دراسة تجريبية لتاثير شكل المسند الطرفي على النحر الموضعي بالقرب من دعامات الجسور

Experimental Investigation for the Effect of Abutment Geometry on the Local Scour at Bridge Piers

Abstract

Local scour at bridge abutments and piers is a major factor contributing to hydraulic bridge failures. Even though a lot of research has been done on this subject, there is a deficiency in research addressing the influence of the interaction between the abutment and pier in terms of their combined effect on scour depth, especially considering the abutment geometry and the space concerning the abutment and the pier. This thesis investigates the impact of various abutment shapes on the volume and shape of local scour development around a single cylindrical bridge pier. Experimental work was conducted under clear-water conditions and steady subcritical flow, using five abutments of different shapes. Three vertical wall abutment models varying in width are (5 cm, 7.5 cm, and 10 cm), the inclined wing-wall abutment, and the trapezoidal abutment. According to the results, the presence of abutment in the vicinity of the bridge pier influences the scour depth of the pier; this influence differs according to abutment shape, abutment width, the distance between them, and hydraulic parameters. Abutment scour decreased, and pier scour increased due to scour hole interference between the two structures when the distance is reduced. Where decreased spacing is from 27 cm to 22.5 cm, 25 cm to 20 cm, and 22.5 cm to 17.5 cm under the same conditions, pier scour increased by about 21%, 29%,3%, and 2% for vertical-wall 1, vertical-wall 2, wing-wall, and trapezoidal abutments model, respectively. In the case of vertical-wall 3, reducing the distance from 22.5 cm to 17.5cm was accomplished by decreasing the percentage of pier scour by 4%. The results show that the pier’s scour depth increases with increasing Froude number and flow intensity, but decreases with increasing flow depth under constant distance. When the abutment is closer to the pier, the scour holes tend to merge and form a larger hole. The comparison between abutment models, according to the value of percentage increase in scour depth, showed that the trapezoidal abutment at x=17.5 cm produced the greatest increase in scour depth, while the vertical-wall 1 abutment at x=27 cm produced the smallest. Furthermore, using the dimensional analysis technique and IBM SPSS 30 new empirical formulas were derived to estimate maximum scour depth around pier affected by abutment shape with a high determination coefficient (R²= 0.948, 0.931, 0.913, 0.916, 0.936) for vertical-wall (V1), vertical-wall (V2), vertical-wall (V3), wing-wall, and trapezoidal models, respectively, displaying a strong relationship between the observed and predicted values. Finally, artificial neural networks (ANNs) were used to simulate the equilibrium scour depth around bridge piers impacted by abutments and have proven to be an excellent tool. The Multi-Layer Perceptron (MLP) model achieved the highest accuracy (R² = 0.983), confirming the reliability of ANN for complicated scour prediction tasks by outperforming nonlinear regression overall. Also, the abutment in both width and length has the greatest influence on the projected amount of scour depth around the pier, per sensitivity analysis. These findings provide a valuable contribution to the understanding of abutment–pier interaction in local scour processes, offering guidance for safer bridge design and more accurate prediction of scour depth. The outcomes are particularly useful for improving hydraulic design criteria, optimizing abutment geometry in relation to pier placement, and enhancing predictive models for bridge foundation stability under varying flow conditions.