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Re: 20' foundation walls

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That's a deep basement!!  With respect to the anticipated water level, it's
best to drain the water away from your structure.  If you expect the water
to be there against the wall, you should head into ACI 350 which will
increase the steel significantly but is geared for environmental concrete
tanks.  Also, watch buoyancy trying to lift up the basement floor.  The
basement floor would have to resist this uplift force and check the factor
of safety against buoyancy to make sure the structure doesn't float at
worst case water level.  Add a water stop between the base foundation and
the base of the wall.

As far as concrete specifications, take a look at ACI 350.  It has
additional requirements for durability.  The 20' pour height is very high.
Consider adding a construction joint at 10' and if that is under
groundwater level, add a PVC waterstop.  I would question the contractors
ability to consolidate the concrete with a 20' high pour.

Use at rest soil pressure (not active pressure) as the basement wall
typically doesn't deflect enough to activate the active soil pressure.  And
if you are in seismic country, you'll need to take that into consideration
too.

Paul.
Phoenix, AZ

On Sat, Oct 20, 2012 at 10:46 PM, Valerie Eskelsen <
v.t.eskelsen(--nospam--at)sbcglobal.net> wrote:

> Dear List,
> 
> Designing 20' deep, restrained foundation walls for a residential
> structure. The water table will not be known until the dig is begun.
> Aggressive timetable of course.
> 
> I am expecting some water regardless, and am concerned that 20' is too 
> deep for a single pour.
> There is rebar, of course, but is there more about the concrete mix or 
> the lift heights  that I should be concerned about and specifying to the
> contractor?
> 
> Thank you for any direction that you might be able to give.
> -----------------------------
> Valerie Eskelsen, P.E.

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