---
title: "Rock classes needing steel rib and shotcrete"
subject: "Engineering Geology II"
chapter: 6
key: "semester_4/engineering_geology_ii:rmr-steel-rib-classes"
source: https://www.bctengineer.com/q/semester_4/engineering_geology_ii/rmr-steel-rib-classes
site: BCT Engineering
---

# Rock classes needing steel rib and shotcrete

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

- Asked in: 2071 Magh, 3 marks
- Video: [NATM Design Secret: Calculating RMR Like a Pro I #nwr #geology #natm #geologist #rmr #gis](https://www.youtube.com/watch?v=hjr7a9ESGPg) — Peaceful.07 and Engineer Geologist
- 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-steel-rib-classes
- Whole chapter: https://www.bctengineer.com/md/semester_4/engineering_geology_ii/chapter-6

## Question

According to the RMR system, which rock mass classes require steel rib and thick shotcrete for support of underground excavation?

## Short answer

Steel ribs and thick shotcrete are required for Class IV (Poor) and Class V (Very poor). Class IV uses light to medium ribs at1.5 m spacing where necessary and 100–150 mm shotcrete; Class V demands medium to heavy ribs at0.75 m and150–200 mm shotcrete applied immediately.

## Answer

According to the RMR system (Bieniawski, 1989) the classes needing steel ribs together with shotcrete are the two weakest:

| Class | RMR | Steel ribs | Shotcrete |
|---|---|---|---|
| IV (Poor) | 21 to 40 | Light to medium ribs at 1.5 m spacing, where required | 100 to 150 mm crown, 100 mm sides |
| V (Very poor) | below 20 | Medium to heavy ribs at 0.75 m with steel lagging and forepoling if required; close the invert | 150 to 200 mm crown, 150 mm sides, 50 mm on the face |

**Class V** definitely needs steel ribs and thick shotcrete, applied immediately after blasting. For **Class IV** ribs are used where required (weak zones, high water or squeezing), with thick shotcrete. Classes I to III do not need steel sets.

## Check yourself

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

1. Which classes need steel ribs with shotcrete?
   A. Classes I and II
   B. Classes II and III
   C. Classes IV and V
   D. Only Class I
   Answer: C. The two weakest classes need ribs. (Key point: Classes needing ribs)

2. What ribs are specified in Class V?
   A. Light ribs at 1.5 m
   B. Medium to heavy ribs at 0.75 m with lagging and forepoling if required; invert closed
   C. No ribs
   D. Spot ribs at 3 m
   Answer: B. Very poor rock requires heavy close-spaced ribs. (Key point: Class V ribs)

3. How are ribs used in Class IV?
   A. Always at 0.75 m
   B. Never
   C. Light to medium at 1.5 m where required, e.g. weak zones, water or squeezing
   D. Only on the invert
   Answer: C. They are used where required. (Key point: Class IV ribs)

## 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 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 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×
- [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×
- [Role of RMR in underground design](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Armr-importance-underground) — Engineering Geology II, chapter 6, asked 2×

## Related in other subjects

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- [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×
- [Underground RCC tank: brickwork, steel (fig a)](https://www.bctengineer.com/question.md?key=semester_7%2Festimating_and_costing%3Aug-rcc-tank-a) — Estimating and Costing, chapter 6