---
title: "Plane failure versus wedge failure"
subject: "Engineering Geology I"
chapter: 6
key: "semester_3/engineering_geology_i:plane-failure-versus-wedge-failure"
source: https://www.bctengineer.com/q/semester_3/engineering_geology_i/plane-failure-versus-wedge-failure
site: BCT Engineering
---

# Plane failure versus wedge failure

Engineering Geology I · Chapter 6: Physical Geology

- Asked in: 2063 Baisakh (old course), 2 marks
- Video: [Types of Slope Failures in Mining || Planar, Wedge, Toppling, Circular, Face, Toe & Base Failures |](https://www.youtube.com/watch?v=algu8ZNvKG8) — Mining Mirror
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/engineering_geology_i/plane-failure-versus-wedge-failure
- Whole chapter: https://www.bctengineer.com/md/semester_3/engineering_geology_i/chapter-6

## Question

Differentiate between plane failure and 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.

## Answer

| 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.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Plane failure slides on:
   A. Two planes, while wedge failure slides on one
   B. Curved circular surface only
   C. No surface in either
   D. One discontinuity plane, while wedge failure slides on two intersecting planes
   Answer: D. A wedge is formed by two planes meeting in a line. (Key point: Sliding surface)

2. In wedge failure the sliding is down:
   A. The line of intersection of the two planes
   B. The dip of one plane only
   C. The strike of the slope
   D. The vertical
   Answer: A. The intersection must plunge toward the face and daylight. (Key point: Direction)

3. The dip condition for plane failure is:
   A. Plane dip > slope face angle
   B. Slope face angle > plane dip > friction angle
   C. Plane dip < friction angle
   D. Friction angle > slope face angle
   Answer: B. psi_f > psi_p > phi, with plane striking within 20 degrees of the face. (Key point: Plane failure condition)

4. Wedge failure analysis generally needs:
   A. Only simple 2-D with one plane
   B. No analysis
   C. 3-D treatment, using a stereonet or vector method
   D. A single friction test only
   Answer: C. Plane failure is simple 2-D analysis. (Key point: Analysis)

## Related in Engineering Geology I

- [Landslide versus slope failure](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Alandslide-versus-slope-failure) — Engineering Geology I, chapter 6
- [Weathering versus erosion](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Aweathering-versus-erosion) — Engineering Geology I, chapter 6, asked 5×
- [Primary versus secondary geological structures](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Aprimary-versus-secondary-geological-structures) — Engineering Geology I, chapter 5, asked 4×
- [Convergent versus divergent plate boundary](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Aconvergent-versus-divergent-plate-boundary) — Engineering Geology I, chapter 2, asked 3×
- [Intrusive versus extrusive igneous rocks](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Aintrusive-versus-extrusive-igneous-rocks) — Engineering Geology I, chapter 4, asked 3×
- [Engineering geology versus geology; disciplines](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Aengineering-geology-versus-geology-disciplines) — Engineering Geology I, chapter 1, asked 2×

## Related in other subjects

- [Conditions for wedge failure](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Awedge-failure-conditions) — Engineering Geology II, chapter 6
- [Conditions for plane failure](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Aplane-failure-conditions) — Engineering Geology II, chapter 6, asked 2×
- [Dry sand failure plane 25°, find σ3](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Afine-dry-sand-failure-plane-25) — Soil Mechanics, chapter 10
- [In-plane, out-of-plane failure and remedial measures](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Ain-plane-out-of-plane-remedial) — Concrete Technology and Masonry Structure, chapter 9