Jim Lux wrote:
>At 08:35 AM 2/16/2006, Ian White GM3SEK 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
>>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
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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