Studies on the effect of an upstream pier as a scour protection measure of a downstream bridge pier

摘要

The safety of a bridge depends on the depth and volume of local scour around the pier. Therefore scour protection measures are most sought after to increase the stability of the bridge foundation. Scour protection measures broadly divided into bed armouring countermeasures and flow-altering countermeasures. Extensive research studies are available on bed armouring counter measures which were proved satisfactory, however flow altering countermeasures are gaining momentum at present. In this research, the effect of placing a pier upstream of a bridge pier as a protective measure to decrease the local scour at the bridge pier was studied. Pier placed upstream of a bridge pier modifies the flow field, thereby reducing the eroding power of flow, and hence it reduces depth of the scour hole at the bridge pier. It works perfectly if the wake region formed by the upstream pier has a lower velocity as compared to the velocity without the upstream pier. In this process, considerable scour was expected at the upstream pier. The present study attempts to quantify the effect of the mutual interference of front pier and bridge pier on local scour at the bridge pier. The mutual interference of the piers on local scour depends on the shape of the piers, spacing of the piers, flow velocity, flow depth, relative diameters of the piers, sediment size, and oblique distance between the piers and flow oblique angle. These studies were accomplished by measuring equilibrium scour depths in clear water scour in carefully designed and controlled experiments in the laboratory. Two identical piers with series and offset arrangement with several pier diameters of spacing between them were studied. It was observed in ongoing experiments that equilibrium scour depth at rear pier increases as the distance between rear and upstream piers increases. However, the equilibrium scour depth at upstream pier was independent of streamwise spacing between the piers. In offset arrangement the equilibrium scour depth at rear pier increases as the offset distance increases between them.