Self Weight of Deep Foundations and Structural Slabs
Self Weight of Deep Foundations and Structural Slabs
(OP)
The traditional method for designing any deep foundation system is to treat the entire system like an elevated platform and all the loading (including self-weight) is ultimately transferred down the load path to the piles or shafts.
However, I wonder how much of the self-weight truly gets transferred to the deep foundation elements (i.e. piles or shafts). When the concrete for the foundation superstructure (i.e. grade beams and slabs) is initially poured, nearly 100% of the dead load has to be soil supported since the foundation superstructure has no spanning capabilities until some curing starts to occur. So, if the soil is initially taking all of the dead load, what is going to cause a transfer in load once the foundation superstructure cures? I can understand how it could easily occur when you are dealing with highly expansive soils, but it soil movement enough of a concern for typical, non-expansive soils?
I'm curious to get your thoughts on this matter. I'm not saying our current practice isn't the right/safe way to do it. However, I wonder about reality.
However, I wonder how much of the self-weight truly gets transferred to the deep foundation elements (i.e. piles or shafts). When the concrete for the foundation superstructure (i.e. grade beams and slabs) is initially poured, nearly 100% of the dead load has to be soil supported since the foundation superstructure has no spanning capabilities until some curing starts to occur. So, if the soil is initially taking all of the dead load, what is going to cause a transfer in load once the foundation superstructure cures? I can understand how it could easily occur when you are dealing with highly expansive soils, but it soil movement enough of a concern for typical, non-expansive soils?
I'm curious to get your thoughts on this matter. I'm not saying our current practice isn't the right/safe way to do it. However, I wonder about reality.






RE: Self Weight of Deep Foundations and Structural Slabs
The soil conditions that exist for structures like this are surely quite varied. But these parameters would be tough to know actually and even tougher to use to determine what the load sharing (between soil and structure) would be.
I think you are correct that we have to design the safe way and put all of the load in the structure.
Most times the "bad" soil won't have much stiffness or capacity to take load at all.
RE: Self Weight of Deep Foundations and Structural Slabs
RE: Self Weight of Deep Foundations and Structural Slabs
RE: Self Weight of Deep Foundations and Structural Slabs
I have seen slabs that were initially cast against the soil but over time the soil compressed and there was a gap between the slab and the soil. In this instance, the foundation system performed properly.
RE: Self Weight of Deep Foundations and Structural Slabs
RE: Self Weight of Deep Foundations and Structural Slabs
That being said, I have seen a project where the geotech report allowed load sharing between the slab and the piles. I'm sure the geotech did an extensive investigation to justify this. But, it's not hard to understand conceptually. In this case, they could easily justify the Geotech's expense because it was an existing structure that could never have been modified without it.