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Precast poles behind retaining wall

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A week or so ago, Sarah Nuttall wrote:

"I am working on a project where the architectural
design calls for direct 
buried precast concrete columns placed on a circle
supporting a lateral 
load of approx. 3000 lbs. each, within 4 ft. of the
face of a retaining 
wall. I am trying to isolate the load from impacting
the retaining wall 
design. I was initially using a mechanically
stabilized earth wall with the 
column extending below the wall. Now that I have to
design a cast in place 
cantilever retaining wall, I have an interference
between the wall 
foundation and the columns. I am considering the
option of providing a 
block-out in the retaining wall foundation for the
width of the precast 
column and a rock sleeve. The columns are spaced more
than 30' apart.

I'd appreciate any ideas on how I can isolate the
retaining wall from the 
column loads. Also, how do I prevent soil structure
interaction between the 
column and the earth behind the wall, something along
the lines of a void 
form, perhaps?"

With the MSE wall you don't have much choice but to
try isolate the column.  That wall can't help you much
because it isn't strong enough, and won't because it
isn't stiff enough.  Personally, I don't think it's a
good idea in that situation.

A cast in place cantilever wall is another story. 
Even if you could effectively isolate the column from
the wall, you have to look at the global effect of the
two structures in the soil beneath and behind the
wall.  If it were me, I'd pick a 15 foot or so zone
around each column and make the retaining wall footing
in that zone big enough to carry the wall and the
column.  I'd also design the wall reinforcing in
perhaps a 5 foot band to carry the moment of the earth
and the column, on the assumption that you're not
going to get perfect fixity between a precast column
and your footing.

Hope that helps.

Mike Hemstad, P.E., S.E.
St. Paul, Minnesota

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