Tunnel ventilation methods; bridge site factors

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.

 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.