EXPERIMENTAL STUDY ON COUNTERMEASURES WITH GROUPE PILES AGAINST LOCAL SCOURING AROUND A BRIDGE PIER

Abstract

 The local scour at bridge pier has known to be caused by the formation of three-dimensional separated vortices, i.e., the horseshoe vortex at the movable sand bed and the impinging down-flow of water at the upstream of the bridge pier. This horseshoe vortex removes sand from the bed, particularly at the region where the piers are in contact with the bed material. Countermeasures against local scouring of bridge piers include bed-armoring for protection against scouring of the river bed and control of horseshoe vortices with strong downflow by means of flow-conducting structures. In this study, focusing on the latter, which has less environmental impact, three types of submerged group piles as flow-conducting structures were installed directly upstream of the piers to reduce scour of the piers by controlling the horseshoe vortex around the pier and the downflow in front of the piers. The experimental results showed that the maximum scour depth around the piers was reduced by about 40% and the volume of scour holes was reduced by about 6% when group piles were installed, compared to the local scouring of the piers without reinforcement, and that the scour holes around the piers and at the top of group piles showed independent forms if their length in the downstream direction was more than four times the pier diameter.