Factors Affecting Critical Shear Stress of Scour of Cohesive Soil Beds

Nearly 60% of bridge failures in the United States are caused by scour. When water flows in a channel, there is a critical value of riverbed shear stress; below this value, scour will not take place. This critical shear stress is an important parameter in scour prediction. However, a standard method to estimate the critical shear stress does not exist. From a study of 33 erosion function–apparatus (EFA) curves derived for cohesive soils, a method is proposed to estimate the critical shear stress using a hyperbola to fit the data, with some constraints. The intercept of the hyperbola on the shear stress axis is the critical shear stress. The ability to use Shelby tubes for EFA testing allows for careful control of properties when cohesive soils are being reconstituted. The hyperbolic approach was used to determine the critical shear stress for four samples of the same soil consolidated to different consolidation stresses. It was observed that the critical shear stress increased with decreasing water content, decreasing void ratio, increasing unit weight, and increasing consolidation stress. That these soil samples were essentially normally consolidated also suggests that the critical shear stress increased with undrained shear strength ( S u ) because S u /p = constant, where p = consolidation stress. This trend was not observed in NCHRP Project 24-15; this factor indicated the lack of a relationship between critical shear stress and some common soil parameters on the basis of observations from a large EFA test database. When soils from multiple sources are used to derive such relationships, a statistical approach that incorporates mixed effects may be needed.