Experimental study on incipient motion of non-cohesive and cohesive sediments.

摘要

The current method to predict the erosion in cohesive sediment adopts the erosion equation of non-cohesive sediment, which is very conservative. Understanding the incipient motion of cohesive sediments is essential to predict erosion and to reduce the design cost in cohesive sediment foundations. Determining forces that dominate incipient motion of cohesive sediment would be a fundamental task.;Based on the observation that the incipient motion of cohesive sediments is similar to the ejection of the wall-packed non-cohesive sediment, the incipient motion of a 5 mm-diameter plastic ball, entrained out of a well-packed bed composed of identical balls, was conducted to understand the role of lift force in the entrainment of soil. It is found at the moment of entrainment, the ball springs upward, and then travels downstream after reaching a certain height. A cosine function and a parabolic function were found to adequately describe the initial vertical and horizontal displacements of the particle. Based on the displacement and a simplified force model, the particle's velocity, acceleration and eventually, the lift were derived. The lift force at the moment of particle entrainment is about 1.4-1.8 times of its submerged weight.;An innovative Ex-situ Scour Test Device (ESTD) was designed for the incipient motion study of cohesive sediments. The Kaolinite and EPK Kaolin were eroded in the ESTD by silicone fluid and water. It is found that the infiltration of the outer flow into the soil's inner structure is essential to separate the soil into small clumps, which is the pre-requirement of the cohesive sediments' entrainment. The erosion of the "artificial clay" mixed with silicone fluid reveal that the entrainment of clumps is randomly distributed on the surface in a form of ejection. The eroded clumps have a wide size distribution. There is no clear evidence of the existence of a critical shear stress since small clumps kept ejecting from random positions at different flow velocities.