Hi Gene & All,
After spending 4 hours trying different techniques this morning I have come
to the conclusion that Gene is right. I was planning to cut the undercut
for the pad with a trapezoidal top (slanted top) rather than a "pure"
rectangle even though the plans show a rectangle. The inspectors never
complain about too much concrete, only too little. I was doing that for
numerous reasons: it's easier to dig, it would be easier to tie the rebar
and it prevents the possibility of trapping air and not getting a
complete-coverage pour of the pad portion of the concrete.
First we tried digging the undercut with the backhoe but the operator could
not see what he was doing and hand signals were only partially successful.
Next we tried the breaker bar and shovel technique, using the backhoe to
clean out the dirt from the center of the hole. That was working, but
taking far too long. Next we tried a Hilti electric demolition driver (mini
jackhammer) with a spade bit. It weighed about 25 pounds and wasn't too
hard to handle. It looked like that was the answer. It really cut into the
heavy clay and the vibration caused the clay to fall off in big chunks... at
first. This would have worked very well for a 1 ft. undercut (7'x7' pad)
and with some difficulty for a 1.5 ft. undercut (8'x8' pad) but for the
A-Type foundation necessary for the HD-80 tower you need the full 9'x9' pad,
requiring a 2 ft. undercut and that last 6 inches proved to be nearly
impossible. Given enough time and a good chiropractor it could probably be
done, eventually, but we finally decided it wasn't practical. Our heavy
clay soil is so "sticky" that I wasn't worried about the overhang
collapsing, but in different soil that would be an issue as well.
We now have a 9'x9' (but not yet 4.5 ft. deep everyplace) hole in the yard.
We still have at least two trailer loads of dirt to go and should be able to
finish the digging tomorrow. That's a lot of wheelbarrow loads. We have to
remove the dirt from the back yard one wheelbarrow at a time. That's one
trip for every full scoop of the backhoe. Two of us can usually keep up
with the backhoe operator. At least we don't have to wheel it very far (~40
ft.).
We are going to form a 5'x5' pier. Not because of the cost of concrete but
to keep the "concrete footprint" in the back yard as small as possible. If
I understand what Bill (William Q Meeker, K0KT) was saying in another post,
you only form the pier and pour down the center and let it flow out into the
pad portion of the foundation and it doesn't keep running out and flow up
the outside of the pier forms. In fact, it seems that more concrete needs
to be added to the pad after the pier is full in order to completely cover
the rebar. Obviously, the concrete has to pass the slump test for this to
work.
I was planning judicious use of a concrete vibrator no matter how we did the
digging. I'm not sure how the vibrator could disturb the tower base (as was
also mentioned by Bill) any more than just pouring the concrete into the
hole. Any other thoughts on this?
That's the report for today. No new pictures. I was going to have my wife
take a picture of me in the hole running the demo hammer but I was having so
much fun I forgot.
73 - JC, K0HPS
-----Original Message-----
From: ersmar@comcast.net [mailto:ersmar@comcast.net]
Sent: Friday, June 30, 2006 6:56 AM
To: JC Smith; 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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