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Anchoring into gravels 26-Nov-2010 At 1:01:50 PM pcb

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On 26/11/2010 patto wrote:

>When it comes to bolts in mediocre rock I trust the strength of the bolt
>long before I trust the adhesion in rock. You could have a long carrot
>bolt that is loose in the rock hold a 8kN fall when it's a downwards force
>but then be able to be pulled out by hand horizontally.

In mediocre rock, as the rock around the bolt fails won't the load on the bolt shift from pure shear, making adhesion significant in the overall strength of the anchor? At the same time the bending of the bolt and loss of rock would reduce adhesion and cyclic loading would presumably make the situation worse. As to whether angling the bolt towards the load to reduce this effect outweighs the initial loss in strength from not having a shear loading is a question I suspect is best answered by experimentation.

>In good quality rock I believe the bolt itself breaks. So certainly angling
>the bolt in the direction of the force will increase the maximum holding
>force. But this is all largely irrelevant because good bolts in good rock
>aren't a problem and are very much strong enough.

Of course it's all irrelevant. There's no way I'm ever going to clip a bolt that is aimed at the ground, no matter how good or bad the rock. I just thought it would be amusing if it turned out that it was better to do so.

>(Think of tent pegs. If you are placing is coarse loose sand you really
>want your tent pegs angled back. In this case there is little adhesion
>and the tent pegs bending from tent forces are not the issue. If you are
>placing them in low plasticity consolidated clay then they'll stay in no
>matter how you angle them!)

My reading of the linked article suggests that, at least in the non-extreme cases, the tent pegs should be vertical and it really does make a difference how you angle them. At least that's something I can test easily, unfortunately the analogy doesn't necessarily hold for bolts in rock.