Angle of discontinuity vs. friction angle.

[HighflyerBE]

Hi,

I'm a student who is studying for a degree of civil engineering. I have the following question:

If you're let's say about 500m in the ground and there's a discontinuity under an angle of 20 degrees. The cohesion of the discontinuity can be neglected and the friction angle is 30 degrees. Can this discontinuity fail?
I was thinking about this. I would say yes because if you look at the shear criterium tau=sigma_n*tg(friction angle), you can have a situation where you have a tau which is bigger or equal to sigma_n*tg(friction angle). But on the other hand I would say no because the angle of the discontinuity is simply to small: 20 degrees<30 degrees, so it can not slide away.

I've been looking for some time on the internet, but I can't find the answer.
Would somebody please take a look at this?

Thanks in regard!

 

[BigH]

You do realize that there are to be no student posts on the site. Fail? Sure- but not 500 m into the ground unless there was a driving force greater than the resisting force (only thing I can think of is if you had a very deep excavation - and then, again, failure would also depend on the strike and dip into the excavation. But level ground - no.

 

[BigH]

Make that very very deep - as in open pit mining

 

[HighflyerBE]

I was not aware of that. I won't make any new topics regarding any type of student questions.

Thank you for your reply. But what about water in the discontinuity?. It could reduce the resisting force so the driving force becomes bigger than the resisting force. Could it slide away? Because the angle of the discontinuity is still smaller than friction angle. (Assuming the wetness of the material does not influence the friction angle)