Differential settlement 1

[longisland]

Hi
I'm designing a multi purpose hall sitting between a cut & fill area. I'm planning to use pad foundating for the cut & piled foundation for the fill area. What's the best approach to design the shallow foundation to avoid differention settlement? The floor will be laid with marble & no expansion joint is allowed. The cut ground is a silty-clay of 1 tsf with no ground water. For the sake of discussion, what if the cut ground is sand?

 

[ishvaaag]

I would say that the best way to avoid differential settlement is to intorduce continuity in the foundation. Make a 3D model on springs or elastic soil, and give the soil different properties to see how your continuous foundation deforms short an longterm under likely irregular loads, in more than those extant. These should give you good mechanical control of differential settlement.

But do not forget to get thorough understanding of te soil under, for there maybe differential properties that you need to enter in your model (softer springs etc).

What said wouldn't change much for sands, it is just generic way to tackle the problem.

 

[ed34]

Hi Longisland,

How deep is the fill layer and what does it consist of? Generally, to avoid differential settlements, you can design your foundations to bear on the same soils, as it sounds like you have done. However, depending on the depth of the fill section, and the length of the piles, the settlement of the pile foundations may be negligible compared to that of the pad foundations. Therefore, your differential settlements over the structure would approach maximum settlements anticipated for the pad foundations. One thing you can do, if the differential settlements are not workable, is to reduce the allowable bearing capacity of the cut section, thereby increasing the foundation area.

Another thing you could do, and this must be looked at carefully because I do not know anything about the soils you are dealing with or the fill, but you could possibly go with shallow foundations over the entire building area (in both cut and fill) if the fill is a structural fill compacted in lifts and monitored during construction. Then, you could reduce the allowable bearing capacity used to design the foundations to coincide with the bearing capacity of the weaker soils. There will be some differential settlement, but it may be in line with the differential settlement you would get from going with a shallow and deep foundation system, and it may be less expensive to construct.

Are the clays expansive? The marble flooring will be very sensitive to any differential vertical movement (settlement or expansion). If you overexcavated in the cut section to place a "cushion" of structural fill, you would address both expansion potential and differential settlements. Otherwise, void forming and a structural slab could be used.

Hope this was helpful

 

[mfrad]

The slab on ground area is not compatible with a structural slab that is pile supported. The slab on ground has control joints that will introduce reflective cracks into the marble. The S.O.G. will deflect differently especially at the edge adjacent to the supported slab area. Without an expansion joint between the two the marble will definitely crack at the interface between the two. Check with the Marble Institute since they have very severe deflection requirements of L/720 and would require an expansion joint at the boundary between the two foundation systems in additon to other control joint spacings. It would be best to pile support both areas and have a supported structural slab system between the pile caps since deflection control is important with regards to a marble floor.

 

[Rowland33]

I totally agree with mfrad. Why take a chance?