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Re: [TowerTalk] (Repost) Engineering advice on above ground foundation

To: Kevin Normoyle <>
Subject: Re: [TowerTalk] (Repost) Engineering advice on above ground foundation
From: "Roger (K8RI)" <>
Date: Sat, 06 Jun 2009 13:15:02 -0400
List-post: <">>

Kevin Normoyle wrote:
> I think David's lack of additional detail, plus focus on little stuff 
> that doesn't matter so much, makes it sound dicier
> than it probably is. For instance, he says only 1.5' of the foundation 
> is below ground, but what was done with the gravel and hole is a key 
> part of the foundation. He doesn't specify that part though.
I agree with the need for more details and at this point an engineer, or 
at least someone from the local zoning board although the second might 
create more problems at this point.

Some where around here I have a writeup on pouring foundations in, for 
lack of a better description, (wet, loose, soil). Although they all say 
"undisturbed soil", this one says to set up the forms with internal 
bracing. This is necessary as the hole just naturally wants to become 
wide and shallow. (Been there and done that.  found one guy anchor hole 
to be about 10'-12' wide and about 8" deep the day after we dug them)  
As the concrete is poured you pull the internal braces, staying just 
above the concrete.  They do not address the issue of braces mixing with 
the rebar cage.  At any rate, once finished and the concrete has reached 
a "preliminary cure" (set up), they pull the forms as well. In this kind 
of soil it naturally fills in.  Even up here in the frozen North this 
does work if you make sure the bottom of the concrete is will below the 
frost line. In most places code will require it.

The water will not hurt the concrete and in many cases will result in a 
better, long term cure.

Up here (Central Michigan) upheaval from frost can be a big problem. For 
example, the apron for my garage is concrete while the driveway is 
asphalt.  In the spring the concrete will rise to the point where the 
underside of the concrete is even or above (some years) the asphalt. 
That's a 4" (or higher) rise in about 12'. Imagine one side of the tower 
base raising 4". Now that would certainly be noticeable.

But which ever the case, one should check with the local zoning  board 
and/or  and engineer _before_ starting. (It would be more than a bit 
inconvenient to finish up and find a tear down order the next day)  In 
my case the board told me what was expected and that the  engineering 
shown in the ROHN catalog was sufficient for our soil.  BTW  ours is a 
mix of alternating sand and clay layers about 4 to 6" thick.  For most 
of the year the water table is within 16 to 18" of the surface and for 
two months is within about 6". Of course by August, you can did a 6' 
hole and find only a little water at the bottom.  OTOH dig much deeper 
and break through the "hard pan" in some areas such as my place at 
Breckenridge about 30 miles SW of here and water would come gushing up 
through the hole.  A 6' hole might be dry and a 7' hole might have 5' of 
water in it. This can be a real worry when installing sumps for sump pumps.

If you have water, you don't just pour the concrete into it. Like the 
bridge builders you use forms (I forget the term bridge builders use) to 
keep the water away from the concrete until the proper cure, then pull 
the forms.  Again, this is where an engineer is important as these are 
non standard, or non typical soil conditions.

Most _small_towers_ can be set up in a wide variety of ways including a 
dirt base if the soil is solid enough to support the weight, but when 
you get into self supporting towers, or towers of any kind much above 
40' then a whole new set of rules apply and the forces involved multiply 


Roger (K8RI)


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