>You'd be surprised at what 1.25 Cu.Yd. of concrete will do in a guy anchor
I know you are in favor of hiring PEs for everything, and that's a
good course, but not necessarily the only valid one. Sometimes overkill
is cheaper than a PE, and just as comforting.
I've got lot's of elevated anchors in my station and have had a chance
to observe the long term performance of them. They are all around 4' deep
and vary from 3/4 yard (approx 2x2x4) to 3.5 yards (5x5x4), and are used on
towers ranging from 100' to 150'. I never worried that the smaller ones
were going to pull out, but the guy wires required occasional re-tensioning
as the anchors took a slight lean. I didn't like that so replaced them
with larger ones. I simply noted by observation that for 4' deep anchors
and square holes, 2 yards or more is needed to maintain perfect rigidity
in the soil here, which is wet gravel/clay mix.
Granted, a 70' tower might not need as much in a particular location, but
concrete is cheap.