Bridge components; tunnel drainage and ventilation

Components of a bridge

A bridge has a superstructure (above the bearings) and a substructure (bearings, piers, abutments and 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: carry the traffic loads (IRC Class A/70R/AA loading as per IRC:6) to the girders.
• Girders/trusses/arches: the main load-carrying members that span between supports.
• Cross girders/diaphragms: give lateral stability and distribute the load.
• Parapets, railings, kerbs and footpaths: provide 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 and rocker types).

Substructure

• Abutments: end supports that also retain the approach fill, with wing walls and return walls that hold back the 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 the loads to sound strata below the scour depth.

Others: approach embankment and approach slab, protective works (guide bunds, spurs, apron, pitching) and river training works. The 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 the ground, damages the lining and reduces working efficiency, so drainage is planned at every stage.

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. The gradient is commonly 0.2–0.5%.
• Sumps and pumping: if the tunnel is 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 and 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, deep wells and drainage holes (or a drainage drift) 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 and cleaning chambers, and sufficient cross-fall (usually 1–2%) on the carriageway.
• Drainage behind the lining: perforated pipes and geotextile fleeces behind the lining carry seepage to the invert drain, so that hydrostatic pressure does not act on the lining.
• Waterproof membrane (PVC/HDPE) between the primary and final linings.
• Portal drainage: cut-off drains above the portal to divert surface water, and a catch drain.
• Fire/oil separators for drained liquids in road tunnels, in line with IRC:SP:91.

Management of ventilation problems

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.