---
title: "Tunnel ventilation methods; bridge site factors"
subject: "Transportation Engineering II"
chapter: 5
key: "semester_7/transportation_engineering_ii:tunnel-ventilation-methods-bridge-site-factors"
source: https://www.bctengineer.com/q/semester_7/transportation_engineering_ii/tunnel-ventilation-methods-bridge-site-factors
site: BCT Engineering
---

# Tunnel ventilation methods; bridge site factors

Transportation Engineering II · Chapter 5: Introduction to Bridge and Tunnel Engineering

- Asked in: 2071 Chaitra, 8 marks
- Video: [Tunnel Engineering | Ventilation in tunnel | Methods of ventilation in tunnel | Natural ventilation](https://www.youtube.com/watch?v=2tRN9ePe84o) — Digitech education
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/transportation_engineering_ii/tunnel-ventilation-methods-bridge-site-factors
- Whole chapter: https://www.bctengineer.com/md/semester_7/transportation_engineering_ii/chapter-5

## Question

What are the methods of providing tunnel ventilation? Explain the major factors controlling the selection of bridge sites.

## Short answer

Tunnel ventilation is natural (piston effect) or mechanical: longitudinal jet fans, semi-transverse, fully transverse, and shaft ventilation. Bridge site selection considers stable straight river reach, adequate waterway for design flood, firm foundation below scour, square alignment, navigation clearance, and economy.

## Answer

#### Methods of tunnel ventilation

Vehicles in a tunnel emit CO, NOx, hydrocarbons and smoke, and blasting and diesel plant add dust and fumes during construction. Ventilation supplies fresh air and dilutes or removes pollutants, keeping visibility and air quality within safe limits (IRC:SP:91 and PIARC give limits, e.g. CO of about 70–100 ppm for normal traffic, and visibility limits for smoke).

**Natural ventilation**
- Relies on the **piston effect** of moving vehicles and on wind and temperature differences between the portals.
- Suitable only for short tunnels (below about 300–500 m) with low traffic; it is not dependable.

**Mechanical (artificial) ventilation**
1. **Longitudinal system:** jet fans hung from the roof push air along the tunnel from one portal to the other. It is simple and economical, and suits one-way tunnels of moderate length.
2. **Semi-transverse system:** fresh air is supplied through a duct along the tunnel and escapes through the portals (or the reverse: an exhaust duct, with fresh air entering at the portals). Used for medium to long tunnels.
3. **Fully transverse system:** separate supply and exhaust ducts; fresh air is supplied uniformly along the length and polluted air is extracted. It gives the best control but is costly; used for long tunnels with heavy traffic, and it also helps with smoke control during a fire.
4. **Shaft ventilation:** vertical or inclined shafts with fans in long tunnels, to remove exhaust at intermediate points.
5. **Ventilation during construction:** forced (blowing) or exhaust ventilation with flexible ducts and fans, often a combined (push-pull) system.

```text
 Longitudinal:  ->J- J- J- J->   (jet fans, air flows end to end)
 Semi-transverse: [supply duct] ==> air leaves at portals
 Transverse:    [supply duct]
                 --- tunnel ---
                [exhaust duct]
```
The choice of system depends on the length, traffic volume, gradient, number of lanes and cost; CO and visibility sensors control fan operation.

#### Factors controlling bridge site selection

A bridge site is chosen so that the bridge is safe and economical, and serves the road alignment and the river with the least disturbance. The factors to consider (IRC:5, IRC:SP:13 and Nepal DoR bridge practice) are:

1. **Stream/river behaviour:** a straight, stable reach with a well-defined, non-meandering channel; avoid bends, confluences, and braided or aggrading reaches.
2. **Waterway and flood data:** a narrow, stable crossing with high, firm banks; adequate waterway for the design flood (normally the 100-year flood as per IRC:5; 50-year for small bridges and culverts as per IRC:SP:13); reliable HFL, discharge and velocity records.
3. **Foundation conditions:** firm strata or rock at a reasonable depth; the scour depth (Lacey's regime theory, IRC:78) must give an affordable foundation depth; avoid soft, liquefiable or landslide-prone soils.
4. **Approaches and alignment:** the bridge should be square (normal) to the river flow; the approach roads should be straight, of limited height and not on unstable slopes. For major bridges the road alignment is adjusted to suit the best bridge site, while for minor bridges the bridge follows the road alignment.
5. **Navigation and clearance:** vertical and horizontal clearance for boats, floating logs or boulders; freeboard above HFL (minimum 0.6 m below the girders; more in Nepal's boulder-carrying rivers).
6. **Economy:** total cost of the bridge, approaches and river training works; nearness of construction materials and labour, and access for equipment.
7. **Geology and seismicity:** avoid active faults and steep gorges with rockfall; apply the seismic provisions of NBC 105 (Nepal) and IRC:6.
8. **Existing structures and utilities:** distance from other bridges, dams and weirs (backwater or draw-down effects), settlements, and land acquisition.
9. **Environmental and social aspects:** minimum afflux, no damage to property upstream, aesthetics, and environmental clearance requirements.
10. **Construction convenience:** space for temporary works and diversions, and a low-flow period for working.

## Check yourself

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

1. In a longitudinal tunnel ventilation system, how is air moved?
   A. Jet fans hung from the roof push air along the tunnel from one portal to the other
   B. A supply duct and an exhaust duct run along the whole length
   C. Vertical shafts extract air at the middle only
   D. Air moves only by the temperature difference between portals
   Answer: A. Longitudinal systems use roof-mounted jet fans to drive air end to end; it is simple and economical for unidirectional tunnels of moderate length. (Key point: Longitudinal system)

2. Natural ventilation of a tunnel is suitable only for:
   A. Long tunnels with heavy traffic
   B. Short tunnels (below about 300-500 m) with low traffic
   C. Tunnels during construction only
   D. Tunnels with fire hazards
   Answer: B. It relies on the piston effect, wind and temperature difference, which is not dependable, so it suits only short, low-traffic tunnels. (Key point: Natural ventilation)

3. Which system has separate supply and exhaust ducts and gives the best control, including smoke control in fire?
   A. Longitudinal
   B. Semi-transverse
   C. Fully transverse
   D. Natural
   Answer: C. A fully transverse system supplies fresh air uniformly and extracts polluted air; it is costly, so used for long, heavily trafficked tunnels. (Key point: Fully transverse system)

4. In a semi-transverse system:
   A. Fresh air is supplied through a duct and escapes through the portals (or the reverse)
   B. Jet fans alone push air end to end
   C. There are two full ducts, supply and exhaust
   D. Ventilation is provided only by the vehicles' movement
   Answer: A. Only one duct (supply or exhaust) is provided and air leaves or enters via the portals. (Key point: Semi-transverse system)

## Close variants set in other exams

- [Tunnel ventilation methods; bridge site selection factors](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Atunnel-ventilation-methods-bridge-site-selection) — Transportation Engineering II, chapter 5, asked 2×

## Related in Transportation Engineering II

- [Bridge site factors; importance of tunnel ventilation](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Abridge-site-factors-tunnel-ventilation-importance) — Transportation Engineering II, chapter 5
- [Ideal bridge site; tunnel ventilation need and methods](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Aideal-bridge-site-tunnel-ventilation) — Transportation Engineering II, chapter 5
- [Bridge site factors and tunnel drainage](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Abridge-site-factors-tunnel-drainage) — Transportation Engineering II, chapter 5
- [Bridge components; tunnel drainage and ventilation](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Abridge-components-tunnel-drainage-ventilation) — Transportation Engineering II, chapter 5
- [Bridge components; tunnel drainage and ventilation](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Abridge-components-tunnel-drainage-ventilation-problems) — Transportation Engineering II, chapter 5
- [Bridge site factors and components with sketches](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Abridge-site-factors-components-sketches) — Transportation Engineering II, chapter 5

## Related in other subjects

- [Geology in tunnel site selection](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Atunnel-site-selection) — Engineering Geology II, chapter 4, asked 4×
- [Geology in bridge site selection](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Abridge-site-selection) — Engineering Geology II, chapter 4, asked 6×
- [Reservoir intake and site selection factors](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Areservoir-intake-site-factors) — Water Supply Engineering, chapter 5, asked 2×
- [Distribution substation site selection factors](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Asubstation-site-location-factors) — Industrial Power Distribution and Illumination, chapter 4