Experimental Investigation of Effect of Upstream Sill on Reduction of the Local Scour Hole around Complex Piers

Bridges are one of the main components of the transportation. One of the main causes of bridges failure, especially during flood times, is the development of scour depth near and below the pier and its foundation. Therefore, protecting the pier from scouring is an important issue in the safety of bridges at design stage . In general, scouring can be divided into three types: general scour, scouring due to the contraction of the river section and local scouring such as around bridges and abutments . The flow pattern around the bridge pier include downflow, horseshoe vortices, and wake vortices. By increasing the local shear stress in the vicinity of the bridge piers, these vortices cause erosion in front of the piers and gradually extend to the sides of those. The separation of the flow at the sides of the pier creates so-called wake vortices, which is unstable and shed alternatively from each side of the pier. These vortices act as little tornadoes lifting the sediment particles from the bed. As a result of these processes, a scour hole is formed around the pier and gradually developed. Regarding to the importance of scour around the bridge piers, many studies have been conducted on understanding of governing process and estimation of the scour depth around bridge pier. Although many literature have been documented for single pier but due to geotechnical and economical reasons, multiple-pile bridge piers and complex piers have become popular in bridge design and have attracted interest of researchers. Due to the importance of reduction of scour around the bridges, different countermeasures have been introduced and evaluated in the literature. In general, the countermeasures against pier scour are broadly classified into two categories: (1) flow-altering countermeasures, and (2) bed-armoring countermeasures.One of the devices to reduce scour depth around the bridges piers is an installation of bed sill in vicinity of the foundation. In this research, the effect of the upstream sill on time development and maximum scour depth around inclined bridge pier group was investigated for different pile cap thicknesses, top level installation of pile caps, array and diameter of piles. The bridge pier consist of two rectangular piers with 2.5 and 3.5 cm dimensions which mounted at an angle of 28 degrees on a pile cap with 10 cm width, 16 cm length, 3 and 5cm thickness which placed on an array of 2×2 and 2×3 piles with different diameters. The experiments were performed for relative flow depth (y/D) 6.42, relative pile cap levels (Z/D) 0, 1, 2, 3, 4, relative pile diameter levels (dp/D) 0.57 and 0.85 in clear water conditions. A total of 72 runs was performed for hydraulic and geometric conditions. Comparison of the results indicated that at the level of Z/D =0, by installation of sill at upstream of pile cap, the maximum scour depth decreases 24.5 percent. At the top installation of pile cap of Z/D =1, the maximum scour depth decreases 29 percent of sill installation location at upstream of pile cap. At the level of Z/D =2, by installation of sill at upstream of pile cap, the maximum scour depth decreases 32.5 percent. At the top installation of pile cap of Z/D =3, the maximum scour depth decreases 35 percent of sill installation location at upstream of pile cap. At the level of Z/D=4, by installation of sill at upstream of pile cap, the maximum scour depth decreases 36.8 percent. Comparison of results indicated that installation of upstream sill length of scour hole decreased 26 percent approximately. However, width of scour hole increased 2-3 times of corresponding scour width of non-sill conditions.