Stability: HS 310/43, BP 306/31

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)

HS 310/43; BP 306/31; J1 350/40; J2 288/37; J3 237/32. In dip/dip direction: HS 43/310, BP 31/306, J1 40/350, J2 37/288, J3 32/237. The friction angle is not stated, so $\phi = 30^\circ$ is assumed.

Single planes

Intersections

Result

• BP dips almost in the same direction as the face (4° difference), daylights ($31^\circ < 43^\circ$) and is just above $\phi$ ($31^\circ > 30^\circ$): plane failure along the bedding is marginally possible.
• J1 (dip direction 40° off) and J2 (22° off) each daylight, but they are outside the $\pm 20^\circ$ range for plane failure. J3 dips across the slope at 32° (73° off).
• The J1 x J2 wedge (plunge $34.1^\circ$ towards 314°) and the BP x J1 wedge (plunge $31.0^\circ$ towards 306°) lie between the friction circle and the slope great circle: wedge failure is possible.

Conclusion: the slope is marginally unstable, with bedding-plane sliding and wedge failure (J1-J2 and BP-J1) as the possible modes. If the true friction angle is $35^\circ$ or more, all of them become stable. Remedies: reduce the slope angle to less than about $31^\circ$, or use rock bolts, benches and drainage.