Jim Lux wrote:
>At 03:33 PM 6/23/2005, K2EK@aol.com wrote:
>>Hello all...
>>
>>Within the next few months, construction begins on my new hilltop QTH.
>>
>>Obviously, I am already planning for the installation of a pair of 45G
>>towers, which
>>for the heights I expect, Rhon specs (4A or 4B) call for guy anchors at a
>>depth of 4 feet.
>><snip>
>>I imagine I could have the rock hammered out, but intuitively that sounds
>>expensive.
>>Rather than removing a lot of very hard rock, to just turn around and re-fill
>>with concrete wouldn't it be possible to drill a suitable hole and set some
>>kind of anchor rod in it?
>
>
>You might be well advised to find a local engineer who can tell you how to
>solve the problem. As you say, if you could just drill an appropriate hole
>in suitably competent bedrock (or even, a big enough boulder), you could
>use a chemical anchor (the fancy name for a bolt and epoxy) or an expansion
>bolt. There's no question that you can do it, there are chemical anchors
>and expansion bolts that have multi-tens of thousands of pound pull out
>forces, but it might be cost prohibitive.
>
>
>However, it's one of those specialty items, and is going to require some
>analysis and local knowledge of the geology and rock properties, who to
>hire to drill the holes (for expansion bolts and chemical anchors, they
>don't have to be very big (if it's more than 1" in diameter, I'd be
>surprised, or necessarily very deep), etc. This isn't a matter of whipping
>on down to home depot and buying some 3/8" masonry anchors.
>
>If it does work out, it's probably a matter of a few hundred dollars for
>the engineer, maybe a hundred bucks for the hammer drill, and a few tens of
>dollars for the bolts themselves. If you need a serious drill:like you
>need to drive a borehole 10 feet deep and 2" in diameter, you're going to
>want a crawler drill (like they use for blasting), and it's going to be a
>bit more, as will the bolts. Either way, done right, the anchor will be
>wicked strong, and the tower will collapse long before the anchor pulls out.
>
I'm facing a similar problem at the new QTH, except that it's for the
base of a free-standing tower. A 4ft cube of concrete is required, with
four 1in studs set in - but as soon as we start to dig, we're sure to
find rocks. These could be anything from football-size up to the bedrock
of the whole hill.
Jim is right about chemical anchors - they're excellent - but the big
unknown is: what size of rock are you anchoring into? The strongest
anchor in the world will be useless if the boulder is too small... and
that's very hard to tell.
One way to find out more would be to probe straight down from the
surface with a long, thin drill bit. On the next visit to the new QTH,
I'm planning to do some probing with the 1000mm x 10mm SDS drill, to get
some idea of what's really under there.
--
73 from Ian GM3SEK
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