JC:
It's nearly impossible to undercut safely the full size of the pad for
this type of foundation. When I was working in the electric utility industry
as an engineer, we used these foundations all the time for our substation
structures. The crews dug out the full size hole for the pad (in your case,
that would be a hole 9 feet by 9 feet by five feet deep.) They set the rebars
for both the pad and pier, the poured the concrete for the pad. Next day they
set the forms for the pier on top of the pad and poured the pier. ANd I
believe there is a material that is applied to the concrete to allow this
second pour to adhere to the first. Anyone help here?
The next step is the trickiest: backfilling the excavation after removing
the pier's form material. In your case you ought to back fill a foot deep
around the pier, then spend a bit of time with a tamper banging the soil in
compaction. You can rent a motorized tamper for this. Then add another foot
and tamp it, etc until the entire excavation is filled and the earth put back
in near-undisturbed state.
My Trylon tower's foundation is similar in construction except that the
undercut is a slanted cut only a foot wider than the main hole. I was able to
use a spade, with proper shoring of course, to loosen the earth and haul it out
with a backhoe. But yours is a bit too wide to safely support the overlying
earth.
I know - a PITA, but it has to be done that way so you don't injure anyone
should the hole collapse.
Good luck, and remember - On towers as in driving a car, there are no such
things as accidents.
73 de
Gene Smar AD3F
-------------- Original message ----------------------
From: "JC Smith" <jc-smith@comcast.net>
> Hello All,
>
> As mentioned previously I am stick-building (like a giant erector set) a new
> AN-Wireless HD-80. I will be posting photos at:
> http://s14.photobucket.com/albums/a317/k0hps/Tower%20Project/
> if anyone cares to watch it go up and the antenna stack go on it. We just
> started digging the foundation yesterday. It's a pad and pier foundation
> (like an inverted tee) with a 9'x9'x1.5' pad and a 5'x5'x3.5' pier.
>
> Has anyone on here ever dug one of these by digging the 5'x5' hole first and
> then undercutting the bottom to create the pad? If so, I'd sure appreciate
> hearing how you did the digging for the undercut. We got the 5'x5' hole no
> problem but our experiments (so far) with the undercutting haven't been very
> successful. We are in clay soil, fortunately with no rocks. If things
> don't go better today we will probably dig out the entire 9'x9' hole, pour
> the pad, form the pier and backfill around it.
>
> Suggestions appreciated.
>
> 73 - JC, K0HPS
>
>
> _______________________________________________
>
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>
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