In a message dated 1/15/2006 4:35:22 P.M. Pacific Standard Time,
> My tower project is starting to come together, but I have a concern that
came about while installing the screw anchors today. Two of the anchors are
in undisturbed soil, while the third is in soil that was disturbed when the
house was built. The soil was used to build up the backyard of the house
about 8 months ago. What prompted the following question was that one
anchor went in CONSIDERABLY easier than the other two. The tower is a
planned 70 foot Rohn 25 installation with two sets of guys with a tribander
and 2 element 40 on top. The anchor is on the downwind side of the
predominate wind here.
> How long before disturbed soil is considered undisturbed? I am concerned
that the anchor in question may pull out due to the soil it was installed
in. Are there any recommendations as to what I can do to improve this
anchor and in turn make me sleep better at night after the tower is up?
Lots of folks have weighed in on this question so I thought I'd throw in
my 2 cents worth.
I'm not a big fan of screw-in anchors, particularly the ones that Rohn
sells. They're pretty small and Rohn says they are rated at "2500 pounds
holding power" plus they're pretty small - 5/8" rod with a 4" auger. With
sets of 3/16" EHS guys on 100' of 25G for instance, the proper tension of 400#
times 3 sets of guys is 1200# with NO WIND - cutting the capacity of the
anchor by almost half. And the other half is going to be used up pretty quick
when a big wind comes up. Obviously you can have less guys per screw-in and I
strongly recommend it.
And of course nothing is said about soil holding capacity which the only way
you're going to know for sure if it's going to work in the first place. And
you have to have a geotechnical survey (not cheap) in order to know more
specifically about your soil conditions.
Concrete anchors OTOH will go just about anywhere - even sand if they're
big enough. If you're going to put in industrial-type screw-ins like AB
Chance, etc., you will increase your reliability over the funky Rohn versions.
And utility pole guying is WAY different than tower guying - different
vectors, forces, heights, etc.
Back to your original question - it takes YEARS for soil to compact
adequately back to where it was before it was excavated and fluffed up so it's
mystery why that screw-in went in so easily and it's pretty obvious that
it'll be a tower failure point waiting to happen. I'd recommend a concrete
anchor for that anchor at least.
Cheers & GL with your installation,
TOWER TECH -
Professional tower services for commercial and amateur
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