---
title: "Geo-mechanics classification of rock mass"
subject: "Engineering Geology II"
chapter: 6
key: "semester_4/engineering_geology_ii:geomechanics-classification"
source: https://www.bctengineer.com/q/semester_4/engineering_geology_ii/geomechanics-classification
site: BCT Engineering
---

# Geo-mechanics classification of rock mass

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

- Asked in: 2078 Chaitra, 3 marks; 2076 Bhadra, 3.5 marks; 2068 Bhadra, 4 marks; 2070 Magh, 2 marks; 2068 Magh, 2 marks (asked 5 times)
- Video: [Rock mass classification](https://www.youtube.com/watch?v=wNcjGXtD8-Q) — Suneel Prema Vlogs
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/engineering_geology_ii/geomechanics-classification
- Whole chapter: https://www.bctengineer.com/md/semester_4/engineering_geology_ii/chapter-6

## Question

Describe the geo-mechanics classification of rock mass (mention the different rock mass classification systems).

## Short answer

The Geomechanics Classification (RMR) by Bieniawski sums ratings of six parameters: intact rock strength, RQD, joint spacing, joint condition, groundwater, and orientation adjustment, yielding classes I–V. Other classification systems include RQD, Q-system, GSI, RSR, and RMi.

## Answer

The **Geomechanics Classification** is the **Rock Mass Rating (RMR) system** of Bieniawski (1973, updated 1989). It classifies a rock mass by adding the ratings of six parameters, so that engineers can estimate stand-up time, shear strength and the support needed.

#### Parameters
1. Uniaxial compressive strength of intact rock (0 to 15)
2. RQD (3 to 20)
3. Spacing of discontinuities (5 to 20)
4. Condition of discontinuities: persistence, aperture, roughness, infilling, weathering (0 to 30)
5. Groundwater (0 to 15)
6. Adjustment for orientation of discontinuities (0 to -12)

$RMR = \sum$ ratings (0 to 100).

#### Classes
| Class | RMR | Rock mass | Stand-up time |
|---|---|---|---|
| I | 81 to 100 | Very good | 20 years (15 m span) |
| II | 61 to 80 | Good | 1 year (10 m span) |
| III | 41 to 60 | Fair | 1 week (5 m span) |
| IV | 21 to 40 | Poor | 10 hours (2.5 m span) |
| V | below 21 | Very poor | 30 minutes (1 m span) |

Each class has a cohesion (from above 400 down to below 100 kPa), a friction angle (from above 45° down to below 15°) and a recommended excavation and support method (bolts, shotcrete, steel ribs).

#### Other rock mass classification systems
| System | Author and year | Basis |
|---|---|---|
| Rock load | Terzaghi, 1946 | Rock load on steel arches |
| Stand-up time | Lauffer, 1958 | Span and unsupported time |
| RQD | Deere, 1967 | Modified core recovery (pieces of 10 cm or more) |
| RSR | Wickham et al., 1972 | Rock structure rating |
| RMR | Bieniawski, 1973 | Six parameters |
| Q-system | Barton et al., 1974 | Six parameters (NGI) |
| GSI | Hoek, 1995 | Structure and surface condition |
| RMi | Palmstrom, 1996 | Rock mass index |

## Check yourself

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

1. What is the Geomechanics Classification?
   A. The Q-system
   B. Terzaghi's rock load
   C. The Rock Mass Rating (RMR) system of Bieniawski
   D. The GSI chart
   Answer: C. Geomechanics Classification is another name for RMR. (Key point: Identity)

2. Which RMR class has stand-up time of about 1 week for a 5 m span?
   A. Class II
   B. Class III (RMR 41-60, Fair)
   C. Class IV
   D. Class I
   Answer: B. Class III Fair rock has stand-up time 1 week for 5 m span. (Key point: Class and stand-up time)

3. What do RMR below 20 and stand-up time of 30 minutes for 1 m span indicate?
   A. Class I, very good
   B. Class II, good
   C. Class IV, poor
   D. Class V, very poor
   Answer: D. Class V is very poor rock. (Key point: Class V)

4. Which pair of system and author is correct?
   A. RQD - Deere 1967
   B. Q-system - Bieniawski
   C. RMR - Barton
   D. Rock load - Hoek
   Answer: A. RQD by Deere 1967; RMR by Bieniawski 1973; Q by Barton 1974; rock load by Terzaghi 1946. (Key point: Other systems)

## Related in Engineering Geology II

- [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×
- [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×
- [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×
- [RMR poor rock support and excavation](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Armr-poor-class-support) — Engineering Geology II, chapter 6, asked 2×
- [Meaning of rock class and shear parameter](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Armr-class-shear-meaning) — Engineering Geology II, chapter 6

## Related in other subjects

- [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 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
- [Soil classification systems](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Acommon-classification-systems) — Soil Mechanics, chapter 3, asked 3×