LOCAL SCOUR PROTECTION METHODS IN BRIDGE PIER.pptx

Seminar project

1. L OCAL SCOUR PROTECTION METHODS IN BRIDGE PIER Guided by Presented by Paul Jose Muhammed Rafi M A Roll no:19 1

2. OVERVIEW Introduction Scour mechanism Scour protection methods Case study 1 Case study 2 Conclusion References 2

3. Introduction Scour is a common soil- structure interaction problem. It is the removal of sediments such as sand and rocks from abutments . Scour can be defined as the excavation and removal of material from bed and banks of streams as a result of erosive action of flawing water . Bridges play an important role in transportation. Due to the increase in traffic volume more bridges have been built across rivers and canals. 3

4. Contd.. Scour caused by swiftly moving water ,can scoop out scour holes and affects the integrity of the structure . Its has been estimated that 60% of bridge failures result from scour. Bridge failure leads to disorder in transportation systems and may cause loss of life and properties. So scouring consideration in the design of bridges is an important issue. 4

5. Failure of bridge due to scour-Sava bridge, Zagreb 5

6. Malahide viaduct, Dublin 6

7. Contd. There are mainly three main types of scour . General scour Contraction scour Local scour 7

8. General scour Occurs naturally in river channels. This includes the aggradation and degradation of river bed occurs as the result of changes in the hydraulic parameters such as changes in flow rate or changes in the quantity of sediment in the channel. 8

9. Contraction scour It occurs as a result of the reduction in channel’s cross sectional area that arises due to the construction of structures such as bridge piers and abutments. It manifests itself as an increase in flow velocity and resulting bed shear stresses. 9

10. Local scour It occurs around individual bridge piers and abutments. Local scouring around the bridge pier occurs because of flow separation and developing several vortexes around bridge piers. Due to the flow and action of vortexes scour hole created around bridge pier. Such scour hole cause failure of the bridge especially during the river floods. 10

11. Local scour mechanism 11

12. Contd. The flow pattern and mechanisms of scouring are very complicated and the complexity of flow increases with the development of scour hole. The basic mechanism causing local scour at piers is the down flow at the upstream face of the pier and the formation of horseshoe vortex (HSV) at the base of the pier. Down flow is believed to be the primary cause. HSV developed due to the separation of flow at the edge of the scour hole upstream role rolls to form a helical flow. HSV is the consequence of scour , not the primary cause of scour. 12

13. Contd. HSV pushes the down flow inside the scour hole closer to the pier. Separation of the flow at the sides of the pier also creates the wake vortices. These are unstable and shed alternatively from each side of the pier. They act as a little tornados lifting the sediment from the bed and form a scour hole downstream of the pier. 13

14. Scour protection methods In order to protect bridge piers from scouring, engineers have designed and used different types of countermeasures. They are mainly divided into two . Armoring and flow altering countermeasures. Armoring includes usage of riprap stones, tetrapods , reno -mattresses etc. Flow altering countermeasures includes sacrificial piles , collars etc. 14

15. Contd. Ring columns as pier scour countermeasures. Scour reduction using collars. Scour reduction using simultaneous use of cable and collar. Protection using tetrahedral frames. 15

16. Ring columns as pier scour countermeasures This method presents the use of intact used tires in a stream channel. The proposed protection structure comprises flexible rubber rings formed into cylinders in groups of interlocking columns placed upstream of the pier . This will shelter the bridge pier and minimizes the flow impact. 16

17. Schematic diagram of different ring columns 17

18. Configuration of ring columns and bridge pier 18

19. Contd. The were interlaced, generating irregularly shaped protective structure. The impact of water flowing around the protective structure gets reduced due to frictional forces generated by the irregular surface of ring columns. Due to permeability of the ring layers, down flow strength gets reduced. Ring column structures absorbs flow energy due to the flexible rubber composition. As water approaches towards the pier, an ill defined flow field is formed, neutralizing upward and downward kinetic energy and minimizing particle entertainment in water ,thereby reducing scouring of the river bed near bridge pier. 19

20. Flow around the ring columns and pier 20

21. Flow inside the ring columns 21

22. Contd. Because the rings are hollow ,some water seeps through the gaps of rings and flow strength gets reduced. Causes sediment deposition behind the ring columns . Scour holes around bridge piers were filled with sediment , thereby reducing depth of bridge pier scour. 22

23. Scour reduction using collars The basic mechanism causing local scour at piers is the down flow at the upstream face of the pier and the formation of horseshoe vortex (HSV) at the base of the pier . Down flow is believed to be the primary cause. Collars are mainly used to reduce the effect of down flow. 23

24. flow mechanism 24

25. A collar at any level above the bed divides the flow into two regions above and below the collar. This countermeasure acts as an obstacle against the down flow due to which the down flow loses its strength on impingement at the bed. 25

26. Contd. For the region below the collar, down flow and HSV strength are reduced. The efficiency of the collar increases at the lower elevations since low flow can penetrate below it. 26

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