Effects of Gradation and Cohesion on Bridge Scour : Volume 4 : Experimental Study of Scour Around Circular Piers in Cohesive Soils

007952841999Laboratory Final ReportPDFTech ReportFHWA-RD-99-186, NCP 3D3C1-582DTFH61-91-C-00004Bridge piersCohesionCohesive soilsFroude numberMontmorilloniteRegression analysisSandy claysScourSubsonic flowSupersonic flowFlow conditionsBridges and other structuresData and Information TechnologyDesignHighwaysHydraulics and HydrologyI24: Design of Bridges and Retaining WallsBridge ScourPier ScourAbutment ScourGradationCoarse SedimentCohesionClay ScourUnited StatesTurner-Fairbank Highway Research CenterUnited States. Federal Highway AdministrationMolinas, AlbertHosni, Magdy M.Colorado State University. Engineering Research CenterTurner-Fairbank Highway Research CenterUnited States. Federal Highway Administration. Office of Research, Development, and TechnologyFederal Highway AdministrationThe effects of cohesion on pier scour was investigated experimentally using four-foot-wide, eight-foot-wide, and twenty-foot-wide test flumes at the Engineering Research Center, Colorado State University. In the first part of the experiments, clay-sand mixtures with varying amounts of clay were subjected to different approach flow conditions. Experiments were normalized with scour experienced in pure sands. For Montmorillonitic clay mixtures, results showed that in sandy clays increasing the clay content to up to 30% may reduce scour by up to 40%. Beyond a certain clay content (30-40% for the present mixture) parameters such as compaction, initial water content, degree of saturation, shear strength, etc., dominate the pier scour. In the second part of the experiments, these effects were investigated for flow conditions with Froude numbers ranging from 0.2 (subcritical) to 1.4 (supercritical). Regression equations reflecting flow and selected cohesive soil parameters to pier scour were derived for the particular pier and flow conditions that were used in the experiments. These experiments showed that pier scour in cohesive material can be expressed in terms of cohesive soil parameters which can be determined in the field. The equations derived from this study do not attempt to relate effects due to pier geometry, flow depth, gradation of sand in the mixtures, etc., and therefore are not intended for general application. These relationships are derived to explain the variability of pier scour with cohesion properties