[TowerTalk] Foundations for Self-Supporting Towers

[Howard Hoyt]

> From: Randy<randy at verizon.net>
>
>
> To the best of my understanding, for freestanding towers, you HAVE TO
> have "undisturbed soil" for the
> pour-- you can not dig a hole and form up for the base and backfill.

Hi Randy, all,

What technique is used depends on the type of foundation used. Referring 
specifically to the documents: "Rohn SSV Foundation Specs.pdf" and 
"Foundation Notes B841300.pdf" there are four basic foundation 
configurations:

Pier & (buried)Pad (Type 1), Drill & Bell (Type 2), Drilled Pier (Type 
3) and Mat (Type 4).

Two of these (Type 1 & 4) rely on mass-loading to achieve OTM 
(over-turning moment) ratings, and also require excavation and by nature 
soil removal and replacement.  The other two drilled foundations (Types 
2 & 3) rely on soil-to-foundation friction as in addition to mass, 
therefore the undisturbed soil requirement.

The amount of concrete needed to give the required load rating for pure 
mass loaded Type 4 Mat foundation vs. Type 3 Drilled Pier is more than 
3:1.  Much of the difference is attributable to soil friction, but the 
equation is more complex than just the one factor.  It is also partly 
due to the additional leverage around the center of OTM due to the deep 
piers.  That certainly helps reduce the mass further for a pier 
foundation, relative to a mat.  Maybe someone with actual foundation 
engineering experience can chime in here and elaborate.

I am in a situation here where I need to erect a self-supporting tower 
as well, and the foundation costs are very high.  In my case I am 
utilizing an SSV N10 bottom section which, were I to use a Mat 
foundation, would require 50yds of concrete, so at $150/yd it would cost 
$7500 for just the 'crete!  Utilizing a drilled pier (3 piers, each 2' 
6" in dia) requires "just" 9yds, costing $1350.  I have yet to compare 
the cost between excavation for a mat and a drilling rig for the piers, 
but I am sure the drilling labor is higher.  I'd opt for a belled pier 
(only 7.6yds) but the only firm I've found with the rig charges $400/hr 
portal-to-portal and is 3 hrs away, so that option could cost around 
$3600 for the labor alone.

Needles to say I am saving my pennies for the project.

Good luck!

Howie - WA4PSC

Howard Hoyt
CE - WXYC-FM 89.3
UNC Chapel Hill, NC
www.wxyc.org