---
title: "Criteria for designing and laying out tunnels"
subject: "Hydro Power"
chapter: 5
key: "semester_6/hydro_power:hydraulic-tunnel-design-layout"
source: https://www.bctengineer.com/q/semester_6/hydro_power/hydraulic-tunnel-design-layout
site: BCT Engineering
---

# Criteria for designing and laying out tunnels

Hydro Power · Chapter 5: Components of Hydropower System

- Asked in: 2071 Bhadra, 6 marks; 2070 Magh, 3+3 marks (asked 2 times)
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/hydro_power/hydraulic-tunnel-design-layout
- Whole chapter: https://www.bctengineer.com/md/semester_6/hydro_power/chapter-5

## Question

What criterias do you consider while designing and laying out a hydraulic tunnel? Explain.

## Short answer

Layout criteria: shortest route through sound rock with adequate cover (C_RM = h γw F/(γr cosβ)) to prevent hydraulic jacking, straight alignment, gentle curves. Design: pressure or free-flow type, economic diameter, velocity 1.5-4 m/s, circular/horseshoe shape, support as per rock class, surge tank for pressure tunnels.

## Answer

A **hydraulic tunnel** is an underground water conductor (headrace, tailrace, diversion or pressure tunnel) used where a surface canal is not feasible because of steep, unstable terrain or to shorten the route. Its layout and design must satisfy geological, hydraulic, structural and economic criteria.

#### Layout (alignment) criteria
1. **Shortest practical route** between intake and surge tank/powerhouse, to reduce cost and head loss.
2. **Good geology:** pass through sound, uniform rock; avoid faults, shear zones, thrusts, weak and squeezing rock, karst and water-bearing zones. Cross weak zones at right angles.
3. **Adequate rock cover:** for unlined pressure tunnels, the weight of the rock above must exceed the internal water pressure. Norwegian criterion: minimum rock cover $C_{RM} = \dfrac{h\gamma_w F}{\gamma_r\cos\beta}$, where $h$ = static water head at that point, $\gamma_r$ = unit weight of rock, $\beta$ = slope angle of the valley side and $F$ ≈ 1.3–1.5 (factor of safety).
4. **Stable portals and adits:** portals in sound rock, away from landslides and flood levels; adits to give more working faces and reduce construction time.
5. **Straight alignment** with gentle curves (radius ≥ 5 times the diameter) to reduce head loss.
6. **Gradient:** enough slope for drainage during construction (about 1:200–1:500); free-flow tunnels follow the hydraulic gradient.

#### Design criteria
1. **Type of flow:** pressure tunnel (full flow, circular or horseshoe) or free-flow tunnel (D-shaped/horseshoe, flowing 70–80% full with free-board area of about 15–25%).
2. **Size:** economic diameter found by comparing excavation/lining cost with value of head loss; minimum size about 2–2.5 m for construction access.
3. **Velocity:** unlined 1.5–2.5 m/s; concrete-lined 2–4 m/s; steel-lined higher. Velocities must avoid erosion but limit head loss.
4. **Shape:** circular best for internal pressure; horseshoe/inverted-D easier to excavate by drill and blast and good for external rock pressure.
5. **Lining and support:** shotcrete, rock bolts, steel ribs, concrete or steel lining depending on rock class (Q-system, RMR), internal pressure and leakage control. Smooth lining reduces Manning's n.
6. **Head loss:** friction and form losses kept within economic limits.
7. **Water hammer and surge:** pressure tunnels need a surge tank; check minimum pressure so the crown never goes under negative pressure.
8. **Sediment and rock traps** near the end of unlined tunnels.
9. **Drainage, grouting and access** for inspection and maintenance.

## Check yourself

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

1. How should a tunnel alignment deal with weak zones such as faults and shear zones?
   A. Avoid them where possible and cross them at right angles
   B. Run parallel inside them
   C. Align through the centre of the weak zone
   D. Ignore them if lined
   Answer: A. Sound uniform rock is preferred; unavoidable weak zones are crossed at right angles to shorten exposure. (Key point: Geology in alignment)

2. For an unlined pressure tunnel, the rock cover must be sufficient so that:
   A. Excavation is cheaper
   B. The tunnel is shallow
   C. Rock weight above exceeds internal water pressure (with a factor of safety)
   D. The velocity is lower
   Answer: C. The Norwegian criterion ensures rock weight resists uplift from internal pressure, with F about 1.3-1.5. (Key point: Rock cover)

3. Which shape is best for internal pressure?
   A. Circular
   B. Inverted D
   C. Rectangular
   D. Horseshoe
   Answer: A. A circular section carries internal pressure most efficiently; horseshoe/D shapes are easier to excavate. (Key point: Shape choice)

4. Why does a pressure tunnel need a surge tank?
   A. To give drainage during construction
   B. To increase velocity
   C. To handle water hammer and surges, and check that crown pressure never goes negative
   D. To cut rock cover
   Answer: C. A surge tank relieves water hammer from load changes; minimum pressure must stay positive at the crown. (Key point: Surge protection)

## Related in Hydro Power

- [Hydraulic tunnels and design features](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Ahydraulic-tunnel-design-features) — Hydro Power, chapter 5
- [Selection criteria of hydraulic turbines](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Aturbine-selection-criteria) — Hydro Power, chapter 7, asked 2×
- [Economic diameter of tunnel](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Aeconomic-diameter-tunnel) — Hydro Power, chapter 5
- [Draft tube function and turbine selection criteria](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Adraft-tube-turbine-selection) — Hydro Power, chapter 7

## Related in other subjects

- [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
- [Principles of laying out water supply system](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Awater-supply-principles) — Building Technology, chapter 12
- [Transition deflection angle derivation and laying out](https://www.bctengineer.com/question.md?key=semester_4%2Fsurveying_ii%3Atransition-deflection-angle-derive-laying-out) — Surveying II, 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×