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

Subject: Re: [TowerTalk] (Repost) Engineering advice on above ground foundation
From: David Gilbert <>
Date: Fri, 05 Jun 2009 15:31:27 -0700
List-post: <">>
I'm sorry if it looks like I'm picking a scab, but ...

If there were concerns about the stability of the soil underneath (sand 
and water), how does putting a 2 foot layer of soft material (gravel) on 
top of it make it any more stable?  Gravel is usually used to provide 
drainage and to equalize forces between the bottom of the foundation and 
the underlying soil ... it doesn't act as any kind of stiffener.  And 
even if it did, it isn't stiffer than the concrete would have been.  You 
just made a foundation with a mushy base.  Gravel doesn't provide any 
resistance unless it doesn't have any place to go.

Also, by putting the foundation above ground you've taken away the 
normal tipping resistance of the side wall dirt and put even more burden 
on the viscosity of the soil underneath to hold the tower up in the 
wind.  Making it 11x11 increases the tipping resistance compared with 
9x9, but I would sincerely doubt that it makes up the difference unless 
I saw an engineering analysis that said differently.

Personally, I'm not sure where your confidence that you can "build a 
solid structure before the tower goes up" comes from.  It sounds like 
mere wishful thinking, because you don't really know what you have and 
there isn't much you can do about it anyway if the soil is a problem 
like you originally suspected.  Nothing you did has moderated that 
issue, and everything you're focusing on now is simply to deal with 
frost issues, not the fundamental stability of water and sand below the 

Just curious ... if you recognize now that you should have hired an 
engineer to deal with the dicey soil conditions, why wouldn't you hire 
one now to find out if you're in trouble?  I'd sure want to know if I 
was putting 90 feet of tower on a concrete boat or not.

Dave   AB7E

Rudy Bakalov wrote:
> I really appreciate all the feedback my post has generated.  I think it will 
> be helpful to add some further background.
> Under normal circumstances I would not trust a builder, but I have known this 
>  guy for a while and, more importantly, he builds foundations for Rogers 
> Wireless in Ontario.  I felt that his experience with what he does for 
> commercial installations is somewhat relevant.
> His comment was that although we can pour concrete into water, we don't 
> really know too much about the soil below as the water is there year around 
> and the soil is very sandy. Thus, the decision to have 2 feet of compacted 
> gravel below the base.  We also chose gravel because of its heaving 
> resistance.
> The original foundation specs call for a base of 9 x 9 x 5. Instead, we built 
> the foundation 11 x 11 x 5, which is close to the specs for the 120' tower, 
> and using rebar specified for the 120' tower.
> Right now there is no tower on the foundation and won't be one until I am 
> confident we have addressed major concerns.  Per my original post, we have 
> identified the combination of water + freezing as #1 concern, which we 
> contemplate addressing through drainage, waterproofing, lots of insulation, 
> and the mound 15' around the base.
> Looking back, I realize I should have hired an engineer.  But I also believe 
> that we can build a solid structure now before the tower goes up.
> Again, thank you all for the tough but helpful feedback.
> Rudy N2WQ

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