SCOUR AROUND BRIDGE PIER GROUP

Abstract

The safe and economical design of bridge piers requires accurate prediction of the maximum scour depth around them. Considering cylindrical pier as the most commonly used shape, many investigators in the past studied the problem in its various aspects like temporal and equilibrium scours, uniform and nonuniform bed materials, clear-water and live-bed scours etc. More recently attempts were also made towards scour protection and reduction by riprap around the pier, circular collar around the pier, rectangular slot through the pier, delta wing-like fin ahead of the pier etc. This thesis proposes to replace the solid cylindrical pier by a group of three smaller diameter piers, as a scour reduction device. The angular spacing of-the three piers is equal and at 120°. The diameter of the group piers is such that any one of them can pass through the gap between the other two. In such an arrangement the ratio of the diameter of group pier to that of the circumscribing circle is 0.302. The results of the pier group stand a comparison with those of solid cylinder of diameter equal to that of the circumscribing circle. The objective of the present study is to compare the scour due to pier group vis-a-vis solid cylinder, identify the best orientation of the pier group with respect to approach flow and also compare its scour reduction efficacy with that of the collar and slot. Since the pier group has more than one gap or slot through out depth of flow, the effectiveness of collar ou pier group is studied against that of solid cylinder with slot and collar. Experiments were conducted for equilibrium scour in clear water flow at critical condition with four sizes of granular noncohesive sediment. For each size, they included different orientations of the pier group of diameter 11.25 cm and also with a collar of twice the diameter at 1.5 cm above bed for the best identified orientation. Experiments were also conducted on a 1 solid cylinder of 11.25 cm and with a slot of 4 diameter for the same sediment sizes and flow conditions. It was found that the pier group at 300 to the flow i.e. with two piers aligned parallel to the flow, is the best in orientation, although the difference in scour depths for different orientations is only marginal. This result seems logical since the group embodies gaps, equivalent to slots, in all its orientations. In comparison to solid cylinder, the scour reduction due to pier group is of the order of 39%, practically in all orientations, i.e. the pier group is more or less equally effective for all directions of.approach flow. This perhaps is an additional advantage of pier group over slot or delta wing-like fin against changing direction of river course upstream of bridge. The scour reduction of solid cylinder with slot of the present study is 15 to 25% and is in line with the results reported in literature. A collar on pier group is more effective than on solid cylinder..