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

# Rock mass classification

Engineering Geology I · Chapter 5: Structural Geology

- Asked in: 2066 Bhadra (old course), 2 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
- Whole chapter: https://www.bctengineer.com/md/semester_3/engineering_geology_i/chapter-5

## Question

Describe the engineering classification of rock mass.

## Short answer

Rock mass classification uses intact rock and discontinuity properties to rate quality for tunnels, slopes, and foundations. Key systems: RQD (core recovery ≥10 cm), RMR (sum of five parameter ratings), and the Q index Q = (RQD/Jn)×(Jr/Ja)×(Jw/SRF).

## Answer

**Rock mass classification** is the grouping of a rock mass into classes according to its quality and engineering behaviour, based on the properties of the intact rock and of the discontinuities (joints, bedding, faults). It is used to estimate rock strength, stand-up time and support requirements in tunnels, slopes and foundations.

Common systems:
- **RQD** (Deere) - percentage of core pieces longer than 10 cm; classes from very poor (<25 %) to excellent (90 - 100 %).
- **RMR** (Bieniawski) - sum of ratings for strength, RQD, joint spacing, joint condition, ground water and orientation; five classes, 100 to below 21.
- **Q system** (Barton) - $Q = \frac{RQD}{J_n}\frac{J_r}{J_a}\frac{J_w}{SRF}$, used in tunnel support design.
- Terzaghi's rock load classification and the GSI (Geological Strength Index).

## Check yourself

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

1. What is rock mass classification used for?
   A. Dating the rocks
   B. Estimating strength, stand-up time and support requirements
   C. Naming minerals
   D. Mapping rivers
   Answer: B. It groups rock by quality for design. (Key point: Purpose)

2. What does RQD measure?
   A. Total core length
   B. Number of joint sets
   C. Percentage of core pieces longer than 10 cm
   D. Water content
   Answer: C. RQD is core-piece percentage. (Key point: RQD)

3. Which system is used mainly for tunnel support design?
   A. RQD alone
   B. Mohs scale
   C. Richter scale
   D. Q system (Barton)
   Answer: D. Q is used for tunnel support. (Key point: Systems)

## 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
- [Define mass movement and its classification](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Adefine-mass-movement-and-its) — Engineering Geology I, chapter 6, asked 5×
- [Engineering significance of three rock types](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Aengineering-significance-of-three-rock) — Engineering Geology I, chapter 4, asked 5×
- [Engineering significance of rock-forming minerals](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Aengineering-significance-of-rockforming-minerals) — Engineering Geology I, chapter 3, asked 3×
- [Petrology; engineering significance of rock classes](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Apetrology-engineering-significance-of-rock) — Engineering Geology I, chapter 4

## 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×
- [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×
- [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×
- [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×