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
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
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
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
Good luck!
Mike
RE: Caisson uplift
thanks. You've put it in perspective.
Alten
RE: Caisson uplift
RE: Caisson uplift
RE: Caisson uplift