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Caisson uplift

Caisson uplift

Caisson uplift

(OP)
We're planning to recommend the sinking of a concrete caisson for a large sewerage manhole.  During winter, it is likely that the water table could rise 8 meter above the bottom of the caisson.   Two questions:

1. In the equilibrium calculations, when comparing gravity stabilising forces versus buoyancy, what would be a reasonable factor of safety to use?  Should the dead load be 10% more than the buoyancy?  Or more?
2. Is there a convenient "skin friction" that can be used to calculate the resistance to uplift.   This to be added to the deadweight to improve the Factor of Safety.

Thanks
Alten Hulme

RE: Caisson uplift

I would ignore skin friction unless you have a competent geotech tell you that you can with saturated soil.

Usual building code combination is 0.6D + W for uplift/overturning with wind.  For buoyancy I'd use the same (0.6D + Buoyancy).  Others use a SF of 1.5 as well.  Very similar.

RE: Caisson uplift

(OP)
JAE

Thanks for your reply.  

1. Yes, the skin friction one does seem a bit iffy, but I'm trying hard to reduce the concrete required.
2.  0.6D seems conservative in that:
2.1 The dead load can be accurately determined
2.2 The water table height being used in the calculation is a conservative height.

For now

Alten


RE: Caisson uplift

I think the 0.6D has more to do with the 1.5 safety factor on 0.9D.  Something like a Caisson floating away is pretty catastrophic, so a 1.5 FS is a minimum at best.  Think about it:  compared to most strucutral calculation, this one has no material safety factor built in... it's not like designing a wood beam to 90% or 95%.  It's merely taking physics:  and so, the 1.5 FS is not necessarilt the case in which you have 60% dead, but actually, for the case where you have an extra 50% of pressure due to unforeseen circumstances... like a flood, hurricane, etc.

Good luck!
Mike

RE: Caisson uplift

(OP)
Mike

thanks.  You've put it in perspective.

Alten

RE: Caisson uplift

If you assume your foundation is submerged, it seems to me that bouyancy is then a type "F" load, "load due to fluids with well defined pressures and maximum heights."  I don't see that ASCE7, article 2.4 calls for 0.6D + Bouyancy.

RE: Caisson uplift

Yes, I think Mike is correct that 0.6D is more of a safety factor on the event rather than a response to uncertainty in the dead load.

RE: Caisson uplift

You could reduce the concrete dead weight (i.e., volume of concrete) by using tie-down anchors through the bottom of the caisson.  

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