Stability: HS 202/65, joints J1 to J4

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/dip direction)

HS 65/202; F 27/350; J1 81/180; J2 69/078; J3 73/021; J4 52/293. The friction angle is not given, so $\phi = 30^\circ$ is assumed (typical for rock joints). (The last digit of F in the scan is unclear; $27^\circ$ is used. As F dips into the hill at less than $\phi$, it does not affect the result.)

Single planes

Intersections

(The other 8 intersections plunge into the hill, away from the face, and cannot slide.)

Result

• J3 (73/021) dips into the hill almost exactly opposite to the face (the face dips towards 202°, so the opposite direction is 022°). Its dip of $73^\circ$ is more than $90^\circ - 65^\circ + 30^\circ = 55^\circ$, so toppling failure is possible.
• J1 strikes nearly parallel to the face but dips $81^\circ$, steeper than the face, so it does not daylight.
• F (27°) dips into the hill and is below $\phi$; J2 and J4 do not daylight individually.
• The intersection J1 x J4 plunges about 47.1° towards 260°, between $\phi$ and the face apparent dip (48.5°), so wedge failure is also possible.

Conclusion: the slope is unstable. The main modes are toppling along J3 and wedge sliding along the J1-J4 intersection. Reduce the slope angle, remove the potential wedge, and use bolts or anchors and drainage.