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Foundation on Expansive Clay -- Recap

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'Love it...

Posting a question to this list is like throwing a hand-grenade into a
confetti factory.

Recap:

Foundations for bark blowline in a paper mill.  Spacing 70 ft max,
supporting 18" diameter blowline 20 ft above grade.  Soil -- high
blow-count (n = 20+) silty/sandy clay with "potential for expansion".

My first cut was a drilled pier, setting an oversize sleeve in the
foundation, and plumbing and placing the mast later.  The spread footing
worked out cheaper until I found out about the "potential for expansion".

'Lots of good suggestions -- I have gone back to a single drilled pier for
each support, setting a 2 foot long piece of 18" pipe in the top as a
sleeve for the 12" diameter mast.  There's no potential for uplift, and the
top of the mast can be a little high, because I'm hanging off the side with
a bracket that we'll set to proper elevation & weld in place.  The pipe
will be hung with a threaded rod and a pipe collar which will give a "fine
adjustment" to the pipe elevation.  Yes, it's eccentric, but the load is
not too large.

On the comments:

1.  To Padmanabhan Rajendran -- What would flyash do to relieve the
expansion?  

2.  For David Finley -- There shouldn't be much potential for caving.

3.  For Eric Green -- 'No trees here.  This is a paper mill; they'd make
paper out of a tree if they had one ;-)

4.  For James Lane -- Lime stabilization (related to 1, above) might work,
but I'd prefer to keep it simple.

5.  For Bill Polhemus -- You recognized that I went "full circle", didn't
you.  "Blessed are they who run around in circles, for they shall be known
as 'wheels'."  If, however you want to use the three-sided wheel, you'll
have to get clearance from Harold Sprague 

Again, thanks to all,

Fountain