---
title: "Planes of discontinuity in rock masses"
subject: "Engineering Geology I"
chapter: 5
key: "semester_3/engineering_geology_i:planes-of-discontinuity-in-rock"
source: https://www.bctengineer.com/q/semester_3/engineering_geology_i/planes-of-discontinuity-in-rock
site: BCT Engineering
---

# Planes of discontinuity in rock masses

Engineering Geology I · Chapter 5: Structural Geology

- Asked in: 2061 Baisakh (old course), 2+6 marks
- Video: [Types of Discontinuities in the Rocks - Structural Geology and Stratigraphy - Engineering Geology](https://www.youtube.com/watch?v=cM1oWyqek8Q) — Ekeeda
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/engineering_geology_i/planes-of-discontinuity-in-rock
- Whole chapter: https://www.bctengineer.com/md/semester_3/engineering_geology_i/chapter-5

## Question

What are the planes of discontinuities in the rock masses? Explain their characteristics.

## Short answer

Discontinuities are planes of weakness such as bedding, joints, faults, and foliation. Their characteristics—orientation, spacing, persistence, roughness, aperture, infill, wall strength, seepage, and number of sets—control rock mass strength and are logged for classification.

## Answer

**Discontinuities** are planes of weakness or breaks in a rock mass across which the rock has little or no tensile strength. They separate the rock material into blocks and control the strength, deformability and permeability of the mass.

#### Types of discontinuities
- **Bedding planes:** surfaces between sedimentary layers.
- **Joints:** fractures with no visible displacement; they occur in sets (joint systems).
- **Faults:** fractures with displacement; they often have gouge, breccia or crushed rock.
- **Foliation and cleavage planes:** planar fabric in metamorphic rocks (slate, schist, gneiss).
- **Shear zones and fracture zones**, and **unconformities** or contacts between rock types.

#### Characteristics (ISRM)
1. **Orientation:** the attitude, given by dip and dip direction (or strike). Together with the slope it decides if a block can slide.
2. **Spacing:** the distance between adjacent discontinuities of one set. Close spacing gives small blocks and a weak mass.
3. **Persistence (continuity):** the length of the discontinuity trace; a persistent plane makes a more continuous failure surface.
4. **Roughness:** unevenness of the surface; rough surfaces give higher shear strength.
5. **Aperture:** the open distance between the walls. A wide aperture means low strength and high permeability.
6. **Filling (infill):** material between the walls (clay, sand, calcite); clay filling reduces shear strength sharply.
7. **Wall strength (weathering):** strength of the rock at the wall surface.
8. **Seepage (water flow):** water in the discontinuities.
9. **Number of sets and block size.**

These characteristics are logged in the field by scanline or window mapping and are the input for rock mass classification (RMR, Q) and slope and tunnel design.

## Check yourself

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

1. What does close spacing give?
   A. Small blocks and a weak mass
   B. Large strong blocks
   C. Higher strength
   D. Lower permeability
   Answer: A. Close spacing weakens the mass. (Key point: Spacing)

2. What does a wide aperture mean?
   A. High strength
   B. Low strength and high permeability
   C. Impermeable rock
   D. No water flow
   Answer: B. Open walls reduce strength. (Key point: Aperture)

3. Which infill reduces shear strength most sharply?
   A. Calcite
   B. Hard quartz cement
   C. Clay
   D. Fresh rock
   Answer: C. Clay filling is weak. (Key point: Infill)

4. What does persistence mean?
   A. The gap between walls
   B. The roughness
   C. The dip angle
   D. The length of the discontinuity trace
   Answer: D. Persistent planes give continuous failure surfaces. (Key point: Persistence)

## 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
- [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
- [Discontinuities; Siwalik and Higher Himalaya lithology](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Adiscontinuities-siwalik-and-higher-himalaya) — Engineering Geology I, chapter 7
- [Discontinuities and engineering problems in Nepal Himalaya](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Adiscontinuities-and-engineering-problems-in) — Engineering Geology I, chapter 7

## Related in other subjects

- [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×
- [Balancing one mass using two planes](https://www.bctengineer.com/question.md?key=semester_7%2Ftheory_of_mechanism_and_machine_ii%3Asingle-mass-two-plane-balance-derivation) — Theory of Mechanism and Machine II, chapter 5
- [Dynamic balancing with two correction planes](https://www.bctengineer.com/question.md?key=semester_7%2Ftheory_of_mechanism_and_machine_ii%3Abalancing-masses-two-planes-numerical) — Theory of Mechanism and Machine II, chapter 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×