Bridge components sketch; river bank protection

Sketch and components of a bridge

A bridge has a superstructure (above the 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 the traffic loads (IRC Class A/70R/AA loading as per IRC:6) to the 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 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.

Methods of river bank protection

River bank protection prevents erosion and scour of the 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 and bio-engineering guidelines). Main methods:

1. Revetments (pitching): stone pitching (dry or grouted), concrete blocks or gabion mattresses laid on a filter layer over a graded bank slope, with a toe wall or launching apron to prevent undermining. Used for banks with 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 the bank into the stream that deflect the flow away from the bank, cause silting between them and train the channel. Types: permeable (pile, timber) and impermeable (stone, boulder); attracting, repelling or deflecting, depending on their angle.
4. Guide bunds: earthen embankments with a stone-pitched river face, curved at the upstream end, on both sides of the bridge waterway to guide the flow centrally through it (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 the slope 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 smaller flows (DoR Roadside Bio-engineering Guideline).
9. Check dams/sills and permeable fences: reduce velocity and trap sediment in steep streams.