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Topic Date User

Fatigue failure of ubolt anchor 2-Nov-2010 At 3:12:48 PM hargs

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On 2/11/2010 Lee C wrote:
> ... Just thinking that the failure
>of the U would have been in part, due to bending that would not occur in
>a +10kn load that lasted only <1 second? Feel free to disregard this potentially
>absurd armchair engineer theory if it's way off

The failure was in the rock, not so much the bolt... It had been building up for some time and I was watching closely when it happened. First the surface 'skin' flaked off exposing sand basically, which just fell out of the rock. Then a weakness appeared around the bolt shafts as if the holes had been drilled oversize, dislodging yet more sand. Some distortion of the bolt was visible. (This happened gradually, over repeated loadings of several different bits of gear.) Finally, a well-defined circular area around the bolt appeared visibly to 'give' a bit, kind of like a mini underground detonation. The rock didn't collapse entirely; the crater you can see mikl standing in front of in the photo didn't happen until he tapped the surface with a spanner after the bolt had been removed. There was some kind of structural failure or loosening of the rock, after which the bolt could be pulled out by hand (or more accurately with pliers.)

There's a whole other bunch of questions to ask now about whether it's the number of load events, or load events above a certain value, or the duration of load event that contributes to anchor failure.