Bridge site factors; importance of tunnel ventilation

Factors for selecting the bridge site

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.

Importance of ventilation in tunnel

Ventilation is important in a tunnel because:

• It dilutes carbon monoxide, NOx, hydrocarbons and diesel smoke from vehicles to safe levels, protecting the health of users and workers.
• It maintains visibility by removing smoke and dust, which is essential for safe driving.
• During construction it clears blasting fumes, dust and gases (methane, H2S, CO2), supplies oxygen for workers and machines, and controls heat and humidity.
• It controls temperature and humidity, avoiding condensation and corrosion of equipment.
• In case of fire it controls the spread of smoke and heat and keeps escape routes clear.
• It keeps air quality at the portals and in the surroundings within environmental limits.