Ideal bridge site; tunnel ventilation need and methods

Characteristics of an ideal bridge site

An ideal bridge site has the following features:

1. Straight, narrow, stable river reach - well-defined banks, parallel flow, stable regime (no meandering, braiding, heavy aggradation or degradation), and no bends, confluence or islands close to the site.
2. Firm foundation - rock or hard strata at a reasonable depth, so that foundations are economical and safe against scour.
3. High, stable banks - above the highest flood level (HFL) so that long high approaches are avoided; no landslides on the banks.
4. Right-angled crossing - the bridge axis normal to the direction of flow, avoiding skew.
5. Good approaches - straight alignment, easy gradients and minimum cut and fill; approach roads should not be flooded.
6. Minimum waterway length and obstruction to flow - small afflux and low scour, with adequate linear waterway.
7. Construction facilities - availability of materials, labour, space for stacking, access to the site, and nearness to the road network and population.
8. Economy - least total cost of bridge, approaches and training works.
9. Free from other limitations - no hindrance from existing utilities or from land acquisition, and in Nepal, away from active landslides, gullies and debris flow paths.
10. Suitable for river training - banks that can be protected economically.

Need and methods of ventilation in tunnels

Need for good ventilation in tunnels

• To supply fresh air (at least 19.5 percent oxygen) to workers and drivers.
• To remove poisonous and dangerous gases: blasting fumes (CO, NOx), diesel and vehicle exhaust (CO, NOx, hydrocarbons), and natural gases such as methane, CO2 and H2S found in some formations.
• To remove dust (silica dust from drilling and blasting causes silicosis) and smoke, which reduces visibility.
• To control temperature and humidity, particularly in deep or long tunnels, and to provide safe conditions during a fire (smoke control).
• To meet statutory limits on gas concentrations (for road tunnels in service, CO of about 50-100 ppm).

Methods of ventilation

1. Natural ventilation - uses the piston effect of moving vehicles, wind and temperature (stack) differences; for short tunnels (below about 400 m).
2. Mechanical ventilation during construction -
   • Forcing (blowing) system: fans push fresh air through ducts to the face; the foul air leaves through the tunnel.
   • Exhaust system: fans suck foul air from the face through ducts; fresh air enters along the tunnel.
   • Combined (push-pull) system with booster fans for long tunnels.
   • Water sprays and wet drilling reduce dust.
3. Mechanical ventilation in operation (road tunnels) -
   • Longitudinal: jet fans in the roof push air along the tunnel; simple and economical for tunnels up to about 2-3 km.
   • Semi-transverse: fresh air is supplied through a duct along the length and escapes through the portals.
   • Fully transverse: separate supply and exhaust ducts give uniform ventilation along the length; costly, used for very long and busy tunnels.

 Longitudinal         Transverse
 ->  jet fans ->      supply duct v v v
 =============        ===================
                      exhaust duct ^ ^ ^