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Check for Floatation

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I would not count on the soil to prevent floatation.  Sewer lines buried 
underground have floated to the surface when the ground was saturated.

I would only consider concrete, lead, or similar material to counter buoyancy.

A. Roger Turk, P.E.(Structural)
Tucson, Arizona

P. Rajendran wrote:

>>I am evaluating the adequacy of a, buried, steel storm shelter manufactured 
by a local steel fabrication shop.  In order to pass the FEMA approval 
criteria, the structure has to be checked for floatation assuming that the 
water table can rise up to the ground level.  For a box, L'XW'XH', the 
buoyancy is 62.4LWH.  The weight of the steel structure is very small in 
comparison to the buoyancy.  Additional weight has to come from soil weight 
on the extended portions of the bottom plate.  For the purposes of further 
discussions, assume that the bottom plate extends 2' beyond the face of the 
walls.

For estimating the weight of soil on the extended plate, should the soil be 
limited to 2' wide column?  It seems to me that the failure of soil will be 
in the form of a wedge, the wedge being bounded by the vertical wall of the 
structure, the 2' projection of the bottom plate, existing ground surface and 
an inclined line emanating from the edge of the extended plate.  The inclined 
line could be assumed to be at 60 deg. from the horizontal.

No site specific soil reports available.  The structure will be installed 
anywhere in Oklahoma and adjoining states.  I will put a disclaimer on the 
drawing to exclude sites that may have expansive clays, silts etc.

Any thoughts?<<

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