---
title: "Types of tunnel supports and necessity"
subject: "Hydropower Engineering"
chapter: 6
key: "semester_7/hydropower_engineering:tunnel-supports-types-necessity"
source: https://www.bctengineer.com/q/semester_7/hydropower_engineering/tunnel-supports-types-necessity
site: BCT Engineering
---

# Types of tunnel supports and necessity

Hydropower Engineering · Chapter 6: Water Conveyance Structures

- Asked in: 2070 Ashad, 3+1 marks
- Video: [CRHTX-31-Tunnel Support Design in Fault Zone in Hydropower Project in the Himalaya: A Case Study](https://www.youtube.com/watch?v=ygEjhptCAmc) — CRHT TTL KU
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/hydropower_engineering/tunnel-supports-types-necessity
- Whole chapter: https://www.bctengineer.com/md/semester_7/hydropower_engineering/chapter-6

## Question

Discuss with sketches the types of tunnel supports and their necessity.

## Short answer

Types of tunnel supports: rock bolts (grouted/friction), shotcrete with mesh/fibres, steel ribs with lagging, cast-in-place concrete lining, wire mesh, and pre-support like forepoling/pipe-roof. Their necessity is to stabilize the opening, prevent rock fall, control deformation and water inflow, and carry load until permanent lining is installed; selection depends on rock quality.

## Answer

#### Types of tunnel supports
```text
  Rock bolts          Steel rib          Shotcrete + mesh
  \  |  /          ___________            ___________
 --\ | /--        /  ribs +   \          / sprayed  \
  /-+-+-\        |  lagging    |        | concrete    |
 /  | |  \       |             |        |   layer     |
```
1. **Rock bolts**: steel bars (fully grouted or friction type) 2-5 m long, installed in a pattern. They bind the loose blocks into the sound rock and form a reinforced ring.
2. **Shotcrete**: sprayed concrete (with mesh or fibres) that seals and supports the surface.
3. **Steel ribs/arches**: I-section or lattice girders, with lagging (timber or steel plates) or shotcrete in between; used for weak rock and fault zones; closed rings used for squeezing ground.
4. **Concrete lining** (plain or reinforced; cast-in-place or precast): the permanent support.
5. **Wire mesh** and **forepoling / spiling / pipe-roof** (pre-support) ahead of the face in very weak ground.
6. **Timber supports** for small, temporary works (rarely in large tunnels).

#### Necessity of supports
They keep the opening stable and safe, prevent fall of rock, control deformation and water inflow, and carry the load until the permanent lining is placed. The type is selected by the rock quality (Q-system/RMR): sound rock needs little or no support, jointed rock needs bolts and shotcrete, and poor rock needs ribs and pre-support.

## Check yourself

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

1. How do rock bolts support a tunnel?
   A. Replace the lining
   B. Bind loose blocks into sound rock and form a reinforced ring
   C. Only measure deformation
   D. Seal water
   Answer: B. Bolts anchor loose blocks to sound rock. (Key point: Rock bolts)

2. Where are steel ribs with lagging used?
   A. Never in hydropower
   B. Sound rock
   C. Only in portals of small canals
   D. Weak rock and fault zones; closed rings in squeezing ground
   Answer: D. Ribs give strong support in poor rock. (Key point: Steel ribs)

3. What is forepoling or pipe-roof?
   A. A blasting pattern
   B. Support installed ahead of the face in very weak ground
   C. A drainage hole
   D. Final lining
   Answer: B. Pre-support spiles are driven ahead of the face before excavation. (Key point: Pre-support)

4. Which match of rock and support is right?
   A. Jointed rock: pipe roof only
   B. Sound rock: little or none; jointed: bolts and shotcrete; poor: ribs and pre-support
   C. Sound rock: ribs; poor: nothing
   D. All rock: concrete lining only
   Answer: B. Support intensity increases as rock quality falls (Q/RMR). (Key point: Support by rock quality)

## Related in Hydropower Engineering

- [Tunnelling methods and supports](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Atunnelling-methods-supports-realized) — Hydropower Engineering, chapter 6
- [Tunnel shapes: advantages and disadvantages](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Atunnel-shapes-advantages) — Hydropower Engineering, chapter 6, asked 2×

## Related in other subjects

- [Types of supports and their reactions](https://www.bctengineer.com/question.md?key=semester_3%2Fstrength_of_materials%3Atypes-of-supports-reactions) — Strength of Materials, chapter 1
- [Necessity and types of shoring](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Ashoring-necessity-types) — Building Technology, chapter 8, asked 2×