---
title: "Selecting support type for underground opening"
subject: "Engineering Geology II"
chapter: 6
key: "semester_4/engineering_geology_ii:support-type-underground"
source: https://www.bctengineer.com/q/semester_4/engineering_geology_ii/support-type-underground
site: BCT Engineering
---

# Selecting support type for underground opening

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

- Asked in: 2075 Baisakh, 4 marks
- Video: [Exam Preparation | Geosynthetics, TBM, RMR System, Q System in Tunnelling | Engineering Geology](https://www.youtube.com/watch?v=R2Zay5MRoFo) — CiviL TechnO BrickS
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/engineering_geology_ii/support-type-underground
- Whole chapter: https://www.bctengineer.com/md/semester_4/engineering_geology_ii/chapter-6

## Question

How do you select the support type for an underground opening? Describe with justification.

## Short answer

Support selection is based on rock mass classification (RMR or Q) and opening span, using design charts and site conditions. Good rock is self-supporting with spot bolts, while poor rock needs immediate systematic bolting, thick shotcrete, and steel ribs to prevent loosening.

## Answer

The support type for an underground opening is selected from the quality of the rock mass, the size of the opening and the expected loads, using classification systems and site conditions.

#### Steps of selection
1. **Investigate and classify:** map the rock, estimate RQD, joint sets, strength and groundwater, and determine RMR and/or Q for each tunnel section.
2. **Find the span and use:** the larger the span (or equivalent dimension $D_e = \text{span}/ESR$), the heavier the support. A permanent hydropower tunnel needs more safety than a temporary adit.
3. **Use the charts and tables:** RMR class guidelines (bolts, shotcrete, ribs) or the Barton Q-chart.
4. **Check special conditions:** high stress, squeezing or swelling ground, running water, fault zones, joint orientation relative to the axis.

#### Typical selection (RMR)
| Class | Support |
|---|---|
| I | None or spot bolts |
| II | Local bolts, 50 mm shotcrete |
| III | Systematic bolts, mesh, 50 to 100 mm shotcrete |
| IV | Bolts, 100 to 150 mm shotcrete, light steel ribs |
| V | Heavy steel ribs, thick shotcrete, forepoling, closed invert |

#### Justification
- Good rock carries itself, so only local support is economical.
- Poor and very poor rock loosens rapidly and needs immediate and stiff support (shotcrete and ribs) to prevent collapse.
- Bolts reinforce the rock so it supports itself, shotcrete seals the surface, and ribs carry the loose rock load.
- The final choice is checked and adjusted by observation and monitoring during construction.

## Check yourself

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

1. What is the first step in selecting support?
   A. Ordering shotcrete
   B. Investigating and classifying the rock mass (RMR and/or Q)
   C. Choosing steel ribs
   D. Closing the invert
   Answer: B. Classification gives a basis for the choice. (Key point: First step)

2. How does span affect support?
   A. Larger span (De = span/ESR) needs heavier support
   B. Larger span needs less
   C. No effect
   D. Only height matters
   Answer: A. Support increases with span or equivalent dimension. (Key point: Span)

3. Why is only local support used in good rock?
   A. It is weak
   B. Water is high
   C. Good rock carries itself so only local support is economical
   D. Steel is unavailable
   Answer: C. Heavy support is wasteful in self-supporting rock. (Key point: Justification for good rock)

4. What does shotcrete do?
   A. Carries the loose rock load
   B. Reinforces rock internally
   C. Drains water
   D. Seals the surface; bolts reinforce, ribs carry loose rock
   Answer: D. Bolts reinforce, shotcrete seals, and ribs carry loosened rock. (Key point: Role of elements)

## Related in Engineering Geology II

- [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×

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