Kelly et al:
I used a concrete vibrator like this one:
http://www.amazon.com/gp/product/B0000224W5/002-7979284-5720834?v=glance&n=228013
to ensure proper concrete flow and filling of the undercut in my foundation.
JC should still use this device, even if he overcuts the hole for a monolithic
pour, to ensure that the concrete spreads quickly and evenly across the entire
area of the excavation. The hose on the vibrator should be long enough that
you don't HAVE to get into the hole while the concrete is falling into it.
The trick is not to overvibrate the mix. In that case you would cause the
aggregate (stones) to fall through to the bottom of the pour, resulting in a
weak overall mixture once it sets up.
73 de
Gene Smar AD3F
-------------- Original message ----------------------
From: <ve4xt@mts.net>
> The other consideration about undercutting for the pad is whether the
> concrete will fully form to the shape of the pad. I certainly wouldn't want
> to
> be in the hole forcing concrete into the undercut areas as it's being
> poured...
>
> Even if you could do that, you still wouldn't be sure it filled out the top
> corners.
>
> The principle, I believe, is that once the dirt has been backfilled and
> properly compacted as Gene describes, there is as close to undisturbed
> earth sitting right on top of the pad..
>
> 73, kelly
> ve4xt
>
>
> > From: ersmar@comcast.net
> > Date: 2006/06/30 Fri AM 08:55:47 CDT
> > To: "JC Smith" <jc-smith@comcast.net>, <towertalk@contesting.com>
> > Subject: Re: [TowerTalk] new AN-Wireless tower, floundaion question
> >
> > 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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> > > TowerTalk@contesting.com
> > > http://lists.contesting.com/mailman/listinfo/towertalk
> >
> >
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> >
> >
> >
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> >
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