How safe are stoppers/rocks when threaded through rock tunnels?

Question

Last year, I learned that you could thread the wire of stoppers/rocks/hexentrics through tunnels in the rock if threading cord or a sling through them would be too difficult. As a reference, see the following image (left: sling/cord, right: stopper):

I used that method a few times for rather non-critical pieces or where the tunnel was too narrow to fit a sling through it. However, I am wondering:

How does that method change the usual safety considerations when using tunnels for protection (thickness of the rock bridge etc)? For example, would additional forces be exerted on the bridge? Or would the forces even be smaller since the stopper distributes them around the hole? (All of this assuming that the stopper is large enough to never slip through the hole.)

Answer 1

Your main concern here is going to be shock loading. Nuts are typically tested to resist static compression load. Any shock is typically adsorbed by the rope and other equipment, so they get the mimimum shock load when/if you fall. If they don't fit well and are loose (or are just passed though a hole and are therefore moving around all over the place) then when/if you fall they will hit the rock and shock load. There is no give in the wire or metal so the full fall factor is concentrated in a very small area. This has the potential to take the equipment past it's limits and therefore break it.

Even ignoring the shock loading on the equipment your also going to shock load the rock, this can make it break. Think pulling on a hold vs hitting it with a hammer.

Then there's your slings, etc. Depending on the material you are again potentially shock loading them due to the movement in the anchor.

All in all, this is likely less secure/safe than a well placed secure nut in a crack and should be avoided.

You'd be better off attaching a nylon sling around the "tunnel".

Here's some work DMM did on the effect of shock loading on slings to give you an idea of how it can affect a system.

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Or would the forces even be smaller since the stopper distributes them around the hole

If anything you're distributing the load less evenly. A well placed nut should have the maximum surface area in contact with the rock. The more surface area the more the load is distributed. Only the top of your knut is in contact with the rock and if it's loose then not even that initially. So your actually concentrating the load!

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What I'm trying to say is this:

Rope tight (looks ok?)

rock->|    |<-rope
nut->C|----|
      |    |

Eveything seems ok in the above scenario

Rope loose (not so good now)

   rock->| /<-rope
nut->C---||
         | \ 

Nut no longer in contact with the rock

Leader falls

          | /
      C---|| ----->rope goes taught (moves right)
       -> | \ 


       |    |
      C|----|
      ^|    |
     /
    *Impact*

Nut impacts with rock!!