Plane failure slides on:

One discontinuity plane, while wedge failure slides on two intersecting planes

Two planes, while wedge failure slides on one

In wedge failure the sliding is down:

The line of intersection of the two planes

The dip condition for plane failure is:

Slope face angle > plane dip > friction angle

Friction angle > slope face angle

Wedge failure analysis generally needs:

3-D treatment, using a stereonet or vector method

2063 Baisakh (old course) · 2 marks

Plane failure versus wedge failure

Short answer

Plane failure occurs on a single discontinuity striking parallel (±20°) to the slope face and dipping out, with dip between slope angle and friction angle, requiring side release surfaces. Wedge failure occurs on the intersection line of two planes that plunges toward the face, daylighting, and also satisfies ψf > ψi > φ, without needing side releases.

Differentiate between plane failure and wedge failure.

Point	Plane failure	Wedge failure
Sliding surface	One discontinuity plane	Two intersecting planes
Direction of sliding	Down the dip of the plane	Down the line of intersection
Condition	Plane strikes parallel (within $\pm20^\circ$ ) to the slope face and dips toward it	Line of intersection plunges toward the face and daylights
Dip condition	$\psi_f > \psi_p > \phi$	$\psi_f > \psi_i > \phi$
Release surfaces	Needs release surfaces at the sides, and often a tension crack	No separate side release needed; the wedge is bounded by the two planes, which also give extra frictional support (wedging)
Occurrence	Bedding, foliation, one joint set	Two joint sets, or a joint and foliation
Analysis	Simple 2-D, $FS$ from one plane	3-D, needs a stereonet or vector method

Here $\psi_f$ is the slope face angle, $\psi_p$ the dip of the plane, $\psi_i$ the plunge of the intersection line and $\phi$ the friction angle.

✓ Answer checked by Claude Opus 5.5 (high effort)

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