From:
Fred Hopengarten K1VR 781/259-0088
Six Willarch Road
Lincoln, MA 01773-5105
permanent e-mail address: fhopengarten@mba1972.hbs.edu
if sending attachments: k1vr@gis.net
On Thu, 28 Oct 1999 11:47:32 -0600 Joe Carvalho <joe.carvalho@wcom.com>
writes:
>I've moved to a very beautiful place. Clean air, cool comfortable
>temperatures, a little snow on occasion, deer and antelope playing in
>the
>yard, elevation 8600' and solid granite 2'-3' below the surface of
>the dirt.
>
>HELP!!! Do I need to blast and pour cement/concrete or can I get away
>with drilling/epoxy/bolting?
>
>Any hardrock miners in the group? --joe KR6NA
Granite Guy Anchors
Bill Myers, K1GQ
[Copyright 1983, The YCCC Scuttlebutt]
Many of us in YCCC territory live in areas dominated by rocks and ledge
outcrops. I've yet to dig a hole without hitting an object as large as my
dog house, and more often as large as my car. Since I couldn't get them
out of the way to build a conventional concrete guy anchor. I decided to
try anchoring directly to granite.
The method I used evolved from one I first saw in a commercial
installation on a bare hilltop not too far from here. Simply put, I
drill a hole in the rock and cement in an eyebolt. The jaw-end of an
eye-and-jaw turnbuckle attaches directly to the eyebolt, with the other
end connected to the guy wire in the usual manner.
Of course, it isn't quite as easily done as described. For example, you
don't use ordinary tools to make holes in granite. I learned the hard
way, by melting my father's half-inch hand drill! When I explained my
problem to the people at Taylor Rental, they taught me how to use a
rotary hammer, and gave me a sharp 3/4-inch carbide-tipped bit. The
rotary hammer is an oversized slow-speed electric drill, which
automatically pounds on the end of the drill bit as it rotates. The
secret is NOT to bear down on the drill, but to let the hammering action
do the work. All you need to do is hold the drill in position so the bit
doesn't bind in the hole. Trying to hurry is what makes the dull bits,
which the rental place hands out to untutored customers (who make them
duller).
I drill a separate anchor hole for each guy wire in different large
boulders or ledge outcroppings whenever possible. For three guy levels,
this adds up to nine holes, which takes me about two hours. Since I
normally rent the tool late Saturday for Monday return (about $25), I've
got all day Sunday to do the job, leaving plenty of time for
unanticipated problems.
The holes are drilled at an angle, not straight up-and-down. The angle
is set so the drill bit shaft is approximately perpendicular to the guy
wire. With this tilt, all of the force in the guy is converted to a
shear force on the eyebolt; there is no component along the axis of the
eyebolt shaft, which would tend to pull the eyebolt out of the hole.
Thus, I DO need very strong eyebolts, and I DON'T need a secure means for
cementing the bolt into the hole. In fact, once the guy is attached I
can't pull the eyebolt out, even with a slack guy and no cement (I
tried).
The eyebolts have 2-inch long shafts with an outside diameter of
3/4-inch, which is why the hole is 3/4 inches. This is the largest size
that will accept the jaw of the Rohn 1/2-inch eye-and-jaw turnbuckle,
which attaches to the eye. The eyebolts of are drop forged, not the
formed type found at the corner hardware. They are typically used as
attachment points for lifting heavy machinery and so forth. I get mine
(which say "Vulcan EB-28") at an industrial supply outlet (Hammer, in
Nashua) for about $6 each. Unless you are lucky, they won't be
galvanized. I let mine rust for a year or so then coat them with a
chemical protective layer. Spray-on galvanizing works too, if you clean
the eyebolt thoroughly first.
I don't believe that using a shaft longer than 2 inches is necessary;
however, I've not checked my intuition mathematically. If this point
makes you nervous, you may want to consult a qualified mechanical
engineer.
There are three things to be careful about. Beware of rocks which aren't
as large as they look. Rather than digging up a partially buried hunk of
granite to see how big it is, try standing on it and clobbering it with a
heavy sledge hammer. If you feel some vibration, or you can't get both
feet on it, look for another anchor. Second, look for fresh granite.
Exposure to the elements for several hundred years breaks down the
surface of granite to the point where it falls apart when you beat on it
or drill through it. Also, be sure you are dealing with granite and not
some sort of soft rock -- if you can't tell the difference, find a Boy
Scout who can.
Finally, the eyebolt should be cemented into the hole. This is necessary
to keep water out, not to keep the eyebolt in. If water gets into the
hole and freezes, the granite may break apart right at your anchor point!
Use hydraulic cement, which expands slightly when it sets, rather then
contracting like ordinary cements. Hydraulic cement is the kind used to
patch leaky basement walls and is sold in hardware stores in powder form.
Mix it with water, fill about 1/4 of the hole (after removing all
debris) with the mixture, and screw in the eyebolt. Don't dally -- the
cement will set in one to three minutes and if you don't get the eyebolt
all the way in you'll have a hell of a time trying to remove it to try
again!
I've now got over 20 anchors set this way, and I'll do my next tower the
same way. Using separate anchors for each guy isn't really necessary,
but it only costs me a little extra time to drill a few more holes, plus
$6 for each eyebolt. I like knowing that even if an anchor fails, the
tower will probably stay up, since only one guy is loose instead of all
of them.
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