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Re: Landslide

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John -

The "piles" I refer to are actually drilled cast-in-place concrete, not
driven.

As to the geotechs "minimum" size, you should treat it as such.  Design what
you need structurally to resist the loads.

Lastly, our office policy is to provide 0.5% minimum steel in the piles since
the axial load is so low, and the pile acts much more like a beam than a
column.  Typically, we spec the reinforcing in a round pattern so that the
contractor can't really screw it up when they try to orient the cage in the
hole.  (We used to spec a square cage with added bars on the tension face, but
I was not convinced that they were always installed in the right direction.)
Since we consider the pile as a beam, we provide ties at d/2 maximum (10" for
a 24" pile).  Lately, we have had some LA City plan checkers require ties at
6" on center over the full height since they consider the pile as a concrete
column which is part of the seismic force resisting system.  Why you would
require the extra ties when the capacity of the concrete is probably 10 times
greater than the capacity of the soil so it is really impossible to generate
enough axial load in the column to cause a concrete failure is beyond me.  But
then again, I'm not a plan checker!

Good luck,

Bruce Resnick, SE
Parker Resnick Str. Eng.