Optimum Design of Riprap Extension with Different Stone Size at Rectangular Bridge Piers with and without Protective Collar

Riprap layer is the most commonly employed countermeasure against scouring around bridge piers. Due to the large area of high intensity flow around a pier, riprap layer extent is large, especially in the case of skewed rectangular piers. Stable riprap size is designed so that it can resist the highest flow forces in the critical region around a pier. However, as the strength of flow forces around a pier is different, smaller riprap sizes can be used in the areas with lower flow forces to get optimum design. In the present study, extent of riprap layer with different sizes around bridge piers is investigated. Three different rectangular piers with or without an attached protective collar aligned with the flow and skewed at different angles are tested. The ranges of aspect ratio for the piers and skew angle were 3 to 7 and 0o to 20o, respectively. The optimal configurations of riprap extent for each pier condition with different sizes were determined. Experiments showed that in the case of aligned rectangular pier without a collar only 8% of the area around the pier is critical and the remaining 92% area can be protected with about 38% smaller riprap stones. As the skew angle of the pier increases, the critical area increases too. In case of protected pier with collar, the collar prevents the region around the pier and therefore, the extent of riprap layer decreases compared to unprotected pier with similar flow condition.