Mike, if you look in the phone book for a geotechnical engineer you
will probably find what you are looking for. A geotechnical engineer
will tell you how to prepare the soil. A structural engineer will do
the design of the concrete and steel in the foundation, but the tower
company has probably already done that and have included it with the
instructions. What you really need is a firm who does both, that is
what I do, both, but it's hard to find. Do a little research, if
you go with a huge company you will likely pay much more than you
would if you were to go with a mom and pop place. Location matters as
well. Way too much of my fee is based on liability insurance, and in
areas with higher insurance fees, the engineers will have to charge
higher fees. This is the reason it's hard to find someone who does
both structural and geotechnical engineering, the structural firms
don't want the liability of the geotechnical engineering, and the
geotechnical engineers don't want the extreme price competition of the
structural engineer. The problem is that it's the melding of these two
disciplines that really presents the best foundation solution for the
client. If only everyone understood that... Do look for a
geotechnical engineer, let them know what you are doing. If your
house is in a fairly new subdivision there is probably already a soils
report floating around that they may have access to that would give
them everything they need to know. In that case they wouldn't have to
tear up the back yard. They will probably want to see what the soil
is like for about 10' below the bottom of your foundation.
As for the auger size, you could ask around. Those can be rented from
an equipment rental place as well, so most fence guys can get whatever
size you need. Typically the sonotube is for the above-ground
portion, it's a round form. You auger the hole and put the cage in
then put the sonotube around the above ground portion of the cage.
It's basically how they put in the lights in parking lots. The nice
thing about a deeper narrower foundation with sufficient steel, it
doesn't need to be as big around. Depending on the loading scheme you
can get away with less concrete and steel in a drilled shaft
foundation than a spread footing, that is why many transmission poles,
and commercial poles, especially the cell tower mono-poles are founded
on drilled shaft foundations. The cost with those is in the
engineering and not the construction.
In reality a properly designed foundation can cost as much if not more
than the tower itself, especially when you get into the 100+ foot
range. With shorter towers the wind loads (moment loads at the base)
are much lower and thus a smaller foundation is required, and they are
significantly less cost. I wish the tower manufacturer's would spend
a bit more effort explaining that each location is different and local
recommendations should be made. Or retain a foundation engineer who
would be able to help the customers with a foundation design adequate
for their conditions.
Brian KF7OVD
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