I based it upon my original decision to locate the tower where it is.
Once we found rock and did the initial hole (such as it is), I decided
it was not practical to continue to chip away rock to replace it with
concrete. The factory dimensions are obviously engineered, and we all
know they are likely slightly over-engineered from a litigious society
legal aspect. They are also engineered for "average soil" - whatever
that is supposed to be - so the design is for sand to rock in it's
spectrum of qualification as a sufficient base for the tower and it's
design windload maximum.
It is impractical to continue as is, and the data relational to towers
and installations considerably outside of what I have here, while
interesting, was not really correlational to what I am doing. I have
solid rock, a tower designed for a relatively light load, and a quad
planned which is less than the max design load for the tower.
At this point - weighing cost, convenience, practicality, etc. - using
it "as is," while likely completely fine under my consideration
criteria, I feel to be undersizing. Drilling 6 foot holes and using 25
yards of concrete at $100 a yard is at the other end of the spectrum.
I will drill side holes into the same solid rock to better tie the
tower base into it (attaching side rebar to the slightly cut-down
factory rebar base and plan on three down holes slightly larger than
the 1" diameter of the factory-designed and built rebar base assembly.
Once completed, the tower base will be put into the hole leveled in a
repeatable manner, and then the legs will be epoxied into the rock.
Sufficient tied-in rebar will be added and then concrete of the right
compressive strength will be poured, jigged, and smoothed.
This will exceed many ham installations and while not in exact
accordance to the manufacturer's specs, it is an installation that has
many of the aspects of similar professional ones on rock, with a
concrete base securely tied to the solid rock shelf vertically and
horizontally.
It should hold a quad and any antenna I would put on it (always
keeping in mind the manufacturer's max load rating).
Again, thanks to all for the help. It is much appreciated.
Michael Goins, k5wmg
Pipe Creek, Texas
former editor, qrp quarterly
Fast cars, slow boats, big dogs, and summers off to write
Michael Goins, Editor/Publisher
Natural Awakenings-San Antonio/Hill Country magazine. Our printed
edition is currently distributed throughout Bexar, Kendall, Comal,
Bandera, Kerr and Gillespie counties - with 20,000 magazines in 450+
locations - and our eco-friendly online edition gets emailed across
the globe every month!
----------------------------------------------------------------------
No trees were destroyed to send this e-mail, but a good many electrons
were seriously agitated.
----------------------------------------------------------------------
On Sun, Feb 14, 2010 at 12:09 PM, jimlux <jimlux@earthlink.net> wrote:
> 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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