[TowerTalk] Towers on Rock Ledge

[Robert Shohet]

Steve presented one option.  Let me present a different one - one that I
have
used successfully at my qth.

I am at the top of a semi-flat hill that consists of ledge out-croppings at
the highest points,
with granite below.

My high tower is 130' and sits on ledge.  I made one edge of the base about
2' away from one of the
outcroppings where there was about 6" of soil and dug a wide square about 4'
x 4'.
The dirt was anywhere from 6" to about 15" deep.  I built a solid wood frame
out of 2x4's and 1x10's.  The top of the frame was about 6" over grade.

I used lots of steel rebar and wired a grid with
rebar in a tic-tac-toe pattern and put that about 3" from the top.
Underneath I used more rebar and wired a smaller grid for additional
support in the deeper parts of the hole.

We then trucked in concrete and  poured to the top, smoothed it level with a
trowel
stuck in a pier pin, measured everything again and then covered it up.

After some consultation, I chose not to use the
special compound that can be employed to better bond cement directly with
rock.
The feeling was that this would not be necessary due to the weight
of the concrete in the large base plus the large downward force of the
tower plus antennas and rotators.

That tower base has been in for almost 5 years through many storms with
wind gusts up the tower of 100+ MPH.  It has also endured more frost
heaves than I can imagine.  The base has seen no cracking or movement of any
kind even
though it supports 130' of Rohn 45' with 3  20-4cd's and 2  40-2cd's, with
three rotators,
two of them on sidemounts.

I think that the use of the pier-pin removed a lot of the twisting stress
that towers and
their bases experience.  I still like this idea better than drilling into
rock and using bolts.

Let us know what you decide and Good luck!

73
Bob KQ2M


> > Is there any general information available on siting towers on rock?  I
> have
> >  granite ledge just a foot or less down, so digging the big hole in the
> > ground won't do the trick here.
>
>     It's pretty simple. What you can do is to use the rock as the base.
> Excavate down to the granite and then drill anchor bolt holes in the rock.
You can
> either use expandable rock bolts or epoxy the anchor bolts right into the
> rock. Pour some concrete on top so it comes to several inches above grade
and
> you'll be done.
>
> Cheers,
> Steve     K7LXC