Have you ever seen a foundation like this??

[Scottpat]

I'm seeing this type of slab foundation more and more, but I do not know anything about this design. Any help would be great. This is being done by several builders, including a couple of national builders.

The center pad is made by stacking bags full of crushed rock. Then the center is filled with crushed rock. A trench is then dug around the perimeter to form the footers. Foundation wall forms are then placed around the footers. Steel is then placed between the forms and the pad. Concrete is then poured into the forms. When it is done it looks like a raised slab foundation.

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[Jim Katen]

Originally posted by Scottpat

. . . The center pad is made by stacking bags full of crushed rock. Then the center is filled with crushed rock.

How do they compact the rock without knocking the bags out of place?

A trench is then dug around the perimeter to form the footers. Foundation wall forms are then placed around the footers. Steel is then placed between the forms and the pad. Concrete is then poured into the forms.

They're wasting a lot of concrete by using those bags as the inner form. They could use much less concrete by forming the walls traditionally and filling the interior with gravel several days after the forms are removed. Gravel is way cheaper than concrete and there's gotta be a significant cost associated with placing all those bags.

The whole thing seems bassackwards to me.

- Jim Katen, Oregon

[Les]

never seen nothing like it.

[Chad Fabry]

They could have a nice basement with less effort.

[kurt]

Exactly. I'm still trying to figure out what's the benefit?

[Scottpat]

What I have been told is that they build this type of slab when the area has a large amount of subsurface rock. It is cheaper than blasting out the foundations.

Some builders use CMU blocks placed on a concrete footer. They still use the crushed rock for the pad but they don't pour the concrete walls. The CMU's will end up being about 4" higher than the crushed rock pad. Once the CMU's are in place, concrete is poured to the top of the CMU walls. My home is built that way.

The two national builders that are using the bags of rock are Beazer and Drees.

[Jim Katen]

Originally posted by Scottpat

What I have been told is that they build this type of slab when the area has a large amount of subsurface rock. It is cheaper than blasting out the foundations. . .

OK. Now I'm really confused. If that's the reasoning, why don't they simply cast the perimeter foundation and frame the first floor over a crawlspace. It would be a whole lot cheaper.

- Jim Katen, Oregon

[Kyle Kubs]

Originally posted by Jim Katen

Originally posted by Scottpat

What I have been told is that they build this type of slab when the area has a large amount of subsurface rock. It is cheaper than blasting out the foundations. . .

OK. Now I'm really confused. If that's the reasoning, why don't they simply cast the perimeter foundation and frame the first floor over a crawlspace. It would be a whole lot cheaper.

- Jim Katen, Oregon

The first thing that comes to my mind is "very, very unskilled labor" needed for something like this = Cheap Cheap.