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

Subject: Re: [TowerTalk] UST HDX589 crank-up tower base on rock?
From: Ian White GM3SEK <>
Reply-to: Ian White GM3SEK <>
Date: Thu, 16 Feb 2006 23:39:38 +0000
List-post: <>
Jim Lux wrote:
>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.
Sorry - I should have said "adhesive anchors".

>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)
Unfortunately my backyard is like that too - a random mix of large and 
small boulders, everything from granite to slate-like shale, all brought 
here by the last great glacier and dumped in a big pile. Exactly what's 
down there is a gamble, but I've drilled enough deep test holes to be 
fairly confident about trusting it for post-mounted verticals.

Since the posts are for a four-square, there's no question of trying 
again somewhere else - all four have got to be right first time. This 
discussion and the one a couple of weeks ago are moving me towards some 
kind of adhesive anchors, because there's no assurance that the rock 
will be strong enough to take four expansive bolts only a few inches 
apart without crumbling.

>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;

Well, that's a problem. There's very little advanced civil engineering 
needs to be done around here, so the local knowledge doesn't exist.

>bearing in mind that it might come to "blasting and refilling the hole 
>with concrete", as inelegant as that might be.
That pretty much describes the local knowledge base. Refilling a big 
hole with concrete is exactly what I'm going to do for the tower, and 
that hole is already dug. The boulders are loose enough to dig out with 
a backhoe, but they only come out in huge lumps - if at all. There's no 
way I could hope to dig four reasonable-sized holes in four specified 
locations, so bolting into the rock seems like the only option... just 
gotta find some way to make it work.

73 from Ian GM3SEK


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