caisson uplif

huangyhg

caisson uplift
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
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.
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
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
mike
thanks.  you've put it in perspective.
alten
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.
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.
you could reduce the concrete dead weight (i.e., volume of concrete) by using tie-down anchors through the bottom of the caisson.