Tunnel ventilation methods; bridge site selection 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 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; 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. Simple and economical; suitable for 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. Gives the best control but is costly; used for long tunnels with heavy traffic, and also helps 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 as 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 depends on 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 both the road alignment and the river with the least disturbance. Factors to consider (IRC:5 / IRC:SP:13 and Nepal DoR bridge practice):

1. Stream/river behaviour: a straight, stable reach with a well-defined, non-meandering channel; avoid bends, confluences, and braided and aggrading reaches.
2. Waterway and flood data: a narrow, stable crossing with high banks; adequate waterway for the design flood (usually a 100-year return period for major bridges as per IRC:5 and DoR, 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 foundation must be taken below the scour depth (Lacey's regime theory, IRC:78) at an affordable cost; avoid soft, liquefiable or landslide-prone soils.
4. Approaches and alignment: the bridge should be square (normal) to the river flow; 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 and culverts the bridge follows the road alignment.
5. Navigation and clearance: vertical and horizontal clearance for boats, floating logs or boulders; vertical clearance between HFL and the soffit of 150 mm to 1.5 m depending on the discharge (IRC:5), with more in Nepal's boulder-carrying rivers.
6. Economy: total cost of 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; follow the seismic provisions of NBC 105 and IRC:6.
8. Existing structures and utilities: distance from other bridges, dams and weirs (backwater/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 diversion, and a low-flow period for working.