---
title: "Conditions for wedge failure"
subject: "Engineering Geology II"
chapter: 6
key: "semester_4/engineering_geology_ii:wedge-failure-conditions"
source: https://www.bctengineer.com/q/semester_4/engineering_geology_ii/wedge-failure-conditions
site: BCT Engineering
---

# Conditions for wedge failure

Engineering Geology II · Chapter 6: Measurement, analysis and interpretation of structural geological data

- Asked in: 2072 Asoj, 2 marks
- Video: [Geology: Kinematics of Rock slope](https://www.youtube.com/watch?v=TnH3_S0z-aA) — Dr. Mohit Kumar Puniya
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/engineering_geology_ii/wedge-failure-conditions
- Whole chapter: https://www.bctengineer.com/md/semester_4/engineering_geology_ii/chapter-6

## Question

Mention the conditions for wedge failure in a rock mass.

## Short answer

Wedge failure occurs when two planes intersect to form awedge; the line of intersection must daylight in the slope, with its plunge less than the face's apparent dip in that direction but greater than the friction angle. The trend of the line must point out of the face.

## Answer

**Wedge failure** is the sliding of a wedge-shaped block, formed by two intersecting discontinuity planes, along their **line of intersection**. The conditions (Hoek and Bray) are:

1. **Two planes** (joints, bedding or foliation) intersect each other and the slope face, forming a wedge whose base (keel) is the line of intersection.
2. **Daylighting:** the line of intersection must daylight in the slope face, i.e. its plunge $\psi_i$ is less than the apparent dip of the slope face measured in the trend direction of the line, $\psi_{fi}$: $\psi_i < \psi_{fi}$.
3. **Friction:** the plunge of the line of intersection is greater than the friction angle, $\psi_i > \phi$. Hence the condition is
$$
\psi_{fi} > \psi_i > \phi
$$
4. **Direction:** the trend of the line must point out of the slope face (roughly toward the slope dip direction, within about $90^\circ$ of it).

```text
        \  J1     J2  /
         \  \    /  /
          \  \  /  /
           \  \/  /  <- wedge slides along
            \ line of intersection
```

**Stereonet test:** the point where the two great circles cross (the line of intersection) must lie in the region between the slope great circle and the friction circle.

## Check yourself

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

1. Along what does a wedge slide?
   A. Along the line of intersection of two planes
   B. Along one plane's dip
   C. Along the strike
   D. Rotating about the base
   Answer: A. Wedges slide along the intersection line. (Key point: Sliding direction)

2. What is the daylighting condition?
   A. Plunge of line greater than face dip
   B. Plunge less than the apparent dip of the face in that direction
   C. Line horizontal
   D. Line plunges into the hill
   Answer: B. The line must emerge in the face. (Key point: Daylighting)

3. Which expresses the full condition?
   A. phi > psi_i > psi_f
   B. psi_i > psi_f > phi
   C. psi_f > psi_i > phi
   D. psi_i = phi
   Answer: C. Slope dip exceeds plunge, which exceeds friction angle. (Key point: Combined condition)

## Related in Engineering Geology II

- [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×
- [Geo-mechanics classification of rock mass](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Ageomechanics-classification) — Engineering Geology II, chapter 6, asked 5×
- [RMR rock mass classification](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Armr-classification) — Engineering Geology II, chapter 6, asked 8×
- [Q-system of rock mass classification](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Aq-system-classification) — Engineering Geology II, chapter 6, asked 7×
- [Rock mass versus intact rock](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Arock-mass-vs-intact-rock) — Engineering Geology II, chapter 6, asked 7×
- [Properties of rock mass and discontinuities](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Arock-mass-properties) — Engineering Geology II, chapter 6, asked 3×

## Related in other subjects

- [Plane failure versus wedge failure](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Aplane-failure-versus-wedge-failure) — Engineering Geology I, chapter 6
- [Rock mass classification](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Arock-mass-classification) — Engineering Geology I, chapter 5
- [Rock mass, rock material, discontinuities](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Arock-mass-rock-material-discontinuities) — Engineering Geology I, chapter 5
- [Rock mass classification and discontinuities](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Arock-mass-classification-and-discontinuities) — Engineering Geology I, chapter 5