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I have installed
many towers and
masts over the
years, and I’m proud
to say I have never
had a tower fail. I
have seen dozens of
towers installed by
others fail,
however. Most
failures are due to
guy line failures
including guy anchor
failures.
Installers,
sometimes those who
profess to be
professionals,
occasionally do
stupid things.
One of the most
common failures is
caused by using the
wrong clamps or
installing clamps
backwards. The
photos below are
from guylines
removed from a
Rohn 45G tower: This
isn’t the only
terrible
installation I’ve
see.
I always wonder
how this stuff stays
up. Imagine $10,000
worth of tower and
antennas guyed this
way, and it really
is very common. In
the 80’s my business
did a lot of tower
work. After storms
we would go out to
remove fallen
towers, and every
single tower I
recall failing
failed because of
improper guying. It
was either too few
saddle clamps
(especially over
over a coated wire),
clamps on backwards,
wrong size clamps,
loose clamps, or
turnbuckles or eye
bolts without forged
closed eyes.
One antenna tower
I removed in
Mississippi (Rohn
45G) had the anchors
in the wrong
direction! They were
not even pointed
towards the tower.
Guy Anchors
Guy anchors
mostly get their
pull-out resistance from
dirt they have to
displace rather than
anchor weight. When
anchor holes are
drilled or bored all
the old well-settled
or compacted dirt is
removed in a line
towards the guy line
pull direction.
Backfilled dirt is
never as resistant
to pulling something
up through it as the
original dirt.
Remember the load on
the soil has to
spread out to
include undisturbed
dirt. The last thing
you want is a solid
walled uniform
diameter plug like
concrete filling the
hole in the
direction of pull.
Filling the anchor
hole with rock or
concrete is a major
problem. If you bore a
hole (such as for a
bust-open anchor),
don’t backfill the
hole with rock or
concrete. This
actually makes it
easy to pull the
plug out of the
earth. Backfill with
dirt and pack the
dirt tight every few
inches of fill.
If you must use
bored holes, use a
“bust open anchor.”
This type of anchor
opens by driving a
pipe down over the
rod. The anchor end
then expands out
into the soil. It
expands and locks in
to the fresh
undisturbed soil.
Remember it is
all about the soil
the anchor is
displacing, not the
weight. My larger
tower uses
commercial anchors
pulling a 6×4 foot
(or larger) rebar
re-enforced wall
through the ground.
We bored the anchor
hole down into the
wall so the wall
pulls against
undisturbed soil. To
the right (your
view) of the
100-pound dog is the
trench for the wall.
It was dug with a
back-hoe at right
angles to the anchor
direction. You can
imagine trying to
pull that wall
through undisturbed
earth!
The anchor is
hot-dipped
galvanized channels
5/16 inch thick and
4 inches wide. TWO
pieces are
back-to-back. The
other end (eight
feet down) has three
rebars through
holes. The rebar
runs the length of
the wall.
On lighter towers
or smaller lines
(under 12,000 pounds
breaking) I use
screw in anchors. I
use an attachment
with my tractors
that will screw the
anchor in. A
good-biting anchor
will almost twist
the shaft while
screwing in. If it
turns in easy
that is a sign
holding power might
not be optimum. I
use a minimum of a
single six inch
blade 66 inches
deep. I have a
jacking fixture we
test the load with,
and can pull at full
pressure (ten tons)
against a screw
anchor without
having it creep
upwards. There is
generally a small
upward shift of a
half inch or so that
stops as the soil
packs, but it won’t
creep out past that
point.
Here is how we
solve creep in wet
holes:
When we hit a
weak hole we back
the anchor out a few
feet.
We bore a 12 inch
hole at right angles
to the anchor rod.
We screw the
anchor back just
past the hole into
the wall edge.
We load the hole
with a few sticks of
heavy rebar making
sure the rebar is
against or near the
anchor shaft.
We backfill with
properly mixed
bagged concrete.
Now the anchor
blade has to drag a
12″ diameter plug of
concrete up through
the undisturbed
earth. I have a Rohn
55G loading two
anchors that are in
a very wet area with
a few thousand
pounds, and those
anchors have not
crept since being
installed. They also
did not jack out
with the 10 ton test
fixture. Prior
to the concrete I
could pull the
anchors up with 1000
pounds of guy
tension!
I don’t hesitate
to use screw anchors
except:
1.) I am leery of
anything that screws
in easily. It has
been my experience
what threads in easy
pulls back up easy.
2.) I am mindful
of saturated soils.
Even sand has not
been a problem, but
wet clay seems to
slide around the
anchor.
3.) I over-pull
to test anchors and
look for creep. I
even built a jacking
fixture.
4.) I mark
anchors with paint
at ground level so I
can monitor for any
movement, and I
watch them the first
few months.
Various Anchor
Types
Above, a pile of
assorted anchors.
Above right, an
expandable
(bust-open) anchor. This type
requires a deep
drilled hole. It is
“busted open” or
expanded with a
pipe slid down over over the rod.
This type of anchor
is well-suited to
problematic soils. A
screw anchor like
the one on the left
works great in my
dense clay soil, not
budging when jacked
with a 10-ton jack.
The Rohn anchor on
the right requires a
backhoe for
installing the
concrete and rebar.
They also require an
equalizer plate.
Screw-in anchor.
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