[TowerTalk] UST HDX589 crank-up tower base on rock?

Jim Lux jimlux at earthlink.net
Thu Feb 16 15:32:00 EST 2006

At 08:35 AM 2/16/2006, Ian White GM3SEK wrote:
>Steve wrote:
> >    You can rent a rock corer to drill the  appropriate holes for the
> >factory anchor bolts - or have someone come out and  do it - and then
> >you can epoxy them in or use rock expansion bolts - or  both. It'll be
> >real strong.
>"Or both"... interesting idea - has anybody here used it?
>On a smaller scale - five posts for verticals, rather than a tower -
>I'll soon have 20 of these anchors to put in, so would welcome any and
>all suggestions.
>It's impossible to dig post-holes here. We tried a little harder with a
>backhoe, but at all five locations it hit solid rock only a few inches
>down. The best it could manage was five shallow scrapes, so I'll now
>have to level each of these back up to grade with concrete, and then use
>bolt-down sockets for the wooden posts. My plan had been to drill
>half-inch holes right through the concrete and some way down into the
>solid rock, and then use half-inch threaded rods anchored by epoxy (or
>similar... whichever kind of resin works best in damp rock).
>Does that make sense, to the folks who have used epoxy?

It's not always epoxy as the adhesive, by the way.

A whole lot depends on the kind of rock.  In my backyard, there are large 
rocks which are  basically impossible to dig in, however, it's a really 
very fragile silt stone or shale, and has almost no structural 
strength.  One can scrape or grind it fairly easily and you can break it, 
across layers.  It is in no way suitable for anchoring into, and I've been 
given to understand that this is quite common in sedimentary 
formations.  (it would be like digging into a phonebook, It's hard to 
penetrate all the pages, but after you can drill it, but it's hard to 
anchor into)

So, you need to talk to a local engineer who's familiar with your 
particular rock, and get him or her to recommend a suitable adhesive; 
bearing in mind that it might come to "blasting and refilling the hole with 
concrete", as inelegant as that might be.

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