Replies

1. 
   FastEddie1 | May 18, 2003 07:09am | #1

   Depends on the intended definition.  For very expansive soils, the house would sit on piers sunk to bedrock and thus be (relatively) immune from movement.  The slab would be suspended from the piers much like there was a basement, except that the 'basement' would only be a few inches deep, just enough to allow the soil to expand without applying pressure to the slab.  One way to do this is to pour the slab on top of cardboard forms that are designed to support the wet concrete, but disentigrate after a few months, leaving a void under the slab.  Another definition is that the slab (and house) is designed to literally float on the soil, moving up and down with the expansion.  That can cause problems with plumbing lines.  Sidewalks are a good example of this, where there is little or no attempt to keep the concrete in a fixed position.  Might be proven wrong on this, but I don't think having the addition be truly floating is such a good idea, because if it moves any differently (or doesn't move) than the main house, there's gonna be a few cracks in the walls and siding at the junction.  The addition need to be tied to the main so they move together.

   Where are you?

   Do it right, or do it twice.

   
   

   ---

   Edited 5/18/2003 12:10:24 AM ET by ELCID72

1. 
   jollygiant | May 18, 2003 07:20am | #2

   Humboldt County in northern California. The plumbing is the point. The plans actually call for pinning the new and old work together with epoxied dowels.

2. 
   Davo304 | May 18, 2003 08:41am | #3

   There may be more than one definition of a floating slab, but the only definition I'm familiar with is where a monolithic slab is poured that sits on top of the soil.

   I've poured many of these type; though not in expansive soil conditions. The main thing is to prepare the ground with proper drainage (layers of sand or gravel) and soil compaction before pouring the slab. ALL floating slabs that I ever poured that was to support a structure ALWAYS had proper rebar (mat) reinforcement installed. WWM was not used. We used  a double layer of 1/2 inch dia rebar positioned 12 inch OC.

   The idea of a floating slab is that if the soil should move, the structure can move with it and not crack..it "floats" on top of the movement.

   If you are using expoxied dowels, your slab is no longer floating. It is tied at the dowled joint. Should movement occur, it would be possible that  cracking could occur at the tied-in (dowled) juncture.  The purpose of doweling  a new foundation to an existing one is  so that both foundations will become one complete unit and behave accordingly..

   Sooo, if you are going to dowel, your slab cannot be referred to as a "floater."

   I don't know what techniques are used to combat expansive soils such as yours, but I do know that when building an addition, its best that both old and new foundations act together in unison. If the existing foundation is a floating slab, then  dowel another floater together so that they  both float as one  big unit. If instead the existing foundation is anchored to a footer or piers, anchor the new one in the same manner...etc. etc.

   One more thing, if your addion is larger than a single story, I would rethink using slab construction altogether .....slabs usually work best in single story structures only.

   Davo

3. 
   Piffin | May 18, 2003 03:01pm | #4

   Excavation contractors are giving free structural advice now, eyah?

   What's next? Electricians giving water-proofing/roofing advice?

   Maybe you know a plumber who can advise how to frame a floor system.

   Floating slabs are great in ceertain situations, but when we do a monolithic pour, we replace expansive soils down 12" to 18" with compacted stone and gravel to controll the floating anyway and reduce cracking. Using the monolithic pour to try to overcome the poor soils is bad advice -0 the cheap way out.

   My greater concern though is that this is an addition to an existing house. The two portions of the house should be on the same kind of foundation whenever possible. if you have a full frost wall foundation for the main house that does not move and a "floating slab" for the addition that is designed to move, you will have a lifetime of maintainance nightmares as a result, at the connection between the two.

   .

   Excellence is its own reward!

1. 
   Rentedmule | May 18, 2003 03:29pm | #5

   Is there ANYTHING that piffin does not know?

   "seldom right but always a fool"

1. 
   Piffin | May 22, 2003 06:00am | #8

   How women think?

   Man, I'm telling you - us know-it-alls have a hard life.

   ;).

   Excellence is its own reward!

2. 
   jollygiant | May 21, 2003 06:40pm | #6

     Thanks for the info Piffin and Davo. The slab addition is to a perimeter foundation tract house. The house is set to low the crawl space wouls become a swimming pool without two sump pumps. To avoid adding to the pumps burden I thought a slab with a foot of excavation compacted, compacted gravel, poly sheeting, sand, doubled rebar mat, dowelled old footing to new footing all along the coomon wall would be more practicle a solution. The flooring spanning old work to new will  be an interlocking laminate plank able to withstand 1/4" subfloor offsets. Biggest worry seems to be drywall cracking as the joists swell and shrink next to the relativly stationary concrete. A lot of assumptions maybe.

1. 
   Piffin | May 22, 2003 05:53am | #7

   That might possibly work if you get below frost level with it. I don't know your climate.

   But you can do a frost wall without a crawlspace. Footings, Walls, infill compacted, slab. No crawl void..

   Excellence is its own reward!