---
title: "RMR fair rock support and excavation"
subject: "Engineering Geology II"
chapter: 6
key: "semester_4/engineering_geology_ii:rmr-fair-class-support"
source: https://www.bctengineer.com/q/semester_4/engineering_geology_ii/rmr-fair-class-support
site: BCT Engineering
---

# RMR fair rock support and excavation

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

- Asked in: 2078 Chaitra, 3 marks; 2078 Baisakh, 2 marks; 2073 Magh, 3 marks (asked 3 times)
- 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-fair-class-support
- Whole chapter: https://www.bctengineer.com/md/semester_4/engineering_geology_ii/chapter-6

## Question

Discuss the excavation and support system in fair rock class according to RMR.

## Short answer

For fair rock (Class III, RMR 41–60), excavate by top heading and bench, 1.5–3 m advance. Systematic 4 m fully grouted bolts at 1.5–2 m spacing, wire mesh in crown, shotcrete 50–100 mm crown and 30 mm sides; no steel sets. Complete support within 10 m of face.

## Answer

Fair rock is **Class III** with RMR 41 to 60 (cohesion 200 to 300 kPa, friction angle 25° to 35°, stand-up time about 1 week for a 5 m span). The guideline (Bieniawski, 1989, for a 10 m span horse-shoe tunnel, drill and blast) is:

#### Excavation
- **Top heading and bench** method.
- Advance of **1.5 to 3 m** in the top heading.
- Support is started after each blast, and the support is completed within **10 m** of the face.

#### Support
| Element | Recommendation |
|---|---|
| Rock bolts (20 mm, fully grouted) | Systematic, 4 m long, spaced 1.5 to 2 m in crown and walls |
| Wire mesh | In the crown |
| Shotcrete | 50 to 100 mm in the crown and 30 mm in the sides |
| Steel sets | Not required |

```text
   Top heading   ____________
                /  bolts 4 m \
   Bench       |  mesh+shotcr |
               |______________|
```

The bolts and shotcrete hold the loosened blocks together and let the rock mass carry itself. Local support is increased where water or weak seams occur.

## Check yourself

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

1. Fair rock under RMR is which class?
   A. Class II (61-80)
   B. Class III (41-60)
   C. Class IV (21-40)
   D. Class I
   Answer: B. Fair rock is Class III. (Key point: Class)

2. What excavation method and advance is recommended in fair rock?
   A. Full face with 3-5 m advance
   B. Multiple drifts with 0.5 m advance
   C. Top heading and bench with 1.5 to 3 m advance
   D. Top heading with 1.0 to 1.5 m advance
   Answer: C. Class III uses top heading and bench, advancing 1.5 to 3 m. (Key point: Excavation)

3. What bolting is recommended in fair rock?
   A. Systematic 4 m bolts at 1.5-2 m spacing
   B. Locally 3 m bolts at 2.5 m
   C. 5 m bolts at 1 m
   D. No bolts
   Answer: A. Class III has systematic 4 m long fully grouted bolts at 1.5-2 m. (Key point: Bolts)

4. Are steel sets needed in Class III?
   A. Yes, heavy ribs at 0.75 m
   B. Yes, light ribs at 1.5 m
   C. Only on the face
   D. No, steel sets are not required
   Answer: D. Fair rock relies on bolts, mesh and shotcrete, with no steel sets. (Key point: Steel sets)

## Close variants set in other exams

- [RMR class IV support and excavation](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Armr-class-iv-support) — Engineering Geology II, chapter 6, asked 6×
- [RMR class II support and excavation](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Armr-class-ii-support) — Engineering Geology II, chapter 6, asked 7×

## Related in Engineering Geology II

- [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×
- [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×
- [Rock classes needing steel rib and shotcrete](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Armr-steel-rib-classes) — Engineering Geology II, chapter 6

## Related in other subjects

- [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×
- [Earthwork excavation in soft soil, hard rock, wet soil](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Aexcavation-soft-hard-wet) — Building Technology, chapter 2