An Experimental Study on Local Scour Protection for Round Slotted Bridge Piers with Collar Variations

Abstract

Structural collapse in bridges frequently results from localized scouring around piers induced by downflow and horseshoe vortices. This study examines the impact of collar diameter and installation height on scour reduction in a cylindrical pier model with circular gaps. A laboratory experiment was performed in a flume utilizing a clear-water scour methodology. The experiment employed a cylindrical pier with a diameter of 3 cm and an average flow velocity of 0.23 m/s. The collar design modifications comprised three outer diameters (2D, 2.5D, and 3D) and two installation heights (hc = 0 cm and hc = 3 cm). The protection efficiency (β) was assessed by evaluating the decrease in the maximum scour depth (dsmax) following an estimated equilibrium period of 45 min. The results demonstrate that the application of collars markedly decreased the scour depth. An inverse relationship exists between the collar installation height and protection efficiency. Specifically, positioning the collar at the channel bottom (hc = 0 cm) was more effective in reducing scouring than positioning it at hc = 3 cm. Moreover, the protection from scouring increases with the outside diameter of the collar. The main findings indicated that the collar with the greatest diameter of 3D and positioned on the channel bottom surface (hc = 0 cm), attained the highest efficiency, namely a scouring reduction exceeding 51.93% relative to unprotected pillars. It was concluded that a large diameter and low placement are essential for effectively disrupting the water vortex, providing a viable solution for bridge pillar scouring issues.

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