A discontinuity dipping out of the slope is unstable when:

Its dip is less than the face angle but more than the friction angle

Its dip is less than the friction angle

Its dip is steeper than the face angle

Which strike relation is most critical?

Strike parallel to the slope (within 20 degrees)

Strike perpendicular to the slope

Discontinuities dipping into the hill are generally:

Stable, though toppling is possible at steep dips

Wedge failure occurs when:

Two joints have an intersection plunging out of the face with psi_f > psi_i > phi

A single joint dips into the slope

Intersection plunge is less than phi

2064 Jestha (old course) · 3 marks

Slope orientation and discontinuities for stability

Short answer

Stability depends on discontinuity orientation relative to slope face. Plane sliding is likely when a discontinuity strikes parallel (±20°) to the face and dips out with slope angle > dip > φ. Opposite dips and dips less than φ are stable; two intersecting joints plunging out with line of intersection between slope angle and φ produce wedge failure.

Discuss in brief the relation between hill slope and orientation of discontinuities to investigate the slope stability condition.

Stability of a rock slope depends on how the discontinuities are oriented relative to the slope face (dip and dip direction), and on the friction angle $\phi$ of the discontinuities.

Relation	Condition	Stability
Dip direction same as slope, discontinuity dip less than slope face angle but more than $\phi$	$\psi_f > \psi_p > \phi$	Unstable (plane sliding)
Dip direction same as slope, dip less than $\phi$	$\psi_p < \phi$	Stable
Dip direction opposite to the slope (dipping into the hill)		Generally stable, but toppling is possible in steep dips
Dip steeper than slope (dip > face angle)		Stable (no daylight)
Strike parallel to the slope	within $\pm20^\circ$	Most critical
Strike perpendicular to the slope		Stable
Two joints whose intersection plunges out of the face	$\psi_f > \psi_i > \phi$	Wedge failure

 UNSTABLE (dip out of slope)    STABLE (dip into slope)
      \                              /
       \ bedding                    /  bedding
  ______\______                ____/_______

In road cuts, the cut slope should not be steeper than the dip of bedding or foliation when these dip out of the slope. This is investigated using a stereonet (kinematic analysis) before design.

✓ Answer checked by Claude Opus 5.5 (high effort)

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Slope orientation and discontinuities for stability

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