Bill Polhemus' suggestion of drilled piers was my first thought, too, and is
a good one. But I would suggest a question you need to resolve first: what
is the magnitude of the expansive soil movement and does this really matter
with a horizontal bark blow line? If it is on the order of an inch or two,
it may not be too significant and your conventional footings may work just
If the vertical movement is critical, you may want to consider drilled pipe
stanchions without the pier cap. Since the piers will extend below the zone
of seasonal moisture change, vertical movements can be limited to a very
small amount. In pouring the piers, you would want to form the tops with
Sonotubes to prevent the mushroom top near the ground surface. This
eliminates the expansion force applied to the piers. In a previous life, I
did overland steam piping for the geothermal industry. We used what we
called support stanchions as the pipe supports (supporting steam piping
ranging from 8" through 48" diameter) for the above ground piping collecting
steam from the well sites and conveying it to the steam power plants. We
basically drilled piers and set a pipe stanchion in the pier. The top was
then cut off to the final pipe grade (ranging from 3 to 5 feet above the
ground typical to over 35 feet in some cases) and a bearing bar (typically
3" pipe) was welded to the top of the pipe stanchion. This bearing pipe
allowed thermal movements to occur with guides provided to resist seismic
and wind loads. The diameter of the stanchion can be increased for tall
supports (although there is economy in minimizing the number of different
pipe sizes used. We commonly had one size that would cover most of the
supports and a second size to accommodate the remainder) or connected to
make a frame (if two legs are used). At 20 feet tall, two legged bents may
make more sense. We found this type of construction quite economical. This
type of construction came out of the "oil patch" where they need to run pipe
overland to collect petroleum products and pipe them to a central facility.
There are some pretty ingenious folks in that industry.
WRT your item 2, if you use drilled piers and pipe stanchions, you have no
need for adjustment.
Bill Cain, S.E.
From: Bill Polhemus [mailto:bill(--nospam--at)polhemus.cc]
Sent: Wednesday, April 04, 2001 8:50 AM
Subject: RE: Foundation on Expansive Clay
I think you should give serious consideration to putting these things on
I'd make a pile cap, maybe 2 feet thick (which will negate any effect of
expansive uplift on the supported equipment) on several drilled piers (may
or may not be belled). I'd get the geotech to tell me the pullout resistance
along with the other data for drilled pier design, design the piers for
gravity and lateral loading, then check for pull-out and adjust as
You'll get a foundation that's a little more expensive, depending on where
you live (in some areas drilled pier installation is so common it's actually
cheaper to do than a bearing foundation), but you'll take care of potential
expansive uplift with no problem.
William L. Polhemus, Jr., P.E.
Polhemus Engineering Company
From: Fountain Conner [mailto:fconner(--nospam--at)pcola.gulf.net]
Sent: Wednesday, April 04, 2001 6:05 AM
Subject: Foundation on Expansive Clay
I need to put some lightly-loaded foundations on clay "with the potential
These foundations will carry a bark blowline in a paper mill.
If all were normal these foundations would be 7 feet long x 8 feet wide x 3
feet thick. They each would have a vertical pipe 20 feet tall, carrying a
horizontal blowline. Maximum soil pressure (max load and wind) would be
about 2000 psf. Static maximum load is about 650 psf. The geotechnical
report allows at least 2500 psf, plus 25 percent for short-term live load
Plain vanilla... Until I factor in the potentially expansive soil.
My thinking is to turn down the outside edge of the footing an additional 6
inches. If this clay is expansive, it shouldn't be given to capillary
action, and will provide a seal, preventing the soil beneath the foundation
from seeing the varying moisture content.
What do you think?
I had intended to place each "mast" in a sleeve, plumb it and grout it
into position -- cheap, but effective. I *can* put a mast on a large,
baseplate, and perch it on nuts below the base plate, with double nuts
above. If I do this, I can allow enough room below the lower nuts to
adjust each mast plumb if there is some movement. I'm hoping you guys can
convince me that this step is overkill.
Fountain E. Conner, P.E.
Gulf Breeze, Fl. 32561