PREDICTING THE EFFECT OF BED MATERIAL SIZE AND GEOGRID ON LOCAL SCOUR AROUND THE BRIDGE PIER IN CLEAR WATER FLOW CONDITIONS
Keywords:
Local scour, Equilibrium scour depth, Equilibrium scour length, Equilibrium scour width, GeogridAbstract
The flow-structure-soil interaction around bridge piers results in an unwanted phenomenon called local scouring. Past evidence shows that local scours are the dominant cause of the bridge pier failures. Equilibrium scour depth in a local scour is the function of the bed material, fluid, and obstruction parameters. This paper presents the effect of the mean size of bed material (D50) on local scour and by choosing the appropriate bed material, the effect of geogrid on local scour was assessed in a clear-water flow condition. Geogrid, having a good capacity for soil reinforcement and stabilization, a high lifetime, low environmental problems, and low cost, has not yet been used to control local scours. So, phase-I of the experiment was conducted in silica sand no. 4 and 5 without using geogrid (Non-Geogrid Case-NG), and the corresponding equilibrium scour hole parameters Sd, Sl, and Sw (Sd, Sl, and Sw are equilibrium depth, length, and width of scour hole respectively) were recorded. In phase-II, geogrid having hexagonal aperture shape with aperture size 4mm×2mm (8 times D50) were used at different placement depths to cover the entire Sd, Sl, and Sw obtained in NG-case. The results showed that the scour hole parameters Sd, Sl, and Sw were found more prominent in silica sand no. 5 than in sand no. 4. Compared to higher placement depths, geogrid was found to be more effective at lower placement depths with maximum efficiency of 67.86% scour reduction at placement depth of 0.2 times Sd. Furthermore, at placement depths equal to or more than Sd, geogrid did not affect local scour.