---
title: "Rock mass classification and discontinuities"
subject: "Engineering Geology I"
chapter: 5
key: "semester_3/engineering_geology_i:rock-mass-classification-and-discontinuities"
source: https://www.bctengineer.com/q/semester_3/engineering_geology_i/rock-mass-classification-and-discontinuities
site: BCT Engineering
---

# Rock mass classification and discontinuities

Engineering Geology I · Chapter 5: Structural Geology

- Asked in: 2059 Chaitra (old course), 2+3+3 marks
- Video: [How to Calculate Rock Mass Rating (RMR) | Real Example + Tunnel Adjustments](https://www.youtube.com/watch?v=jvEQeZuqydI) — Soil Mechanics and Engineering Geology
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/engineering_geology_i/rock-mass-classification-and-discontinuities
- Whole chapter: https://www.bctengineer.com/md/semester_3/engineering_geology_i/chapter-5

## Question

Describe the planes of discontinuity in rock masses and write brief notes on the Q and RMR systems of engineering classification of rock masses.

## Short answer

Discontinuities such as joints, faults, bedding, and foliation control rock mass behaviour. RMR rates unixal strength, RQD, joint spacing, condition, groundw ater, and orientation to give five classes. Q = (RQD/Jn)×( Jr/Ja)×(Jw/SRF) combines block size, shear strength, and stress for tunnel support selection.

## Answer

#### Planes of discontinuity
A **discontinuity** is any plane of weakness or break in the continuity of a rock mass that has little or no tensile strength. Main types:
- **Bedding planes** in sedimentary rocks.
- **Joints** - fractures without displacement.
- **Faults** and shear zones - fractures with displacement.
- **Foliation and cleavage planes** in metamorphic rocks.
- **Unconformity surfaces** and contacts between rock types.
- Boundaries of dykes and sills, and sheeting joints in massive igneous rocks.

Their orientation, spacing, persistence, aperture, roughness, infilling and water condition control the strength and permeability of the rock mass.

#### RMR system (Bieniawski)
Rock Mass Rating gives a rating (0 - 100) as the sum of the ratings for five parameters, followed by an adjustment for joint orientation:
1. Uniaxial compressive strength of the intact rock (0 - 15)
2. RQD (3 - 20)
3. Spacing of discontinuities (5 - 20)
4. Condition of discontinuities (0 - 30)
5. Groundwater (0 - 15)
6. Orientation adjustment (0 to -12 for tunnels)

Classes: I very good (81 - 100), II good (61 - 80), III fair (41 - 60), IV poor (21 - 40), V very poor (<21). It is used to find the support requirement and stand-up time.

#### Q system (Barton et al.)
The tunnelling quality index:

$$
Q = \frac{RQD}{J_n} \times \frac{J_r}{J_a} \times \frac{J_w}{SRF}
$$

where $J_n$ = joint set number, $J_r$ = joint roughness number, $J_a$ = joint alteration number, $J_w$ = joint water reduction factor and SRF = stress reduction factor. The three quotients represent block size, inter-block shear strength and active stress. Q ranges from 0.001 to 1000 on a logarithmic scale: exceptionally poor (0.001 - 0.01), extremely poor (0.01 - 0.1), very poor (0.1 - 1), poor (1 - 4), fair (4 - 10), good (10 - 40), very good (40 - 100), extremely good (100 - 400) and exceptionally good (400 - 1000). Q is used to select the support type in tunnels and caverns.

## Check yourself

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

1. What is a discontinuity?
   A. A strong intact rock
   B. A plane of weakness with little or no tensile strength
   C. A mineral grain
   D. A type of fold
   Answer: B. Discontinuities break rock continuity. (Key point: Discontinuity)

2. What is the range of RMR, and what class is 61 - 80?
   A. 0 - 1000; class V
   B. 0 - 100; class IV poor
   C. 0 - 100; class II good
   D. 0 - 50; class I
   Answer: C. RMR is 0 to 100 and 61 - 80 is good. (Key point: RMR range)

3. What do the three Q quotients represent?
   A. Strength, RQD and water
   B. Spacing, aperture and dip
   C. Depth, width and age
   D. Block size, inter-block shear strength and active stress
   Answer: D. RQD/Jn is block size, Jr/Ja shear strength, Jw/SRF stress. (Key point: Q formula)

4. Which is NOT a main RMR parameter?
   A. Joint set number Jn
   B. Uniaxial compressive strength
   C. RQD
   D. Ground water
   Answer: A. Jn belongs to the Q system. (Key point: RMR parameters)

## Related in Engineering Geology I

- [Planes of discontinuity in rock masses](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Aplanes-of-discontinuity-in-rock) — 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](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Arock-mass-classification) — Engineering Geology I, chapter 5
- [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
- [RMR system and its parameters](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Armr-system-and-its-parameters) — Engineering Geology I, chapter 5, asked 2×

## Related in other subjects

- [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×
- [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×
- [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×
- [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×