Floating Foundation

[bylar]

I live in NC and several engineers in the area design a floating founation when the encounter poor bearing soils. They design the slab to carry all the gravity loads.
I don't care for this approach since if the slab encounters a hard or rocky section then the entire slab will tilt.
Has anyone used this method or what are the pro's and con's.

 

[F1955]

I live in NC & have designed floating foundations using one-way & slabs & grade beams on pile fdns without any problems.

 

[JRGse]

I have also used this method, without problems, in several regions of the US. IBC 2003 allows this method even in cold regions.

 

[BigH]

Just a question - do you mean by floating slab as a slab on grade - rigid enough that it reduces the loads to very low values - or are you talking about digging out basements where the weight of soil removed in basement is equivalent to the load of the structure - hence a net force of, say, nearly nothing? The latter has been used many many places - one of the classic ones is a case history in Guyana (by Golder I believe - or it could have be RM Hardy). This is in a number of books. As for the first approach, this is what we have used on my current job to support brick buildings - we used a slab on grade lying on 3m of sand fill overlying 5m of very soft to soft clay. We used the rigidity so that if there was a tilt, it would be as a unit - hence little cracking of the brick walls. If you did have "hard" points as you have indicated, this would also be a problem with normal spread foundations too.

 

[bylar]

BigH

The case most often is where a basement is dug. The inspection department comes out to inspect the footings and probes the ground by pushing a steel rod into the ground. If it has very low resistance he requires an engineered designed foundation. I suggest to the clients that they have a soil bearing test run at which point i have no problem with a design. Another engineer in the are desings the slab on grade stong enough to distribute the load to a low level. I have never checked their designs but my concern is that if one area has a lot lower soil bearing value than another area that the entire slab will have a tendency to tip at the lower value area.
Am i missiing something?

 

[BigRosie]

A foundation won’t “tip” just because the bearing resistance of the soil under portions of the slab are different. The shape and stiffness of the slab is used to distribute the load such that it is less than the bearing capacity of the soil.

Example: Take a square building on a floating foundation. If half the building is on poor soil then the shape of the foundation wouldn’t be square and the location of the building on that foundation wouldn’t be in the middle …

 

[Focht3]

Stiffened slabs are quite effective on soft ground. By reducing the average pressure to something significantly below the allowable bearing pressure, you limit the soil settlements that the slab will experience. (In reality, the choice of an allowable bearing pressure is really an effort to limit foundation settlement.) That doesn't mean it won't tilt; it's just that the tilt won't be particularly noticeable.

Hint: most (all?) residential floor slabs are tilted. But no one really cares, because the tilt isn't appreciable.



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See faq158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"

 

[BigH]

Bylar - thanks. WIth basements, you can use compensated or buoyancy raft foundations - where the weight of the building is the same as that of the soil removed and you are okay. Likely not many inspectors are aware of this. You may see M.J. Tomlinson's Foundation Design and Construction 6th Edition (also in earlier ones). He has a whole chapter on buoyancy rafts and basements (box foundations). "The true buoyancy raft, however, is a foundation which is designed solely for the purpose of providing support to the structure without regard to utilizing the space for any other purpose. To this end the raft is designed to be as light and rigid as possible. Lightness combined with stiffness is best achieved by cellular or 'egg-box' construction. A good chapter and a good book.