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Re: Check for Floatation

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With wet wells (typically no more than 12' diameter by 15' deep) I consider
only the soil that is directly above any extension of the concrete footing.
The actual failure surface would probably be more of a cone as you suggest -
I prefer to treat that additional capacity as simply an extra factor of
safety especially since here in Florida, seasonal water tables near the
ground surface are not uncommon.

However if the structure is fairly large, this approach may be overly
conservative.  Perhaps a geotech could offer some advice.  If you want to
think a little bit "outside of the box", can some sort of geotextile be
placed in the backfill to assure a conical soil failure surface?

David Finley, P.E.


-----Original Message-----
From: RajSTC(--nospam--at)aol.com <RajSTC(--nospam--at)aol.com>
To: seaint(--nospam--at)seaint.org <seaint(--nospam--at)seaint.org>
Date: Tuesday, August 15, 2000 5:21 AM
Subject: Check for Floatation


>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?
>
>P. Rajendran
>Service & Technology Corporation
>105 SW Penn
>Bartlesville, OK 74003
>Ph: 918 336 8161  FAX: 918 336 8265
>
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