From: Randy<randy@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
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