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

# RMR poor rock support and excavation

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

- Asked in: 2075 Baisakh, 3+3 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_4/engineering_geology_ii/rmr-poor-class-support
- Whole chapter: https://www.bctengineer.com/md/semester_4/engineering_geology_ii/chapter-6

## Question

What are the support system and excavation method of poor rock class according to the RMR system? Mention.

## Short answer

Poor rock (Class IV) needs systematic bolts 4–5 m long at1–1.5 m spacing with mesh, 100–150 mm crown shotcrete, 100 mm side shotcrete, and light ribs at1.5 m if needed. Excavation uses top heading and bench with1–1.5 m advance, and support is completed within10 m of the face.

## Answer

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

#### Support system (10 m span tunnel)
- **Rock bolts:** systematic, 4 to 5 m long, spaced 1 to 1.5 m in crown and walls, with wire mesh.
- **Shotcrete:** 100 to 150 mm in the crown and 100 mm in the sides.
- **Steel ribs:** light to medium ribs at 1.5 m spacing where required.

#### Excavation method
- Top heading and bench.
- Advance of 1.0 to 1.5 m in the top heading.
- Support installed together with excavation and completed within 10 m of the face.

Short rounds and quick support are needed because the rock deteriorates fast after excavation.

## Check yourself

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

1. Poor rock under RMR is which class?
   A. Class III
   B. Class V
   C. Class II
   D. Class IV (RMR 21-40)
   Answer: D. Poor rock is Class IV. (Key point: Class)

2. What bolts are used in poor rock?
   A. Systematic 4 to 5 m long at 1 to 1.5 m spacing with wire mesh
   B. Spot bolts only
   C. 3 m at 2.5 m
   D. None
   Answer: A. Systematic, longer and closer bolts are used. (Key point: Bolting)

3. What shotcrete and steel are used?
   A. 50 mm shotcrete, no steel
   B. 100-150 mm crown and 100 mm sides, light to medium ribs at 1.5 m where required
   C. 150-200 mm, heavy ribs at 0.75 m
   D. No shotcrete
   Answer: B. Thick shotcrete with light-medium ribs where required. (Key point: Shotcrete and ribs)

## Close variants set in other exams

- [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 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 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 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