3. Now draw two lines which are tangent to the Mohr circles, one above and one below the 0.

axis. These lines define the 'Coulomb failure envelope'. A Mohr circle that plots inside the

envelope, without touching it, represents a state of stability with no failure; Mohr circles

which plot on or beyond the envelope represent unstable stress states, which exceed the rock

rupture strength and will result in failure by fracturing. Label these zones on your plot.