventually want to jack up the garage and add a footer, you have room to get a piece of equipment between the house and the garage. Otherwise you are stuck digging a 4' deep trench in an alley only 1' wide between the house and the garage.