Roof vs face bolt strength

[Andy Davies]

I read with interest the current debate around the pulled bolt on "Lip Service" at Sandrift. I previously posted an article on the difference in strength for a roof and face bolt (but am too incompetent to go find it). So again here we go from the Fischer catalogue;

FBN A4 stainless 90mm (express anchor):
Ultimate tensile strength: 23.9 kN (ie roof placement)
Ultilmate shear strength: 24.0 kN (ie face placement)

FIS A4 stainless glue-in 90mm:
Ultimate tensile strength: 41.0 kN (ie roof placement)
Ultilmate shear strength: 20.3 kN (ie face placement)

So I am sure there will be some questions - here are some answers;
1] A roof bolt is stronger than a face bolt as the rock failure cone is greater
2] The glue-in is stronger than the express anchor in a roof as it has a a longer effective anchoring depth
3] I suspect the express anchor is stronger than the glue in rod as it is rolled and not cut.

There are other failure modes apart from the usual ones we see in a controlled lab:
a] Express anchors can slide out in soft materials - ie the rock doesn't fail (see Lip Service)
b] Glue-ins are not placed properly - ie plonkers hand mix the local DIY glue and then try and shove it in the hole
c] Plonkers try and save money and place cheap bolts that will fail over time (normal corrosion and stress corrosion cracking)

[mokganjetsi]

he andy, thanks for the info

so the face position has only more merit in a) where sliding out obviously happens easier if the fall force is in the same direction as the hole (i.e. lip service).
and due to max impact force on single ropes being around 9kn these days, the shear strength is more than adequate.

don't forget those plonkers that used bolts slightly smaller than the drill size in Oz - the bolt pulled; guy fell over a sharp edge on pitch 2 or 3; rope cut; climber dead

[noki]

Hi Andy

I still think that in the Lip Service context, having seen the pic, I would have placed that bolt on the face myself. That would just make more sense, from a rope management perspective, and the face placement will always be more forgiving of adverse conditions (soft rock, incompetent bolter--not that I am saying LS's bolter was incompetent, I don't even know who bolted it, etc.). I have placed a few roof bolts myself, but only when there really was no choice, and if faced with a choice, would almost always go for the face bolt.
In short, the face bolt is always strong enough, and is more forgiving. So face bolts just make more sense...

[angiebuds]

The roof bolt is really stronger. It has the strongest capability to handle that kind of pressure. The roof bolt is common here in our city.

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