Stability: NS 320/70, F 155/68

Method (lower-hemisphere stereonet, equal-angle): plot the great circle of the slope face and of each discontinuity, draw the friction circle of radius $\phi$, and mark intersections of great circles. Then apply the kinematic tests below.

Data (dip direction/dip)

NS (natural slope) 320/70; F 155/68; J1 240/80; J2 310/35; J3 010/85; $\phi = 30^\circ$. In dip/dip direction: NS 70/320, F 68/155, J1 80/240, J2 35/310, J3 85/010.

Single planes

Intersections

(The other 2 intersections plunge into the hill, away from the face, and cannot slide.)

Result

• J2 dips at $35^\circ$, only $10^\circ$ off the face direction: $30^\circ < 35^\circ < 70^\circ$, so plane failure is possible.
• F dips into the hill towards 155° (the direction opposite to the face is 140°, a 15° difference) and is steep ($68^\circ \geq 90^\circ - 70^\circ + 30^\circ = 50^\circ$): toppling of slabs is possible.
• J1 and J3 are steeper than the face ($80^\circ$ and $85^\circ > 70^\circ$), so they do not daylight and cannot slide alone.
• J1 x J2 (plunge $34.3^\circ$/323°) and J2 x J3 (plunge $32.0^\circ$/283°) lie between $\phi$ and the face apparent dip: wedge failure is possible.

Conclusion: the slope is unstable; plane failure on J2, wedge failure (J1-J2, J2-J3) and toppling on F can occur. Flatten the slope to less than $35^\circ$ (the J2 dip), install bolts and anchors, and provide drainage.