The relationship between the shear stresses vs the normal stresses will determine the type of fracture seen and it is called the composite failure envelope- KNOW THIS PAGE

Coulomb failure criterion: shear stress = Cohesion + The multiplication between the normal stress and mu is the coefficient of internal friction.

KNOW THE SHEAR FAILURE CRITERIA PAGES !!!

sigma 1- sigma3= sigma failure..... There is a straight line between the failure point of shear stress and the difference between sigma 3 and sigma 1.

There's failure points of larger differences will be of the same angle... in the diagram it;s about 60 degrees. this is the coefficient of internal friction or mu.

IF you generate a fault is always 30 degrees to sigma 1 and 60 degrees to sigma 3