Introduction to Bridge and Tunnel Engineering

Methods of tunnel ventilation

Vehicles in a tunnel emit CO, NOx, hydrocarbons and smoke, and blasting/diesel plant add dust and fumes during construction. Ventilation supplies fresh air and dilutes/removes pollutants, keeping visibility and health within limits (IRC:SP:91 and PIARC give limits, e.g. CO in the range 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 difference between 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 unidirectional tunnels of moderate length.
2. Semi-transverse system: fresh air is supplied through a duct along the tunnel and escapes out through the portals (or the reverse: exhaust duct with fresh air entering at 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 extracted. Best control but costly; for long tunnels and heavy traffic; also helps in smoke control during 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]

Selection depends on length, traffic volume, gradient, number of lanes and costs; CO and visibility sensors control fan operation.

Factors controlling bridge site selection

A bridge site is chosen so that the bridge is safe, economical and serves the road alignment and the river with 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, braided and aggrading reaches.
2. Waterway and flood data: narrow, stable crossing with high banks; adequate waterway for the design flood (usually 50-year for major bridges, 100-year check flood as per IRC:SP:13 / DoR); reliable HFL, discharge and velocity records.
3. Foundation conditions: firm strata or rock at reasonable depth; scour depth (Lacey's regime theory, IRC:78) must be affordable; 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. Road alignment should not be bent to suit the bridge for major bridges but the opposite for minor ones.
5. Navigation and clearance: vertical and horizontal clearance for boats, floating logs or boulders; freeboard above HFL (minimum 0.6 m for girders; more in Nepal's boulder-carrying rivers).
6. Economy: total cost of bridge, approaches and river training works; nearness of construction material, labour and access for equipment.
7. Geology and seismicity: avoid active faults, steep gorge with rockfall; Nepal Building Code/NBC 105 and IRC:6 seismic provisions.
8. Existing structures and utilities: distance from other bridges, dams, 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, diversion, and low flow period for working.

Methods of tunnel ventilation

Vehicles in a tunnel emit CO, NOx, hydrocarbons and smoke, and blasting/diesel plant add dust and fumes during construction. Ventilation supplies fresh air and dilutes/removes pollutants, keeping visibility and health within limits (IRC:SP:91 and PIARC give limits, e.g. CO in the range 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 difference between 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 unidirectional tunnels of moderate length.
2. Semi-transverse system: fresh air is supplied through a duct along the tunnel and escapes out through the portals (or the reverse: exhaust duct with fresh air entering at 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 extracted. Best control but costly; for long tunnels and heavy traffic; also helps in smoke control during 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]

Selection depends on length, traffic volume, gradient, number of lanes and costs; CO and visibility sensors control fan operation.

Factors controlling bridge site selection

A bridge site is chosen so that the bridge is safe, economical and serves the road alignment and the river with 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, braided and aggrading reaches.
2. Waterway and flood data: narrow, stable crossing with high banks; adequate waterway for the design flood (usually 50-year for major bridges, 100-year check flood as per IRC:SP:13 / DoR); reliable HFL, discharge and velocity records.
3. Foundation conditions: firm strata or rock at reasonable depth; scour depth (Lacey's regime theory, IRC:78) must be affordable; 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. Road alignment should not be bent to suit the bridge for major bridges but the opposite for minor ones.
5. Navigation and clearance: vertical and horizontal clearance for boats, floating logs or boulders; freeboard above HFL (minimum 0.6 m for girders; more in Nepal's boulder-carrying rivers).
6. Economy: total cost of bridge, approaches and river training works; nearness of construction material, labour and access for equipment.
7. Geology and seismicity: avoid active faults, steep gorge with rockfall; Nepal Building Code/NBC 105 and IRC:6 seismic provisions.
8. Existing structures and utilities: distance from other bridges, dams, 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, diversion, and low flow period for working.

Steps in tunnel alignment survey

A tunnel alignment survey fixes the centre line and levels of the tunnel on the ground and then transfers them underground so that headings driven from both ends meet correctly.

1. Reconnaissance - study of topographic maps, aerial photographs and geological information; walk-over survey to select the possible routes and portal locations.
2. Preliminary (route) survey - topographic map of the area at suitable scale; geological and geotechnical investigation (boreholes, geophysical surveys, rock mass rating) along the alternatives; select the best alignment (shortest, stable rock, safe portals, minimum cover problems).
3. Surface (final location) survey - set up a network of triangulation stations or a precise traverse using total station/GPS joining the two portals; mark the centre line by pillars on each side of the hill; measure the baseline precisely; fix reference marks outside the portals.
4. Levelling - precise levelling from benchmarks to the portals and shafts to find the invert levels and gradient (minimum gradient about 1 in 250-500 for drainage); check by double levelling.
5. Transfer of alignment underground - from the portal by theodolite sighting on the two reference pillars; through shafts by two plumb wires or optical/laser plummets, or by gyro-theodolite; then extend the centre line along the heading with laser guidance.
6. Check and control - repeat the survey from both ends, correct closing errors (permissible error in meeting of headings is a few centimetres), and keep checking lines and levels while the tunnel advances.

Features of an ideal bridge location

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.

Methods of tunnelling in hard rock

1. Drill and blast (conventional) method - the cycle is drilling holes (jumbo), charging with explosives, blasting, ventilation, mucking out (loaders and trucks), scaling and rock support. It can be done by:
   • Full-face method - the whole section is excavated at once; suitable for small, good-rock tunnels.
   • Heading and bench method - top heading is advanced first and the bench below is excavated later; common for medium-large sections.
   • Pilot (drift) method - a small centre or side drift is driven first, then widened to full section; used when rock is fractured.
   • Top heading and multiple-drift methods for very large sections or poor rock.
2. Tunnel Boring Machine (TBM) method - a rotating cutter head with disc cutters cuts the rock mechanically; open-gripper TBMs are used in hard rock. Fast and gives smooth walls but needs high capital cost and long straight alignments.
3. NATM / sprayed concrete lining (SEM) - excavate by drill-blast or roadheader and support at once by shotcrete, rock bolts and steel ribs, using the rock itself as the main support.

Types of lining used in tunnels

Lining supports the rock, controls seepage, protects against weathering and gives a smooth surface; in sound hard rock it may be limited to local support.

Major characteristics of an ideal bridge

An ideal bridge is one that serves its purpose safely and economically throughout its life. Its major characteristics (IRC:5 and Nepal DoR bridge standards) are:

1. Safety and structural adequacy - designed for IRC loads (Class A, 70R) and for flood, earthquake (NBC 105), wind and temperature effects with required factors of safety.
2. Serviceability - adequate carriageway width, vertical and horizontal clearance, smooth riding surface and limited deflection and vibration.
3. Economy - minimum total cost including initial cost, maintenance and approaches; use of local materials; standard designs where possible.
4. Adequate waterway and free board - passes the design flood (50 to 100 years) with little afflux and scour; foundation depth below scour level.
5. Durability and low maintenance - good materials, drainage, bearings and expansion joints that need little upkeep.
6. Ease of construction - suitable for the available equipment, labour and construction period; minimum temporary works.
7. Aesthetics - pleasing appearance fitting the surroundings.
8. Environmental compatibility - minimum disturbance to the river and surrounding environment.
9. Suitable site and good approaches, as described under ideal bridge site.

Drainage during tunnelling

Water in the tunnel heading (seepage, groundwater and drilling water) disturbs work and can destabilise rock. It is managed as follows:

1. Gravity drainage - when the tunnel is driven upgrade from the portal (positive gradient 0.2-0.5 percent), water flows back to the portal through a side or central drain, so no pumping is required.
2. Pumping from sumps - when driving downgrade (or in shafts), water collects in sumps at the face and is pumped by stages to the outlet through pipes.
3. Probe holes and pre-grouting - holes drilled ahead of the face (probing) to detect water; cement or chemical grout injected to seal fissures before excavation.
4. Drainage holes and dewatering wells - relief holes drilled into the rock to lower the water pressure; wellpoints or deep wells from the surface for shallow tunnels.
5. Temporary drains - ditches along the invert, covered or piped, to carry the water from the face.
6. Permanent drainage after lining - weep holes and drain pipes behind the lining, longitudinal side drains, catch pits and cross-drains, and waterproof membrane to stop leakage; a drainage layer prevents water pressure on the lining.
7. Portal and cut drainage - catch drains above portals and slope drainage to prevent surface water from entering the tunnel.

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 (oxygen at least 19.5 percent) 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 in some formations.
• To remove dust (silica from drilling and blasting) and smoke, which causes silicosis and bad visibility.
• To control temperature and humidity, particularly in deep or long tunnels, and to provide safe conditions in fire (smoke control).
• To meet statutory limits on gas concentrations (for road tunnels, CO about 50-100 ppm in service).

Methods of ventilation

1. Natural ventilation - uses the piston effect of moving vehicles, wind and temperature (stack) difference; for short tunnels (below about 400 m).
2. Mechanical ventilation in construction -
   • Forcing (blowing) system: fans push fresh air through ducts to the face; the foul air leaves by the tunnel.
   • Exhaust system: fans suck foul air from the face through ducts; fresh air enters along the tunnel.
   • Combined (push-pull) system for long tunnels and booster fans.
   • Water spray 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 economic for tunnels up to about 2-3 km.
   • Semi-transverse: fresh air supplied through a duct along the length and escapes through the portals.
   • Full transverse: separate supply and exhaust ducts give uniform ventilation along the length; costly, for very long and busy tunnels.

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

Factors for selecting the bridge site

A bridge site is chosen so that the bridge is safe, economical and serves the road alignment and the river with 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, braided and aggrading reaches.
2. Waterway and flood data: narrow, stable crossing with high banks; adequate waterway for the design flood (usually 50-year for major bridges, 100-year check flood as per IRC:SP:13 / DoR); reliable HFL, discharge and velocity records.
3. Foundation conditions: firm strata or rock at reasonable depth; scour depth (Lacey's regime theory, IRC:78) must be affordable; 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. Road alignment should not be bent to suit the bridge for major bridges but the opposite for minor ones.
5. Navigation and clearance: vertical and horizontal clearance for boats, floating logs or boulders; freeboard above HFL (minimum 0.6 m for girders; more in Nepal's boulder-carrying rivers).
6. Economy: total cost of bridge, approaches and river training works; nearness of construction material, labour and access for equipment.
7. Geology and seismicity: avoid active faults, steep gorge with rockfall; Nepal Building Code/NBC 105 and IRC:6 seismic provisions.
8. Existing structures and utilities: distance from other bridges, dams, 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, diversion, and low flow period for working.

Drainage in tunnel construction

Water entering a tunnel from rock joints, springs or the portals makes excavation unsafe, softens ground, damages lining and reduces working efficiency, so drainage is planned at all stages.

During construction

• Gravity drainage: if the tunnel is driven up-gradient (rising heading), water flows out by itself through a side ditch/gutter or pipe at the invert. Gradient is commonly 0.2-0.5%.
• Sumps and pumping: if driven down-gradient (falling heading), water collects at the face in a sump and is pumped out in stages through pipes with booster pumps.
• Pre-grouting/probe holes: cement or chemical grout is injected ahead of the face to seal water-bearing zones; probe drilling detects water in advance.
• Dewatering of the ground: well points or deep wells and drainage holes in poor ground.
• Sealing: shotcrete, waterproof membrane and concrete lining with good control of leakage.

Permanent drainage (road tunnel)

• Side/central drains with covered manholes, cleaning chambers and sufficient cross-fall (usually 1-2%) on carriageway.
• Drainage behind lining: perforated pipes and geotextile fleeces behind the lining carry seepage to the invert drain so hydrostatic pressure does not act on the lining.
• Waterproof membrane (PVC/HDPE) between primary and final lining.
• Portal drainage: cut-off drains above the portal diverting surface water, and a catch drain.
• Fire/oil separators for drained liquids in road tunnels, in line with IRC:SP:91 and the DoR road tunnel guideline.

Components of a bridge

A bridge has a superstructure (above bearings) and a substructure (bearings, piers, abutments, foundations), plus approaches and protective works.

 Approach   Parapet/Railing    Deck slab   Approach
 slab        ________________________      slab
 ____   ____|_____|_____|_____|_____|____   ____
      \  |  [girder][girder][girder]    |  /
 Wing  \ |Bearing        Bearing  |  / Wing
 wall   |Abutment  [ PIER ]   Abutment| wall
        |   |         |  |          |
 HFL ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 Scour level - - - - - - - - - - - - - - - -
     [Footing]    [Pile/Well]    [Footing]

Superstructure

• Deck/slab and wearing coat: carries traffic loads (IRC Class A/70R/AA loading per IRC:6) to girders.
• Girders/trusses/arches: main load carrying members that span between supports.
• Cross girders/diaphragms: give lateral stability and distribute load.
• Parapets, railings, kerbs, footpaths: safety for vehicles and pedestrians.
• Expansion joints and drainage spouts: allow thermal movement and remove surface water.

Bearings: transmit reactions from the superstructure to the substructure and permit rotation and movement (elastomeric, roller, rocker types).

Substructure

• Abutments: end supports that also retain approach fill; with wing walls and return walls that hold back earth.
• Piers: intermediate supports carrying the vertical load, with a pier cap and bed block.
• Foundations: open (spread) footings, piles, wells or caissons that carry loads to sound strata below scour depth.

Others: approach embankment and approach slab, protective works (guide bunds, spurs, apron, pitching) and river training works; design HFL, LWL and scour level are marked on the section.

Drainage in tunnelling

Water entering a tunnel from rock joints, springs or the portals makes excavation unsafe, softens ground, damages lining and reduces working efficiency, so drainage is planned at all stages.

During construction

• Gravity drainage: if the tunnel is driven up-gradient (rising heading), water flows out by itself through a side ditch/gutter or pipe at the invert. Gradient is commonly 0.2-0.5%.
• Sumps and pumping: if driven down-gradient (falling heading), water collects at the face in a sump and is pumped out in stages through pipes with booster pumps.
• Pre-grouting/probe holes: cement or chemical grout is injected ahead of the face to seal water-bearing zones; probe drilling detects water in advance.
• Dewatering of the ground: well points or deep wells and drainage holes in poor ground.
• Sealing: shotcrete, waterproof membrane and concrete lining with good control of leakage.

Permanent drainage (road tunnel)

• Side/central drains with covered manholes, cleaning chambers and sufficient cross-fall (usually 1-2%) on carriageway.
• Drainage behind lining: perforated pipes and geotextile fleeces behind the lining carry seepage to the invert drain so hydrostatic pressure does not act on the lining.
• Waterproof membrane (PVC/HDPE) between primary and final lining.
• Portal drainage: cut-off drains above the portal diverting surface water, and a catch drain.
• Fire/oil separators for drained liquids in road tunnels, in line with IRC:SP:91 and the DoR road tunnel guideline.

Ventilation in tunnelling

Vehicles in a tunnel emit CO, NOx, hydrocarbons and smoke, and blasting/diesel plant add dust and fumes during construction. Ventilation supplies fresh air and dilutes/removes pollutants, keeping visibility and health within limits (IRC:SP:91 and PIARC give limits, e.g. CO in the range 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 difference between 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 unidirectional tunnels of moderate length.
2. Semi-transverse system: fresh air is supplied through a duct along the tunnel and escapes out through the portals (or the reverse: exhaust duct with fresh air entering at 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 extracted. Best control but costly; for long tunnels and heavy traffic; also helps in smoke control during 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]

Selection depends on length, traffic volume, gradient, number of lanes and costs; CO and visibility sensors control fan operation.

Lighting: importance and methods

Day-time driving causes a "black-hole" effect: eyes adapted to bright outdoor light cannot see the dark inside, and at night the sudden change in lighting upsets the driver. Tunnel lighting is therefore provided in zones.

Importance

• Enables drivers to see the road, obstacles and signs; reduces accidents.
• Provides gradual eye adaptation between outside and inside luminance.
• Gives comfort, safety, and sense of direction, and helps rescue and maintenance.
• Provides visibility during emergencies and evacuation.

Methods/zones (CIE 88 / IRC:SP:91 approach)

1. Access zone: outside the portal, where the driver sees the tunnel entrance.
2. Threshold zone: the first part with the highest luminance (length = stopping sight distance), about 30-100% of the outside luminance at the portal, proportional to it.
3. Transition zone: luminance is gradually reduced (stepwise or smoothly) to the interior level so eyes adapt.
4. Interior zone: constant lower level depending on speed and traffic.
5. Exit zone: higher luminance, to help drivers adapt to outdoor light; at night this is kept low.

Lighting is provided using high pressure sodium or LED luminaires, in symmetric, counter-beam (against traffic) or pro-beam arrangements; emergency lighting supply is separate.

Ventilation: importance

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) and supplies oxygen for workers and machines, 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 the portal and surrounding air quality within environmental limits.

Ventilation: methods

Vehicles in a tunnel emit CO, NOx, hydrocarbons and smoke, and blasting/diesel plant add dust and fumes during construction. Ventilation supplies fresh air and dilutes/removes pollutants, keeping visibility and health within limits (IRC:SP:91 and PIARC give limits, e.g. CO in the range 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 difference between 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 unidirectional tunnels of moderate length.
2. Semi-transverse system: fresh air is supplied through a duct along the tunnel and escapes out through the portals (or the reverse: exhaust duct with fresh air entering at 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 extracted. Best control but costly; for long tunnels and heavy traffic; also helps in smoke control during 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]

Selection depends on length, traffic volume, gradient, number of lanes and costs; CO and visibility sensors control fan operation.

Factors affecting bridge site selection

A bridge site is chosen so that the bridge is safe, economical and serves the road alignment and the river with 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, braided and aggrading reaches.
2. Waterway and flood data: narrow, stable crossing with high banks; adequate waterway for the design flood (usually 50-year for major bridges, 100-year check flood as per IRC:SP:13 / DoR); reliable HFL, discharge and velocity records.
3. Foundation conditions: firm strata or rock at reasonable depth; scour depth (Lacey's regime theory, IRC:78) must be affordable; 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. Road alignment should not be bent to suit the bridge for major bridges but the opposite for minor ones.
5. Navigation and clearance: vertical and horizontal clearance for boats, floating logs or boulders; freeboard above HFL (minimum 0.6 m for girders; more in Nepal's boulder-carrying rivers).
6. Economy: total cost of bridge, approaches and river training works; nearness of construction material, labour and access for equipment.
7. Geology and seismicity: avoid active faults, steep gorge with rockfall; Nepal Building Code/NBC 105 and IRC:6 seismic provisions.
8. Existing structures and utilities: distance from other bridges, dams, 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, diversion, and low flow period for working.

Essential components of a bridge

A bridge has a superstructure (above bearings) and a substructure (bearings, piers, abutments, foundations), plus approaches and protective works.

 Approach   Parapet/Railing    Deck slab   Approach
 slab        ________________________      slab
 ____   ____|_____|_____|_____|_____|____   ____
      \  |  [girder][girder][girder]    |  /
 Wing  \ |Bearing        Bearing  |  / Wing
 wall   |Abutment  [ PIER ]   Abutment| wall
        |   |         |  |          |
 HFL ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 Scour level - - - - - - - - - - - - - - - -
     [Footing]    [Pile/Well]    [Footing]

Superstructure

• Deck/slab and wearing coat: carries traffic loads (IRC Class A/70R/AA loading per IRC:6) to girders.
• Girders/trusses/arches: main load carrying members that span between supports.
• Cross girders/diaphragms: give lateral stability and distribute load.
• Parapets, railings, kerbs, footpaths: safety for vehicles and pedestrians.
• Expansion joints and drainage spouts: allow thermal movement and remove surface water.

Bearings: transmit reactions from the superstructure to the substructure and permit rotation and movement (elastomeric, roller, rocker types).

Substructure

• Abutments: end supports that also retain approach fill; with wing walls and return walls that hold back earth.
• Piers: intermediate supports carrying the vertical load, with a pier cap and bed block.
• Foundations: open (spread) footings, piles, wells or caissons that carry loads to sound strata below scour depth.

Others: approach embankment and approach slab, protective works (guide bunds, spurs, apron, pitching) and river training works; design HFL, LWL and scour level are marked on the section.

Importance of tunnel

A tunnel is an underground passage excavated through hill, mountain or under water, without removing the overlying ground, to carry traffic.

Importance of road tunnel

• Shortens distance and travel time by avoiding steep climbs and long detours; especially useful in Nepal's hilly terrain.
• Lowers gradient, vehicle operating cost and fuel use.
• Avoids unstable slopes, landslides and snow/avalanche-prone passes; provides all-weather connection.
• Reduces environmental damage from large cuttings and hill-cut roads; saves land in congested cities.
• Provides crossing under rivers, cities or sensitive areas without disturbing the surface; also useful for defence and utilities.
• Reduces accidents on hairpin bends.

Components and cross sections

          Crown / Arch (roof)
        ___________________
      /       Extrados       \
     |   ___ Intrados ___     |
 Haunch |  |  Springing line | |  Haunch
     |  | /               \ |  |
 Side   |/    CARRIAGEWAY    \|  Side
 wall   |  Footpath/Walkway   | wall
        |___Invert (floor)____|
        [Side drain]  [Side drain]

Components

• Portal: entrance/exit structure with wing walls and retaining wall that protects the face from erosion and slides.
• Crown/roof (arch): top part of lining; its inner surface is intrados, the outer surface extrados.
• Springing line: where the arch meets the side walls.
• Side walls (haunches): vertical or curved support for the roof.
• Invert (floor/bottom arch): floor, flat or curved, carrying the pavement; it resists upward pressure in squeezing ground.
• Lining: shotcrete, cast-in-place concrete or segmental lining supporting the ground.
• Drainage: side drains, sumps and cross-drains.
• Services: ventilation, lighting, emergency exits and cross-passages, signage, and fire safety systems.

Tunnel cross sections (shapes)

1. Circular: best structural shape against all-round pressure, used in soft ground, shield and TBM tunnels.
2. Horseshoe (D-shaped/arch): common for road tunnels in rock; flat floor with arched roof.
3. Semi-circular (arched) with vertical walls: used where side pressure is small.
4. Egg-shaped/ovoid: for water or sewage tunnels.
5. Rectangular/box: used in cut-and-cover tunnels in cities.
6. Elliptical: for large lateral pressures.

The size follows the carriageway width (Nepal Road Standard 2070, plus shoulders and walkways), vertical clearance (about 5.0 m), and space for services.

Factors for selecting the bridge site

A bridge site is chosen so that the bridge is safe, economical and serves the road alignment and the river with 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, braided and aggrading reaches.
2. Waterway and flood data: narrow, stable crossing with high banks; adequate waterway for the design flood (usually 50-year for major bridges, 100-year check flood as per IRC:SP:13 / DoR); reliable HFL, discharge and velocity records.
3. Foundation conditions: firm strata or rock at reasonable depth; scour depth (Lacey's regime theory, IRC:78) must be affordable; 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. Road alignment should not be bent to suit the bridge for major bridges but the opposite for minor ones.
5. Navigation and clearance: vertical and horizontal clearance for boats, floating logs or boulders; freeboard above HFL (minimum 0.6 m for girders; more in Nepal's boulder-carrying rivers).
6. Economy: total cost of bridge, approaches and river training works; nearness of construction material, labour and access for equipment.
7. Geology and seismicity: avoid active faults, steep gorge with rockfall; Nepal Building Code/NBC 105 and IRC:6 seismic provisions.
8. Existing structures and utilities: distance from other bridges, dams, 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, diversion, and 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) and supplies oxygen for workers and machines, 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 the portal and surrounding air quality within environmental limits.

Sketch and components of a bridge

A bridge has a superstructure (above bearings) and a substructure (bearings, piers, abutments, foundations), plus approaches and protective works.

 Approach   Parapet/Railing    Deck slab   Approach
 slab        ________________________      slab
 ____   ____|_____|_____|_____|_____|____   ____
      \  |  [girder][girder][girder]    |  /
 Wing  \ |Bearing        Bearing  |  / Wing
 wall   |Abutment  [ PIER ]   Abutment| wall
        |   |         |  |          |
 HFL ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 Scour level - - - - - - - - - - - - - - - -
     [Footing]    [Pile/Well]    [Footing]

Superstructure

• Deck/slab and wearing coat: carries traffic loads (IRC Class A/70R/AA loading per IRC:6) to girders.
• Girders/trusses/arches: main load carrying members that span between supports.
• Cross girders/diaphragms: give lateral stability and distribute load.
• Parapets, railings, kerbs, footpaths: safety for vehicles and pedestrians.
• Expansion joints and drainage spouts: allow thermal movement and remove surface water.

Bearings: transmit reactions from the superstructure to the substructure and permit rotation and movement (elastomeric, roller, rocker types).

Substructure

• Abutments: end supports that also retain approach fill; with wing walls and return walls that hold back earth.
• Piers: intermediate supports carrying the vertical load, with a pier cap and bed block.
• Foundations: open (spread) footings, piles, wells or caissons that carry loads to sound strata below scour depth.

Others: approach embankment and approach slab, protective works (guide bunds, spurs, apron, pitching) and river training works; design HFL, LWL and scour level are marked on the section.

Methods of river bank protection

River bank protection prevents erosion and scour of banks near bridge approaches and guides the flow to pass smoothly through the waterway (IRC:89 "Guidelines for design and construction of river training and control works for road bridges"; Nepal DoR River Training/Bio-engineering guidelines). Main methods:

1. Revetments (pitching): stone pitching (dry or grouted), concrete block or gabion mattress laid on a filter layer over a graded bank slope, with a toe wall or launching apron to prevent undermining. Used for banks of moderate flow.
2. Gabion/crate walls and mattresses: wire-mesh boxes filled with boulders; flexible, free draining, cheap and widely used in Nepal. Gabion mattress aprons adjust to scour.
3. Spurs/groynes: short structures projecting from bank into the stream that deflect flow away from bank, cause silting between them and train the channel. Types: permeable (pile, timber) and impermeable (stone, boulder); attracting, repelling or deflecting by angle.
4. Guide bunds: earthen embankments with stone-pitched river face, curved at the upstream end, on both sides of the bridge approach to guide the flow through the waterway centrally (IRC:89).
5. Retaining walls/embankment walls: masonry, RCC or gabion walls along the bank where space is limited.
6. Longitudinal dykes/marginal bunds: embankments parallel to the flow to confine the river.
7. Launching apron and toe protection: loose stones at the toe that launch down when scour occurs, protecting the foundation of the protection work.
8. Bio-engineering: vegetation (grass, bamboo, shrubs, trees) planted on the bank, often combined with gabions, cheap and environment-friendly for lesser flows (DoR Roadside Bio-engineering Guideline).
9. Check dams/sills and permeable fences: reduce velocity and trap sediment in steep streams.

Factors affecting the choice of bridge site

A bridge site is chosen so that the bridge is safe, economical and serves the road alignment and the river with 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, braided and aggrading reaches.
2. Waterway and flood data: narrow, stable crossing with high banks; adequate waterway for the design flood (usually 50-year for major bridges, 100-year check flood as per IRC:SP:13 / DoR); reliable HFL, discharge and velocity records.
3. Foundation conditions: firm strata or rock at reasonable depth; scour depth (Lacey's regime theory, IRC:78) must be affordable; 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. Road alignment should not be bent to suit the bridge for major bridges but the opposite for minor ones.
5. Navigation and clearance: vertical and horizontal clearance for boats, floating logs or boulders; freeboard above HFL (minimum 0.6 m for girders; more in Nepal's boulder-carrying rivers).
6. Economy: total cost of bridge, approaches and river training works; nearness of construction material, labour and access for equipment.
7. Geology and seismicity: avoid active faults, steep gorge with rockfall; Nepal Building Code/NBC 105 and IRC:6 seismic provisions.
8. Existing structures and utilities: distance from other bridges, dams, 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, diversion, and low flow period for working.

River bank protection structures

River bank protection prevents erosion and scour of banks near bridge approaches and guides the flow to pass smoothly through the waterway (IRC:89 "Guidelines for design and construction of river training and control works for road bridges"; Nepal DoR River Training/Bio-engineering guidelines). Main methods:

1. Revetments (pitching): stone pitching (dry or grouted), concrete block or gabion mattress laid on a filter layer over a graded bank slope, with a toe wall or launching apron to prevent undermining. Used for banks of moderate flow.
2. Gabion/crate walls and mattresses: wire-mesh boxes filled with boulders; flexible, free draining, cheap and widely used in Nepal. Gabion mattress aprons adjust to scour.
3. Spurs/groynes: short structures projecting from bank into the stream that deflect flow away from bank, cause silting between them and train the channel. Types: permeable (pile, timber) and impermeable (stone, boulder); attracting, repelling or deflecting by angle.
4. Guide bunds: earthen embankments with stone-pitched river face, curved at the upstream end, on both sides of the bridge approach to guide the flow through the waterway centrally (IRC:89).
5. Retaining walls/embankment walls: masonry, RCC or gabion walls along the bank where space is limited.
6. Longitudinal dykes/marginal bunds: embankments parallel to the flow to confine the river.
7. Launching apron and toe protection: loose stones at the toe that launch down when scour occurs, protecting the foundation of the protection work.
8. Bio-engineering: vegetation (grass, bamboo, shrubs, trees) planted on the bank, often combined with gabions, cheap and environment-friendly for lesser flows (DoR Roadside Bio-engineering Guideline).
9. Check dams/sills and permeable fences: reduce velocity and trap sediment in steep streams.

Components of a bridge

A bridge has a superstructure (above bearings) and a substructure (bearings, piers, abutments, foundations), plus approaches and protective works.

 Approach   Parapet/Railing    Deck slab   Approach
 slab        ________________________      slab
 ____   ____|_____|_____|_____|_____|____   ____
      \  |  [girder][girder][girder]    |  /
 Wing  \ |Bearing        Bearing  |  / Wing
 wall   |Abutment  [ PIER ]   Abutment| wall
        |   |         |  |          |
 HFL ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 Scour level - - - - - - - - - - - - - - - -
     [Footing]    [Pile/Well]    [Footing]

Superstructure

• Deck/slab and wearing coat: carries traffic loads (IRC Class A/70R/AA loading per IRC:6) to girders.
• Girders/trusses/arches: main load carrying members that span between supports.
• Cross girders/diaphragms: give lateral stability and distribute load.
• Parapets, railings, kerbs, footpaths: safety for vehicles and pedestrians.
• Expansion joints and drainage spouts: allow thermal movement and remove surface water.

Bearings: transmit reactions from the superstructure to the substructure and permit rotation and movement (elastomeric, roller, rocker types).

Substructure

• Abutments: end supports that also retain approach fill; with wing walls and return walls that hold back earth.
• Piers: intermediate supports carrying the vertical load, with a pier cap and bed block.
• Foundations: open (spread) footings, piles, wells or caissons that carry loads to sound strata below scour depth.

Others: approach embankment and approach slab, protective works (guide bunds, spurs, apron, pitching) and river training works; design HFL, LWL and scour level are marked on the section.

Management of drainage problems

Water entering a tunnel from rock joints, springs or the portals makes excavation unsafe, softens ground, damages lining and reduces working efficiency, so drainage is planned at all stages.

During construction

• Gravity drainage: if the tunnel is driven up-gradient (rising heading), water flows out by itself through a side ditch/gutter or pipe at the invert. Gradient is commonly 0.2-0.5%.
• Sumps and pumping: if driven down-gradient (falling heading), water collects at the face in a sump and is pumped out in stages through pipes with booster pumps.
• Pre-grouting/probe holes: cement or chemical grout is injected ahead of the face to seal water-bearing zones; probe drilling detects water in advance.
• Dewatering of the ground: well points or deep wells and drainage holes in poor ground.
• Sealing: shotcrete, waterproof membrane and concrete lining with good control of leakage.

Permanent drainage (road tunnel)

• Side/central drains with covered manholes, cleaning chambers and sufficient cross-fall (usually 1-2%) on carriageway.
• Drainage behind lining: perforated pipes and geotextile fleeces behind the lining carry seepage to the invert drain so hydrostatic pressure does not act on the lining.
• Waterproof membrane (PVC/HDPE) between primary and final lining.
• Portal drainage: cut-off drains above the portal diverting surface water, and a catch drain.
• Fire/oil separators for drained liquids in road tunnels, in line with IRC:SP:91 and the DoR road tunnel guideline.

Management of ventilation problems

Vehicles in a tunnel emit CO, NOx, hydrocarbons and smoke, and blasting/diesel plant add dust and fumes during construction. Ventilation supplies fresh air and dilutes/removes pollutants, keeping visibility and health within limits (IRC:SP:91 and PIARC give limits, e.g. CO in the range 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 difference between 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 unidirectional tunnels of moderate length.
2. Semi-transverse system: fresh air is supplied through a duct along the tunnel and escapes out through the portals (or the reverse: exhaust duct with fresh air entering at 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 extracted. Best control but costly; for long tunnels and heavy traffic; also helps in smoke control during 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]

Selection depends on length, traffic volume, gradient, number of lanes and costs; CO and visibility sensors control fan operation.

Methods of river bank protection

River bank protection prevents erosion and scour of banks near bridge approaches and guides the flow to pass smoothly through the waterway (IRC:89 "Guidelines for design and construction of river training and control works for road bridges"; Nepal DoR River Training/Bio-engineering guidelines). Main methods:

1. Revetments (pitching): stone pitching (dry or grouted), concrete block or gabion mattress laid on a filter layer over a graded bank slope, with a toe wall or launching apron to prevent undermining. Used for banks of moderate flow.
2. Gabion/crate walls and mattresses: wire-mesh boxes filled with boulders; flexible, free draining, cheap and widely used in Nepal. Gabion mattress aprons adjust to scour.
3. Spurs/groynes: short structures projecting from bank into the stream that deflect flow away from bank, cause silting between them and train the channel. Types: permeable (pile, timber) and impermeable (stone, boulder); attracting, repelling or deflecting by angle.
4. Guide bunds: earthen embankments with stone-pitched river face, curved at the upstream end, on both sides of the bridge approach to guide the flow through the waterway centrally (IRC:89).
5. Retaining walls/embankment walls: masonry, RCC or gabion walls along the bank where space is limited.
6. Longitudinal dykes/marginal bunds: embankments parallel to the flow to confine the river.
7. Launching apron and toe protection: loose stones at the toe that launch down when scour occurs, protecting the foundation of the protection work.
8. Bio-engineering: vegetation (grass, bamboo, shrubs, trees) planted on the bank, often combined with gabions, cheap and environment-friendly for lesser flows (DoR Roadside Bio-engineering Guideline).
9. Check dams/sills and permeable fences: reduce velocity and trap sediment in steep streams.

Tunnelling in hard soil/rock

Hard soil/ rock is tunnelled using full-face, heading and bench, or multiple-drift methods depending on size, rock quality and stability. Classical methods are:

1. Full-face method: the entire section is excavated at once, using drill-and-blast or a TBM. It gives rapid progress but needs good strong rock and large equipment; used for small tunnels.
2. Heading and bench method: the upper half (heading) is advanced ahead and the lower part (bench) excavated behind. It is common for medium to large rock tunnels, making work flexible and allowing early roof support.
3. Drift (pilot) methods: small pilot tunnels (centre, side or top drift) are driven first and then enlarged to full section. Examples include the English method (crown first, lining from top), Belgian method (top heading first and arch concreted first), German method (side drifts), Austrian method, Italian method.
4. Drill and blast cycle: drilling, charging, blasting, mucking, ventilation, scaling, and support (rock bolts, shotcrete, steel ribs). Controlled blasting (smooth blasting) minimizes overbreak.
5. Tunnel boring machine (TBM): continuous mechanical cutting for long, uniform tunnels, giving smooth walls and high rate.
6. New Austrian Tunnelling Method (NATM): the rock is made to support itself by rock bolts and shotcrete with monitoring of deformation; widely applied in Nepal's Himalayan tunnels.
7. Road header/ripping: for softer rocks and weathered zones.

Factors in tunnel lighting

Factors to consider in tunnel lighting:

1. Speed of traffic and stopping sight distance, which govern the threshold zone length and level.
2. Outside (portal) luminance at the approach, which depends on the sky, surroundings and orientation of the portal; the threshold luminance is a fraction of this.
3. Length of the tunnel: short tunnels (below about 50-100 m) may need only daylight; long ones need full zone lighting.
4. Traffic volume and type (one-way/two-way, heavy vehicles, pedestrians).
5. Eye adaptation time: transition zone must reduce luminance gradually.
6. Day and night requirements, with separate day/night circuits.
7. Wall/road surface reflectance, cleanliness, tunnel finish (light-coloured walls help).
8. Luminaire type, mounting position, glare and uniformity (uniformity of luminance, flicker avoidance below about 2.5-15 Hz).
9. Power supply and emergency lighting, maintenance and cost, energy saving through dimming.
10. Safety and geometry: curves, sight distance and tunnel shape.

Methods of river bank protection

River bank protection prevents erosion and scour of banks near bridge approaches and guides the flow to pass smoothly through the waterway (IRC:89 "Guidelines for design and construction of river training and control works for road bridges"; Nepal DoR River Training/Bio-engineering guidelines). Main methods:

1. Revetments (pitching): stone pitching (dry or grouted), concrete block or gabion mattress laid on a filter layer over a graded bank slope, with a toe wall or launching apron to prevent undermining. Used for banks of moderate flow.
2. Gabion/crate walls and mattresses: wire-mesh boxes filled with boulders; flexible, free draining, cheap and widely used in Nepal. Gabion mattress aprons adjust to scour.
3. Spurs/groynes: short structures projecting from bank into the stream that deflect flow away from bank, cause silting between them and train the channel. Types: permeable (pile, timber) and impermeable (stone, boulder); attracting, repelling or deflecting by angle.
4. Guide bunds: earthen embankments with stone-pitched river face, curved at the upstream end, on both sides of the bridge approach to guide the flow through the waterway centrally (IRC:89).
5. Retaining walls/embankment walls: masonry, RCC or gabion walls along the bank where space is limited.
6. Longitudinal dykes/marginal bunds: embankments parallel to the flow to confine the river.
7. Launching apron and toe protection: loose stones at the toe that launch down when scour occurs, protecting the foundation of the protection work.
8. Bio-engineering: vegetation (grass, bamboo, shrubs, trees) planted on the bank, often combined with gabions, cheap and environment-friendly for lesser flows (DoR Roadside Bio-engineering Guideline).
9. Check dams/sills and permeable fences: reduce velocity and trap sediment in steep streams.

A bridge consists of a superstructure that carries the traffic and a substructure that transfers the load to the ground.

Superstructure

• Deck slab with wearing coat carrying traffic loads.
• Main girders (RCC, PSC or steel) or truss/arch; cross girders for lateral distribution.
• Kerbs, footpath, railing/parapets, expansion joints and drainage spouts.

Bearings between superstructure and substructure transmit reactions and permit rotation and thermal movement.

Substructure

• Piers: intermediate supports with pier cap.
• Abutments: end supports retaining approach fill; wing/return walls retain the earth.
• Foundations: footing, pile, well or caisson, taken below scour depth.

Others: approach slab and embankment, guide bunds, apron and protection works. Loads and dimensions follow IRC:6, IRC:5 and the Nepal DoR bridge standards.

  Railing  Deck + girders     Railing
  ____________________________
 |Abutment|__Bearing__|Pier|Abutment|
  Wing wall  HFL ~~~~~~~  Wing wall
  Foundation below scour level

Ventilation is essential in tunnelling because the air in a tunnel quickly becomes unsafe.

• Removes harmful gases: blasting fumes, CO, NOx, diesel exhaust, and natural gases (methane, H2S, CO2) are diluted or exhausted.
• Supplies oxygen for workers and combustion engines, protecting health and life.
• Clears dust and smoke after blasting and drilling, improving visibility and speed of work.
• Controls temperature and humidity, which rise at depth, and reduces worker fatigue.
• Prevents explosive gas build-up and helps in firefighting and rescue.
• In the operating road tunnel, it dilutes vehicle emissions to permissible limits (IRC:SP:91, PIARC), maintains visibility and controls smoke in fire. Methods include natural, longitudinal (jet fan), semi-transverse and transverse systems, and ducted forced/exhaust ventilation during construction.

Classification by superstructure (type of structure)

Classification by span length (IRC / DoR practice)

• Culvert: total length below 6 m (IRC definition, span below 6 m).
• Minor bridge: length 6 m to 60 m.
• Major bridge: above 60 m.
• Long-span bridge: single spans above about 120 m.
• Nepal DoR classifies small bridges (below about 25 m) and large bridges.

Sketch of bridge sections and plan

 LONGITUDINAL SECTION
       Deck slab + wearing coat       Railing
  ___________________________________  ___
 |==================================|
  Abutment  Bearing  Girder  Pier   Abutment
  |  |       ^   ^                  |  |
  Wing   HFL ~~~~~~~~~~~~~~~~~~~~~~~~  Wing
 wall    Foundation level below scour
 CROSS SECTION
  Kerb  Carriageway  Kerb  Footpath
 |_|______________|__|_|
   ===== deck slab ======
    [girder] [girder] [girder]
 PLAN
 ----------------------------------
  W.wall |  carriageway          | W.wall
 ----------------------------------

Elements shown: deck slab, girders, bearings, piers, abutments, wing walls, foundations, kerbs, footpath, railings, approach slabs and HFL/scour levels.

Classification of bridges

By structure (static system)

• Simply supported, continuous and cantilever bridges.
• Arch, rigid frame, truss, girder, suspension and cable-stayed bridges.
• Beam/slab, box girder bridges.

By material

• Timber bridge: temporary or light bridges.
• Masonry (stone/brick): arches and piers of low spans.
• Reinforced concrete (RCC): slab and T-beam bridges.
• Prestressed concrete (PSC): long girders and box sections.
• Steel: truss, plate girder and Bailey bridges; composite (steel with concrete deck).
• Others: FRP, bamboo/rope (suspension bridges in rural Nepal).

By loading (IRC:6 / Nepal Bridge Standard)

• Highway bridge (carries road vehicles: IRC Class AA, 70R, A, B).
• Railway bridge (railway loading).
• Footbridge (pedestrians, light loads, 5 kN/m² type load).
• Pipeline, aqueduct, viaduct and combined road-rail bridges.
• Class A is for standard live load; Class 70R for heavy tracked/wheeled vehicles on national highways, and Class B for temporary bridges.

Sketch of bridge with components

A bridge has a superstructure (above bearings) and a substructure (bearings, piers, abutments, foundations), plus approaches and protective works.

 Approach   Parapet/Railing    Deck slab   Approach
 slab        ________________________      slab
 ____   ____|_____|_____|_____|_____|____   ____
      \  |  [girder][girder][girder]    |  /
 Wing  \ |Bearing        Bearing  |  / Wing
 wall   |Abutment  [ PIER ]   Abutment| wall
        |   |         |  |          |
 HFL ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 Scour level - - - - - - - - - - - - - - - -
     [Footing]    [Pile/Well]    [Footing]

Superstructure

• Deck/slab and wearing coat: carries traffic loads (IRC Class A/70R/AA loading per IRC:6) to girders.
• Girders/trusses/arches: main load carrying members that span between supports.
• Cross girders/diaphragms: give lateral stability and distribute load.
• Parapets, railings, kerbs, footpaths: safety for vehicles and pedestrians.
• Expansion joints and drainage spouts: allow thermal movement and remove surface water.

Bearings: transmit reactions from the superstructure to the substructure and permit rotation and movement (elastomeric, roller, rocker types).

Substructure

• Abutments: end supports that also retain approach fill; with wing walls and return walls that hold back earth.
• Piers: intermediate supports carrying the vertical load, with a pier cap and bed block.
• Foundations: open (spread) footings, piles, wells or caissons that carry loads to sound strata below scour depth.

Others: approach embankment and approach slab, protective works (guide bunds, spurs, apron, pitching) and river training works; design HFL, LWL and scour level are marked on the section.

Lighting in a tunnel is important for safe driving and easy adaptation of the eyes.

• Daytime drivers moving from bright outside into a dark tunnel suffer the black-hole effect; lighting reduces temporary blindness and accidents.
• Gradual luminance reduction (threshold, transition, interior and exit zones) lets the eye adapt.
• It lets drivers see the road, obstacles, signs, and pedestrians, so traffic flows at the design speed.
• It gives comfort and reduces fatigue in long tunnels, and helps maintenance staff work.
• Emergency lighting helps evacuation and rescue in power failure or fire.
• Good lighting design follows CIE 88 and IRC:SP:91 with energy-efficient luminaires like LED lamps.