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

# RMR class IV support and excavation

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

- Asked in: 2078 Poush, 4.5 marks; 2076 Baisakh, 3 marks (asked 2 times)
- 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_4/engineering_geology_ii/rmr-class-iv-support
- Whole chapter: https://www.bctengineer.com/md/semester_4/engineering_geology_ii/chapter-6

## Question

What are the support system and excavation methods of rock class IV according to the RMR system?

## Short answer

Poor rock (Class IV, RMR 21–40): top heading and bench, 1.0–1.5 m advance, immediate concurrent support. Systematic bolts 4–5 m at 1–1.5 m spacing, wire mesh in crown and walls, shotcrete 100–150 mm crown, 100 mm sides, and light to medium steel ribs at 1.5 m spacing if required.

## Answer

Class IV is **poor rock** with RMR 21 to 40 (cohesion 100 to 200 kPa, friction angle 15° to 25°, stand-up time about 10 hours for a 2.5 m span).

#### Excavation method
- **Top heading and bench**.
- Advance of **1.0 to 1.5 m** in the top heading.
- Install support **concurrently with excavation**, and complete it within **10 m** of the face.

#### Support system (10 m span tunnel)
| Element | Recommendation |
|---|---|
| Rock bolts (20 mm, fully grouted) | Systematic, 4 to 5 m long, spaced 1 to 1.5 m in crown and walls |
| Wire mesh | In crown and walls |
| Shotcrete | 100 to 150 mm in the crown and 100 mm in the sides |
| Steel sets | Light to medium ribs spaced 1.5 m where required |

```text
        ___________________
      /  shotcrete 100-150  \
     |  bolts 4-5 m @1-1.5 m |
     |   light-medium ribs   |
     |_______________________|
```

Because the rock stands only a few hours, short rounds and immediate shotcrete are needed, and the invert may need support if the floor is weak.

## Check yourself

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

1. What is Class IV?
   A. Fair rock, RMR 41-60
   B. Poor rock, RMR 21-40
   C. Very poor rock, below 20
   D. Good rock, 61-80
   Answer: B. Class IV is poor rock with stand-up time about 10 hours for 2.5 m span. (Key point: Class IV identity)

2. What excavation method and advance suits Class IV?
   A. Full face 1.0-1.5 m
   B. Top heading and bench, advance 1.5-3 m
   C. Top heading and bench, advance 1.0 to 1.5 m, with support concurrent with excavation
   D. Multiple drifts, 0.5-1.0 m
   Answer: C. Short rounds in top heading and bench with support within 10 m of the face. (Key point: Excavation)

3. Which support is recommended?
   A. 3 m bolts at 2.5 m, 50 mm shotcrete
   B. Systematic 4-5 m bolts at 1-1.5 m, shotcrete 100-150 mm crown and 100 mm sides
   C. No bolts, 20 mm shotcrete
   D. Heavy ribs at 0.75 m only
   Answer: B. Class IV needs systematic bolts, thick shotcrete and light-medium ribs where required. (Key point: Bolts and shotcrete)

4. Why are short rounds and immediate shotcrete needed?
   A. To save cost
   B. Because the rock stands only a few hours
   C. Because the rock is very strong
   D. Because water is absent
   Answer: B. Stand-up time is only about 10 hours so the rock must be supported quickly. (Key point: Reason for short rounds)

## Close variants set in other exams

- [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×
- [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