---
title: "Slope orientation and discontinuities for stability"
subject: "Engineering Geology I"
chapter: 6
key: "semester_3/engineering_geology_i:slope-orientation-and-discontinuities-for"
source: https://www.bctengineer.com/q/semester_3/engineering_geology_i/slope-orientation-and-discontinuities-for
site: BCT Engineering
---

# Slope orientation and discontinuities for stability

Engineering Geology I · Chapter 6: Physical Geology

- Asked in: 2064 Jestha (old course), 3 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_3/engineering_geology_i/slope-orientation-and-discontinuities-for
- Whole chapter: https://www.bctengineer.com/md/semester_3/engineering_geology_i/chapter-6

## Question

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

## 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.

## Answer

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 |

```text
 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.

## Check yourself

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

1. A discontinuity dipping out of the slope is unstable when:
   A. Its dip is less than the friction angle
   B. Its dip is steeper than the face angle
   C. It dips into the hill
   D. Its dip is less than the face angle but more than the friction angle
   Answer: D. psi_f > psi_p > phi gives plane sliding. (Key point: Unstable condition)

2. Which strike relation is most critical?
   A. Strike parallel to the slope (within 20 degrees)
   B. Strike perpendicular to the slope
   C. Strike always irrelevant
   D. Strike opposite to the sky
   Answer: A. Perpendicular strike is stable. (Key point: Strike)

3. Discontinuities dipping into the hill are generally:
   A. Always the most unstable
   B. Stable, though toppling is possible at steep dips
   C. Failing as wedge only
   D. Failing by plane sliding
   Answer: B. Dip direction opposite to the slope favours stability. (Key point: Dip into slope)

4. Wedge failure occurs when:
   A. A single joint dips into the slope
   B. The joints are horizontal
   C. Two joints have an intersection plunging out of the face with psi_f > psi_i > phi
   D. Intersection plunge is less than phi
   Answer: C. The line of intersection must daylight in the face. (Key point: Wedge)

## Related in Engineering Geology I

- [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

## Related in other subjects

- [Slope stability: N40W/55 slope, two joints](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Aslope-stability-n40w-55) — Engineering Geology II, chapter 6
- [Canal slope stability: HS 138/45](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Acanal-slope-stability-hs138-45) — Engineering Geology II, chapter 6
- [Reciprocal leveling, RL of hill top relation](https://www.bctengineer.com/question.md?key=semester_4%2Fsurveying_ii%3Areciprocal-leveling-when-hill-top-relation) — Surveying II, chapter 3
- [Hill road earthwork, cross slope 1 in 5 (0-60)](https://www.bctengineer.com/question.md?key=semester_7%2Festimating_and_costing%3Ahill-road-r-cross-slope-1-5) — Estimating and Costing, chapter 6