Fw: [TowerTalk] Re: Tower base hole?

[Jim Lux]

Indeed, for hand excavation, some of the hole designs seem a bit weird.
However, deep but small diameter holes are a very common form for concrete
piers, where the expectation is that someone erecting a multi-hundred
thousand dollar structure supported by multiple piers isn't going to kick at
spending a few thousand bucks to have the company come out with the "big
drill" and sonotube.

For example, I've been watching them install some traffic signals at a local
intersection over the past few weeks.  Even though the poles aren't all that
tall, they do have a huge canteliever load from supporting the horizontal
member over the roadway with the signals mounted on it. And, inasmuch as
they are in Southern California, they probably need to take a 0.6 g
horizontal earthquake load They are bolted to a pier about 3-4 feet in
diameter and 15 or so feet deep, with a very sturdy rebar cage in it.

It takes the crew about 30 minutes to setup and about 15 minutes to drill
the hole.  Most of the time seems to be occupied in blocking off the traffic
with cones, cleanup, etc.  I don't know the cost, but I suspect it's in the
<$500-1000/hole range (Figure a crew of 3 for  2 hours@  $30/hr, that's only
$180 in labor... I'd find it hard to believe that the effective rental cost
of the drill rig is more than $1000/day, or $250/2 hours)

About 10 years ago, I was involved in design and construction of a fairly
large (about 50 feet across and 30 feet high) structure supported on top of
3 steel pipe columns 20 feet tall.  We supported this on drilled concrete
piers as well. I don't recall the dimensions, but I seem to remember that
they were something like 25-30 feet deep and less than 3 feet in diameter
(we had to trade more depth for less diameter, because there were other
structural footings and grade beams close by).   The challenges were getting
sonotube in the right diameters and getting the holes drilled and concrete
poured in amongst the rain which always seemed to come at the wrong days.
Again, out of this million dollar scale project, the hole drilling cost
wasn't a big factor.

I think Steve's advice to hire a P.E. to look at your specific situation and
design an appropriate footing for your construction technique (be it hand
digging, backhole, or earth drill) and soil conditions is probably a good
idea.  This isn't a huge design challenge for the engineer, who will be
familiar with local conditions and such, and he can basically cookbook it
from experience, revising the calcs to fit your situation.  A couple hundred
bucks to the engineer might save you a bunch o' bucks in yards of concrete,
rebar, and digging costs.

----- Original Message -----
From: <K7LXC at aol.com>
To: <ve4xt at mts.net>; <towertalk at contesting.com>
Sent: Sunday, September 07, 2003 6:17 AM
Subject: Re: Fw: [TowerTalk] Re: Tower base hole?


> In a message dated 9/6/03 9:12:07 PM Pacific Daylight Time, ve4xt at mts.net
> writes:
>
> > My bigger concern with an 8-foot deep hole isn't with how to dig it,
but,
> >  depending on the soil, how to keep it from collapsing in on me as I do.
> >
> >  For that reason, anything that allows you to dig the hole without being
in
> >  the hole is a good thing.
>
>     Good point. US OSHA rules say that any hole deeper than 4-feet needs
to
> be shored up to prevent cave-in injury. Any time you're in over your head,
it's
> dangerous.
>
>     I have a big problem with difficult and/or dangerous hole designs. US
> Tower and Trylon have some stupid designs. Trylon wants you to bell out
the
> bottom of their bases which means you have to be in a 5-1/2 to 6-foot-plus
hole and
> dig the bell portion out by hand. In some soil conditions this is almost
> impossible. In all cases it's potentially dangerous. UST wants you to dig
a
> 4x4x7-feet deep (or even 3x3x-6-feet deep) hole. These bases were designed
by
> engineers sitting in air-conditioned offices who never have tried to build
one of
> these things. I much prefer having a shallower but wider base - easier to
> excavate and not dangerous at all. Don't get me started!!!!
>
>     You can have your PE design the base to your particular configuration
and
> requirements so you don't have to put up with a stupid factory design.
>
> Cheers,
> Steve     K7LXC
> TOWER TECH --
> Professional tower services for commercial and amateur
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