---
title: "Tunnelling methods and supports"
subject: "Hydropower Engineering"
chapter: 6
key: "semester_7/hydropower_engineering:tunnelling-methods-supports-realized"
source: https://www.bctengineer.com/q/semester_7/hydropower_engineering/tunnelling-methods-supports-realized
site: BCT Engineering
---

# Tunnelling methods and supports

Hydropower Engineering · Chapter 6: Water Conveyance Structures

- Asked in: 2072 Chaitra, 4+2+2 marks
- Video: [Exploring Hydraulic Tunnels: Shapes, Design, Methods, and Support Structures ⛏️🌊 #tunnel #design#tbm](https://www.youtube.com/watch?v=pI254zTUVE8) — Hydropower
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/hydropower_engineering/tunnelling-methods-supports-realized
- Whole chapter: https://www.bctengineer.com/md/semester_7/hydropower_engineering/chapter-6

## Question

Discuss various tunnelling methods used in hydropower projects. Why do you provide tunnel supports? How are they realized?

## Short answer

Tunnel methods include drill-and-blast, TBM, NATM, and mechanical excavation. Supports prevent rock fall, control deformation, and stop water inflow, protecting workers and machines. They are realized by scaling, immediate shotcrete with mesh/fibres, rock bolts, steel ribs with lagging, pre-support in weak zones, and final concrete lining, all selected based on rock classification.

## Answer

#### Tunnelling methods in hydropower projects
1. **Drill and blast (conventional method)**: the most common method in Nepal. The cycle is drilling holes, charging with explosives, blasting, ventilation, mucking (removal of rock), scaling and installation of support. Large sections are excavated in stages: top heading then benching.
2. **Tunnel boring machine (TBM)**: a rotary cutter-head that excavates the full circular section and installs the precast segments or supports; fast and gives a smooth surface, but costly and not suitable in squeezing or highly variable ground. Used for long, uniform tunnels.
3. **NATM / sprayed-concrete method (New Austrian Tunnelling Method)**: the excavation is supported at once by shotcrete, rock bolts and mesh, and the rock itself carries the load; the deformation is monitored.
4. **Mechanical excavation (road-header)** in soft rock, and **cut-and-cover** for shallow sections.
5. Adits and shafts are used to open extra faces; **raise boring** is used for shafts.

#### Why tunnel supports are provided
- To prevent the fall of rock and collapse of the opening, and so protect workers and machines.
- To control the deformation and the stress redistribution, particularly in weak, jointed or squeezing rock and fault zones.
- To stop the loosening of the rock and the inflow of water.
- To provide a base for the final lining.

#### How the supports are realized
1. **Scaling** of loose rock after each blast.
2. **Shotcrete** (5-15 cm) with wire mesh or fibre is sprayed on the crown and walls straight after excavation.
3. **Rock bolts** (grouted or friction), 2-4 m long at 1-2 m spacing, are installed through the shotcrete to bind the rock.
4. **Steel ribs/arches (I-sections or lattice girders)** with lagging in poor rock, installed close to the face and encased in shotcrete.
5. **Pre-support** (spiles, forepoling, pipe umbrella) in very weak ground and fault zones, ahead of the face.
6. **Final concrete lining** (and steel lining for pressure sections) after the excavation is finished, followed by contact and consolidation grouting.

The type and amount of support follow the rock mass classification (Q-system, RMR) and are adapted to the rock seen at the face.

## Check yourself

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

1. Which is the correct sequence of the drill and blast cycle?
   A. Drill, charge, blast, ventilate, muck, scale, support
   B. Support, drill, blast, scale
   C. Blast, drill, support, muck
   D. Muck, charge, drill, blast
   Answer: A. After blasting the fumes are ventilated, the rock mucked, scaled and then supported. (Key point: Drill and blast cycle)

2. When is a TBM unsuitable?
   A. Long uniform tunnels
   B. Squeezing or highly variable ground
   C. Where smooth surface is needed
   D. Circular sections
   Answer: B. TBMs are costly and do not suit squeezing or highly variable ground. (Key point: TBM limitation)

3. What is the principle of NATM?
   A. Excavate with a cutter-head only
   B. Fill the tunnel with timber
   C. Support immediately with shotcrete, bolts and mesh so that the rock carries the load, monitoring deformation
   D. Rely only on thick final lining
   Answer: C. NATM lets the rock itself share the load, with monitored deformation. (Key point: NATM principle)

4. Which is a reason for tunnel supports?
   A. To reduce the discharge
   B. To prevent rock fall and control deformation and inflow, and give a base for final lining
   C. To increase tunnel velocity
   D. To avoid grouting
   Answer: B. Supports keep the opening stable and safe. (Key point: Why supports)

5. How is the type and amount of support decided?
   A. By contractor preference
   B. By rock mass classification (Q, RMR) and the rock seen at the face
   C. Fixed by tunnel length
   D. By discharge
   Answer: B. Support follows rock classification and is adapted to the rock found. (Key point: Support selection)

## Related in Hydropower Engineering

- [Types of tunnel supports and necessity](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Atunnel-supports-types-necessity) — Hydropower Engineering, chapter 6
- [Tunnelling methods; shotcreting](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Atunnelling-methods-shotcrete) — Hydropower Engineering, chapter 6
- [Optimization of hydraulic tunnel diameter](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Atunnel-optimization-diameter) — Hydropower Engineering, chapter 6, asked 2×
- [Hydraulic design of tunnel; alignment criteria](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Atunnel-hydraulic-design-alignment-selection) — 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×
- [Surge numerical: 4 m tunnel, 4000 m, 20 m³/s](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Asurge-numerical-tunnel-4m-4000m-20cumec) — Hydropower Engineering, chapter 6, asked 2×

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- [RA: projects where all used bolts size 6](https://www.bctengineer.com/question.md?key=semester_6%2Fdatabase_management_system%3Ara-projects-all-used-bolts) — Database Management System, chapter 3
- [Equipment used in hydropower construction](https://www.bctengineer.com/question.md?key=semester_8%2Fconstruction_management%3Ahydropower-equipment) — Construction Management, chapter 4
- [Tunnelling and rock drilling equipment](https://www.bctengineer.com/question.md?key=semester_8%2Fconstruction_management%3Atunnel-rock-drilling-equipment) — Construction Management, chapter 4, asked 3×