---
title: "Meaning of rock class and shear parameter"
subject: "Engineering Geology II"
chapter: 6
key: "semester_4/engineering_geology_ii:rmr-class-shear-meaning"
source: https://www.bctengineer.com/q/semester_4/engineering_geology_ii/rmr-class-shear-meaning
site: BCT Engineering
---

# Meaning of rock class and shear parameter

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

- Asked in: 2074 Bhadra, 1 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/engineering_geology_ii/rmr-class-shear-meaning
- Whole chapter: https://www.bctengineer.com/md/semester_4/engineering_geology_ii/chapter-6

## Question

Describe the meaning of rock mass class with respect to shear parameters.

## Short answer

Each RMR rock mass class is assigned cohesion and friction angle values, decreasing from over 400 kPa and 45° for very good rock to below 100 kPa and 15° for very poor rock. These parameters are used in the Mohr-Coulomb failure criterion for stability design.

## Answer

In the RMR system each rock mass class is given shear strength parameters of the rock mass, **cohesion $c$** and **angle of internal friction $\phi$**. They decrease as the class goes from very good (I) to very poor (V):

| Class | Rock mass | Cohesion (kPa) | Friction angle |
|---|---|---|---|
| I | Very good | above 400 | above 45° |
| II | Good | 300 to 400 | 35° to 45° |
| III | Fair | 200 to 300 | 25° to 35° |
| IV | Poor | 100 to 200 | 15° to 25° |
| V | Very poor | below 100 | below 15° |

These values are used in the Mohr-Coulomb relation $\tau = c + \sigma\tan\phi$ for stability and support design.

## Check yourself

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

1. Which shear parameters does each RMR class give?
   A. Cohesion c and angle of internal friction phi
   B. Poisson's ratio and modulus
   C. UCS and RQD
   D. Density and porosity
   Answer: A. These are the rock mass shear strength parameters. (Key point: Parameters)

2. How do c and phi change from Class I to V?
   A. Increase
   B. Stay constant
   C. Decrease
   D. Vary randomly
   Answer: C. Quality falls from very good to very poor so strength parameters fall. (Key point: Trend)

3. Which values belong to Class III (fair)?
   A. c above 400 kPa, phi above 45°
   B. c 200-300 kPa, phi 25°-35°
   C. c 100-200 kPa, phi 15°-25°
   D. c below 100 kPa, phi below 15°
   Answer: B. Class III is 200-300 kPa and 25-35°. (Key point: Class III values)

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

## Related in other subjects

- [Meaning of shear stress reduction factor K](https://www.bctengineer.com/question.md?key=semester_4%2Fhydraulics%3Ashear-reduction-factor-k-meaning) — Hydraulics, chapter 10, asked 3×
- [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, 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 and discontinuities](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Arock-mass-classification-and-discontinuities) — Engineering Geology I, chapter 5