Caisson uplift 1

[wildehond]

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

 

[JAE]

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.

 

[wildehond]

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

 

[seattlemike]

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

 

[wildehond]

Mike

thanks. You've put it in perspective.

Alten

 

[miecz]

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.

 

[JAE]

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.

 

[BigH]

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