[TowerTalk] Forms for tower base

[N8GS]

K7LXC@aol.com wrote:
> 
> In a message dated 98-07-14 16:02:12 EDT, brian_smithson@vds.com writes:
> 
> << It appears from the pictures I've seen that a big hole is dug, a form is
>  built and suspended in the hole. The concrete is poured and the form
>  is left in the hole and it is backfilled around the form. I've always heard
>  that undesturbed earth is the best thing to support something like this,
>  not backfilled.
> 
>       Yes, undisturbed earth is the preferred situation. The only reason why
> you would need forms in the hole that I'm aware of would be if the walls were
> sloughing into the hole. For sandy or unconsolidated soil, this would be a
> problem but usually isn't for 'normal' soil.
> 
>  >>What if the form were 'short' - just a couple of feet to square off the top
>  (visible) part and the rest of the concrete was poured against the bare
>  undesturbed earth below? Then I'd only be backfilling the top couple of
>  feet to neaten it up. Is the chunk so big that it doesn't really
>  matter? Seems like a way to make the form smaller and easier to
>  handle. >>
> 
>       Sure, but I would just have 2x4's or similar right at the grade level.
> Then there's no need to backfill the hole at all.
> 
>       Soil engineers? Geotechnical folks?
> 
> Cheers,  Steve  K7LXC

The use of forms saves a lot of concrete.  the support provided by
undisturbed walls is compensated for by proper soil compaction after the
forms have been removed.  If the forms are left in place they will rot. 
When they rot away we now have a chunk of concrete that is standing in a
hole with little side wall contact with the soil except on the side that
the tower tipped toward because the wood rotted away. 

Soil compaction is usuclly done with a mechanical engine driven
compactor.  These can be rented a most rental shops.  The soil should be
moistened somewhat and compcted with the machine in 6" to 9" "lifts". 
There is a standard for determining just how good the compaction is but
a discussion of the "Modified Proctor" test is uncalled for here.  

I can go into more detail if you want, or contact your friendly local
soil testing company for more info.

Gale

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