STUDY OF SCOUR REDUCTION AROUND THE CIRCULAR PIER USING COLLAR AND SLOT

Abstract

The erosion of the bed material surrounding any substructure or obstruction is known as scouring. The extent of erosion of the soil bed around the structure due to the score is called scour depth. When bed particles washed away from the bed near the obstruction resulting in the formation of depression in the bed, this depression is called scour hole. Removal of the soil particles from the adjacent area of the bridge supporting structure called pier or abutment is called the local scour. It is due to the meeting obstruction of supporting structure or abutment to the flow. Due to this interference of the of the obstruction to the flow, results in acceleration of the flow, which forms vortex flow around the supporting structure that washes away the bed particle from the adjacent area of the pier or abutment. Scouring process depends on the factors, which include shape of the pier, type of the flow, type of fluid, bed characteristics. If flow forms the vortices around the substructure than it results in scouring. If the fluid posses the characteristics like erosive capacity, then it will lead the washing away of the bed particles from the area surrounding the pier or abutment, shape of the pier also affects scouring. Various shapes have different extent of the scouring, and if the bed material have non cohesion material, then there is more possibility of the individual particle of being getting washed, when the stabling force of the particle is overcome by the force of the approaching flow. On the other hand, if the bed material has cohesion material like clay it requires large force to move the individual particle. If the flow which is coming towards the pier or abutment, carries any soil particle from the past then it is called live bed scour, and if it does not contain any soil particle from the past then it is called clear water scour. When the flow comes with a certain velocity then there is stagnation of the flow at front face of the pier, when this flow comes in contact with the soil bed, it results in the formation of depression at the front of the pier. Due to this, vortices are created in the area near the piers. This vortices goes downstream of the pier and then passes through the sides of pier. This type of vortex flow is called the horseshoe vortex because it resembles with horseshoe. Due to this type of the vortex flow, hole which was formed earlier will go on increasing depth wise, till the shear stress on the soil bed particles becomes less than the critical shear stress. This vortex flow results in the formation of the two slots in the bed, which results in the movement of the sediment particle which are removed from the soil bed. This phenomenon causes the formation of the scour depression at the front face of the supporting . 8 substructure. Now, there is a separation of the flow from the sides of the piers which results in the formation of whirlpool which removes the soil particle from the bed. These vortices are not stable at all. These vortices results in the formation of the scour hole of itself on d/s of the substructure. Now, we have been already discussed the live scour. Now, when the velocity of the flow is more than the velocity of the particles transported by the flow then there is occurrence of live scour. And when the flow velocity is 0.3 times the velocity of the flowing sediments then the clear water scour takes place. There are various other factor like pipe geometry, angle of attack etc which also affects the scouring.