The ability to dig an "undercut" hole no doubt depends on the soil. I
gave a local concrete contractor the AN Wireless foundation plan, and he
said there was no way to dig such a hole. He said the only way to do a
pad and pier foundation around here would be to dig the hole the size of
the pad, construct a form the size of the pier, then backfill.
(The photographs on the AN Wireless Web site show both undercut and
backfilled mathods, IIRC.)
He initially proposed a two-stage pour, but when I told him that my
understanding was that it was supposed to be a monolithic pour, he was
quite happy to do it that way.
It had occurred to me that the higher concrete cost of a solid-block
foundation might be more than offset by the saving of formwork and
backfilling, but I thought that the smaller exposed concrete area of the
pad and pier foundation would win on esthetic grounds.
73
Alan NV8A
On 07/05/05 01:09 pm ersmar@comcast.net tossed the following ingredients
into the ever-growing pot of cybersoup:
> Digging this kind of excavation is a royal PITA. First, you dig the
> hole the size of the bottom pad, e.g., 5X5 feet, to the final depth of the
> hole. You place rebar for the pad AND for the pier. Then you pour only the
> bottom pad concrete and let it set for a day. The next day you form up the
> pier and pour it.
>
> After this entire block cures for another day or two, you can remove the
> forms. Now comes the fun part.
>
> You'll have to back fill the hole around the pier. This means you must
> place some earth into the hole around the sides of the pier, covering the pad
> in the process. THEN you must ensure that the replaced earth is as compacted
> as undisturbed soil by pounding on it with, for example, a 4X4 or 2X4 piece
> of wood. Once that layer of soil (abt 6 inches or so) has been compacted,
> then you add another layer of earth and repeat the pounding (compaction)
> process until you reach grade. One might have a tendency to slack off a bit
> on compacting the upper levels of earth due to fatigue. This won't give you
> the mechanical performance from the earth that your tower will need.
>
> I used the Trylon foundation design, that is, a monolithic concrete
> pour, but with an undercut at the bottom of the hole. I did this by having
> the backhoe dig the basic hole (5.5 X 5.5 feet six feet deep) and then
> shoring up the sides. I then went INTO the hole (ever look up from the
> bottom of a narrow hole in the earth?) and hand-dug the undercut. The
> backhoe bucket was small enough that it was in the hole with me, but I had
> very little room to maneuver.
>
> Thinking a bit about this compacting, if you decide to go the way of the
> two-pour pad and pier design, you might want to really overdig the hole and
> use a gasoline powered rented compactor to do the job with less human effort
> (and fatigue.) It'll sure beat using a two-b'-four!
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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