Ivan Shapiro (KB1STH) wrote:
> My QTH/property is on rock. In fact in many places the rock protrudes above
> the grass.
> We put up a 120 foot free-standing tower for a Jacobs wind turbine 11 years
> ago.
> After getting down as deep as we could with a big backhoe I had a company
> come in with their rock-boring machine.
> We had three cylindrical holes drilled. Each one is 18 inches in diameter
> by eight feet deep.
> Filled this with rebar first.
> Tied the rebar to the rebar cage inside the forms. Forms were four feet
> high, with about 2 feet protruding below grade.
> Forms were nineteen feet on a side. We poured around 27 or 28 cubic yards
> of concrete. Of course we insured that the anchors and first 20 feet of the
> tower were plumb".
>
> I never wanted to have to do any repairs to the base, and I think we have
> achieved that goal.
> Just wondering, however, whether you think this was way too much overkill.
> Thanks
> Ivan
> KB1STH
I'd think that a wind turbine is a fairly hefty load.. and on a 120ft
pole, too!.. just a bit more load than a 50 foot tower in a 50 mi/hr
wind<grin>. That's similar to what they do around here in Southern
California (rock is close to surface, but is often "loosely consolidated
quaternary sediment"... basically 20,000 year old dry mud and gravel),
particularly in non-flat areas.. they drill a 18-48" diameter hole, drop
in a rebar cage, fill with concrete, bolt to top. Designwise, it's the
same sort of idea as driving piles in areas with more soil, except you
drill the hole and then build the column in place, rather than hammering
it into the ground like a giant tent stake.
Here's the question: How did you decide on what to do?
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