Indian Railways Bridge Manual Chapter 8
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Questions and Answers

What is the typical bed slope of rivers in mountainous regions?

  • 1 in 50 to 1 in 500 (correct)
  • 1 in 10 to 1 in 20
  • 1 in 1000 to 1 in 5000
  • 1 in 100 to 1 in 1000

    • What is the primary reason for sediment deposition in braided streams?

  • Insufficient flow to transport sediment (correct)
  • Change in river course due to flooding
  • Excessive flow causing erosion
  • Sudden increase in sediment load

    • What is the term used to describe rivers that progressively raise their beds by sediment deposition?

  • Aggrading (correct)
  • Eroding
  • Meandering
  • Braided

    • What is the primary method of erosion in braided streams?

    <p>All of the above</p> Signup and view all the answers

    Why is it not desirable to locate bridges in braided stream reaches?

    <p>Due to the unpredictability of the river course</p> Signup and view all the answers

    What is the term used to describe streams with a network of channels and islands in between?

    <p>Braided</p> Signup and view all the answers

    What happens to the river bed during high floods in braided streams?

    <p>The river bed gets wider and shallower</p> Signup and view all the answers

    What can cause a sudden change of course in braided streams?

    <p>Abnormal floods</p> Signup and view all the answers

    What is the recommended angle of sweep for the curved head of a guide bund?

    <p>120° to 145°</p> Signup and view all the answers

    What is the primary purpose of providing a graded filter below the stone pitching?

    <p>To prevent displacement of the pitching stones by the current</p> Signup and view all the answers

    Why is it necessary to provide stone pitching on the rear side of the guide bund in some cases?

    <p>For important rivers or large ponding areas</p> Signup and view all the answers

    What is the recommended thickness of the stone pitching for small works?

    <p>20cm to 30cm</p> Signup and view all the answers

    Why is it necessary to provide longitudinal and cross drains for the protection of slopes?

    <p>To prevent rain cuts on high banks</p> Signup and view all the answers

    What is the recommended top width of the shank of the guide bund?

    <p>6m to 9m</p> Signup and view all the answers

    What is the primary purpose of model studies for important rivers?

    <p>To select multi radii for smoother flows</p> Signup and view all the answers

    What is the recommended size of the pitching stones used for stone pitching?

    <p>35kg to 55kg</p> Signup and view all the answers

    What should be inspected soon after the monsoon every year?

    <p>The track on the guide bund</p> Signup and view all the answers

    What is the purpose of plotting the levels of the apron?

    <p>To assess the efficacy of the launching</p> Signup and view all the answers

    What should be done if the pitching stone on the slope is disturbed?

    <p>Add more stones to the apron immediately</p> Signup and view all the answers

    What happens if the launching takes place beyond the capacity of the stone in the apron?

    <p>More stone is added to the apron</p> Signup and view all the answers

    Why should a substantial reserve of pitching stone be maintained on the guide bund?

    <p>For use during emergency situations</p> Signup and view all the answers

    What is the consequence of a steep sub-soil water gradient resulting from a rapidly falling flood in the river?

    <p>Slips and blow-outs occur in the bund</p> Signup and view all the answers

    What should be done to the bank if slips and blow-outs occur due to a steep sub-soil water gradient?

    <p>Widen the bank to reduce the hydraulic gradient</p> Signup and view all the answers

    What should be done if stones are carried away by high velocity current from the launching apron?

    <p>Strengthen the apron against severe attack by laying large sized stones</p> Signup and view all the answers

    What is the typical spacing of spurs on a curved channel?

    <p>2 to 3 times the length</p> Signup and view all the answers

    What is the typical projection of the spurs into the river channel?

    <p>11% to 15% of width of channel</p> Signup and view all the answers

    How many spurs are normally provided for a specific reach to be protected?

    <p>3</p> Signup and view all the answers

    What is the direction of alignment of spurs?

    <p>Towards upstream</p> Signup and view all the answers

    What is the typical arrangement of porcupines for depth of flow up to 3m?

    <p>Two rows of porcupines on either side</p> Signup and view all the answers

    What is the purpose of permeable screens?

    <p>To choke the secondary channels</p> Signup and view all the answers

    What is the typical arrangement of counter weights in porcupines or cribs?

    <p>In both the central box of the porcupines and bottom tray of the cribs</p> Signup and view all the answers

    Is bed protection necessary for structures made up of porcupines and cribs?

    <p>No</p> Signup and view all the answers

    What is the primary purpose of constructing a marginal bund?

    <p>To contain the spread of the river during floods</p> Signup and view all the answers

    Where should the earth for the construction of marginal bunds be obtained from?

    <p>From the river side</p> Signup and view all the answers

    How often should marginal bunds be inspected?

    <p>Every year along with the annual bridge inspection</p> Signup and view all the answers

    What is the purpose of a closure bund?

    <p>To prevent the discharge of channels developing into a main river channel</p> Signup and view all the answers

    What should be done to protect the slope of a marginal bund?

    <p>It should be well turfed</p> Signup and view all the answers

    What should be watched and repaired in marginal bunds?

    <p>Rodent holes and cattle crossing</p> Signup and view all the answers

    Why is a cut-off channel dug?

    <p>To prevent heavy meandering from encroaching into the still water area</p> Signup and view all the answers

    When should necessary repairs be carried out on marginal bunds?

    <p>Before the onset of monsoon</p> Signup and view all the answers

    What is the primary purpose of providing guide bunds in river training and protection works?

    <p>To constrict the river width</p> Signup and view all the answers

    What is the characteristic of an alluvial river?

    <p>It flows on a flat bed built by its alluvium</p> Signup and view all the answers

    What is the purpose of providing a flexible type apron in river training and protection works?

    <p>To reduce the erosive action on the marginal bund</p> Signup and view all the answers

    What is the primary difference between alluvial and quasi-alluvial rivers?

    <p>Alluvial rivers meander as a whole within their khadir</p> Signup and view all the answers

    What is the purpose of undertaking hydraulic model studies in river training and protection works?

    <p>To design the guide bunds and marginal bunds</p> Signup and view all the answers

    What is the term used to describe the strip of low land within which a river meanders and its flood rises?

    <p>Khadir</p> Signup and view all the answers

    What is the purpose of providing toe protection in river training and protection works?

    <p>To prevent erosion at the toe of the marginal bund</p> Signup and view all the answers

    What is the characteristic of the channel in the reach where bridging is required?

    <p>It has a well-defined course with small size gravel and medium sand</p> Signup and view all the answers

    What is the primary purpose of providing an apron beyond the toe of the slope of the guide bund?

    <p>To protect the scoured face when the bed is scoured</p> Signup and view all the answers

    What is the recommended thickness of the apron in the straight portion of the guide bund?

    <p>1.5T</p> Signup and view all the answers

    What is the recommended width of the apron?

    <p>1.5 times the difference between the deepest known scour level and low water level</p> Signup and view all the answers

    Why should guide bunds and approach embankments in khadir of the river be constructed in one go in one season?

    <p>To ensure stability of the structure</p> Signup and view all the answers

    What is the importance of removing a wedge size equal to angle of internal friction of the old construction?

    <p>To ensure stability of the new construction</p> Signup and view all the answers

    What is the purpose of providing spurs projecting from the guide bunds?

    <p>None of the above</p> Signup and view all the answers

    What is the recommended reference for design and construction of guide bunds/launching aprons?

    <p>IRC: 89-1997</p> Signup and view all the answers

    What is shown in Annexure 8/4?

    <p>A typical lay out of a guide bund</p> Signup and view all the answers

    What is the purpose of adopting slope pitching as a remedial measure?

    <p>To prevent disturbance caused by wave lash or other causes</p> Signup and view all the answers

    What type of spur is constructed by driving wooden bullies or bamboos?

    <p>Permeable spur</p> Signup and view all the answers

    What is the function of repelling spurs?

    <p>To deflect the current towards the opposite bank</p> Signup and view all the answers

    What is the recommended action to take when disturbance is noticed in the rear of the guide bund?

    <p>To adopt slope pitching as a remedial measure</p> Signup and view all the answers

    What is the condition required for an apron to launch satisfactorily?

    <p>The material should scour easily and evenly, and the angle of repose of the underlying material is not steeper than that of the stone</p> Signup and view all the answers

    What type of spur is made of solid core, constructed of stones or earth and stones with exposed faces protected by pitching?

    <p>Impermeable spur</p> Signup and view all the answers

    What is the purpose of dumping boulders on the toe of the bank and making up an irregular surface?

    <p>To take action to prevent scouring of the material</p> Signup and view all the answers

    What is a spur/groyne in the context of river training and protection works?

    <p>A structure constructed transverse to the river flow</p> Signup and view all the answers

    What is the primary purpose of permeable structures in river training and protection works?

    <p>To dampen the velocity of flow along the bank</p> Signup and view all the answers

    What is the typical size of a porcupine structure?

    <p>2m to 3m</p> Signup and view all the answers

    What is the purpose of filling the central box of a porcupine with stones?

    <p>To provide stability to the individual unit during floods</p> Signup and view all the answers

    What is the recommended slope of the pilot channel cut?

    <p>At least three times the river’s straight regime slope</p> Signup and view all the answers

    What is the typical height of a crib structure?

    <p>3m to 4m</p> Signup and view all the answers

    What is the purpose of a bell-shaped entrance to the pilot cut?

    <p>To facilitate entry of water into the channel</p> Signup and view all the answers

    What is the purpose of using cross ballies in bally frame structures?

    <p>To provide stability to the structure</p> Signup and view all the answers

    What is used to form boulder crates?

    <p>Hot dip galvanized wire</p> Signup and view all the answers

    How are permeable structures classified according to the type of structure provided?

    <p>Spur type, screen type, and revetment type</p> Signup and view all the answers

    What is the formula used to determine the size of stone required to resist mean design velocity?

    <p>V=4.893d</p> Signup and view all the answers

    What is the purpose of creating a slack flow to induce siltation in the vicinity of permeable structures?

    <p>To prevent erosion of the river bank</p> Signup and view all the answers

    What is the diameter range of crated boulders of circular cross-section?

    <p>600mm to 900mm</p> Signup and view all the answers

    What is the primary purpose of permeable structures in river training and protection works?

    <p>To dampen the velocity of flow along the bank</p> Signup and view all the answers

    What is the recommended chord loop ratio for a successful channel development?

    <p>Greater than 1 to 5</p> Signup and view all the answers

    What is the specific gravity of stones assumed for calculating their weight?

    <p>2.65</p> Signup and view all the answers

    When should a pilot channel cut be made?

    <p>During low flow in the river</p> Signup and view all the answers

    Study Notes

    River Training and Protection Works

    • River training and protection works are essential for railway bridges to prevent erosion and damage from rivers.
    • Characteristics of rivers in mountainous regions:
      • Flatter bed slopes (1 in 50 to 1 in 500)
      • Reduced velocity and sediment transporting capacity
      • Medium size boulders, gravel, and coarse sand in the beds
      • Flashy floods
      • Highly erosive and prone to widening and deepening the channel
    • These rivers are called "Braided" streams and are prone to progressively raising their beds by sediment deposition, also known as "Aggrading" type.

    Protective Measures

    • It is not desirable to locate bridges in reaches with braided streams.
    • Suggested protective measures:
      • Designing chutes with paved aprons and adequate freeboard to lead water down hill slopes
      • Constructing suitably designed catchwater drains on the watershed side
      • Using guide bunds with stone pitching and graded filters to prevent erosion
      • Providing longitudinal and cross drains for good drainage
    • Angle of sweep of curved head and tail head:
      • 120° to 145° for curved head
      • 45° to 60° for tail head

    Guide Bunds

    • Top width of the shank of the guide bund:
      • 6m to 9m to permit plying of trucks and keeping reserve boulders
    • Side slopes of guide bund:
      • Need protection from wave action, water current, wind action, and rain cuts
      • Stone pitching with 20cm to 30cm thick graded filter is recommended
    • Pitching should continue up to the top of the formation for the river side and up to the freeboard level for the approach embankment.

    Maintenance and Failure

    • Maintenance of guide bunds:
      • Reserve of pitching stone should be maintained for use during emergency
      • Track on the guide bund should be maintained in a satisfactory condition
      • Inspect and repair the track after the monsoon every year
    • Failure of guide bunds:
      • If the launching takes place beyond the capacity of the stone in the apron
      • If stones are carried away by high velocity current
      • If slips and blow-outs occur in the bund due to steep sub-soil water gradient
    • Remedial measures:
      • Adding more stone to the apron
      • Strengthening the apron against severe attack
      • Widening the bank to reduce the hydraulic gradient

    Spurs and Porcupines

    • Spurs:
      • Normally spaced at 2 to 3 times the length of the channel
      • Projection of the spurs into the river channel is normally 11% to 15% of the width of the channel
      • Alignment of spurs is kept pointing towards upstream
    • Porcupines:
      • Used for choking secondary channels
      • 4 to 6 rows of porcupines or 6 to 9 rows of cribs are normally used
      • Due care is necessary to tie the weights to the main body of the elements
      • No bed protection is needed for structures made up of porcupines and cribs

    Marginal Bunds and Closure Bunds

    • Marginal bunds:
      • Provided to contain the spread of the river when it spills over its banks
      • Built well away from the active area of the river
      • Slope should be well protected by turfing
      • Inspected every year and necessary repairs carried out
    • Closure bunds:
      • Used to entirely block one or more channels of the river
      • Designed as an earthen dam
      • Inspected every year and necessary repairs carried out

    Cut-off Channels

    • Cut-off channels:
      • Used to divert water away from heavily meandering rivers
      • Help to prevent encroachment into the still water area or Railway embankment

    River Training and Protection Works

    • To shift the point of aggradation downstream, training measures in the form of marginal bunds are required, extending right up to the high ground in the hills.
    • To reduce the erosive action on the marginal bund, suitable slope protection with boulders or concrete slabs, adequate toe protection, and boulders in wire crates forming a flexible type apron may be provided.

    Characteristics of Rivers

    • The bed slope varies from 1 in 500 to 1 in 2,500, and the bed consists of small size gravel and medium sand.
    • The channel has a well-defined course.
    • In some reaches, the river bed slope varies from 1 in 2,500 to 1 in 25,000, and the river flows on an almost flat bed built by its alluvium.
    • The alluvial river meanders as a whole within its “Khadir” (a strip of low land within which a river meanders and its flood rises).

    Suggested Protective Measures

    • Bridging such rivers normally involves constriction in width and provision of guide bunds.
    • Assistance of specialized agencies undertaking hydraulic model studies may be availed of, as considered necessary.
    • Guide bunds and approach embankments must be constructed in one go in one season.
    • In case this is not possible, at least a wedge size equal to the angle of internal friction of the old construction should be removed, and the next construction should be done with proper benching.

    Apron Protection for Guide Bunds

    • An apron is provided beyond the toe of the slope of the guide bund, so that when the bed is scoured, the scoured face will be protected by launching of the apron stone or wire crate containing stone.
    • The thickness of apron is governed by the thickness of pitching on the slopes of the guide bund (T).
    • The width of apron is determined by the depth of scour and is generally kept as 1.5 times the difference between the deepest known scour level and low water level.

    Spurs/Groynes

    • A spur/groyne is a structure constructed transverse to the river flow and projected from the bank into the river.
    • There are two types of spurs: permeable and impermeable.
    • Permeable spurs are constructed by driving wooden bullies or bamboos, filled in with brush wood, with sarkanda mattresses or other suitable material.
    • Impermeable spurs are made of solid core, constructed of stones or earth and stones with exposed faces protected by pitching.

    Permeable Structures

    • Permeable structures can be classified according to function, material, and conditions encountered.
    • Many elements, made up of bamboos, ballies, RCC poles, etc., are arranged in a specific pattern and linked together to form a permeable structure.
    • Different types of elements used for making permeable structures are porcupines, cribs, and bally frames.

    Boulder Crates

    • Boulder crates are formed by filling boulders in a crate made of hot dip galvanized wire.
    • The size of the crate is dependent on the discharge and should be such that the crate is not lifted by water flow.
    • The size of stone required to resist mean design velocity can be assessed by the formula: V=4.893d, where V is the mean design velocity, and d is the equivalent diameter of stone.

    Crated Boulder

    • Crated boulder is a type of boulder crate with a circular cross section of diameter varying from 600mm to 900mm.

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