Characterizing incipient motion of low fines content soils with varying compositions, water contents, and relative densities

Laboratory flume experiments were conducted to quantify the effects of the soil characteristics on the critical shear stress of low fines content soil samples collected from the Montauk shores in New York. The collected soils were reconstituted at five different fines contents, ranging between 0 and 20%. These soil mixtures were composed of two initial water contents, dry of optimum and optimum moistures, and two relative densities, one moderate dense and the other dense. The strength indices of the soils, including the effective cohesion and effective angle of internal friction, were measured using the consolidated undrained (CU) triaxial test. The initiation of erosion tests was conducted on the soil mixtures under a unidirectional steady current condition. The near-bed flow velocity, at the onset of erosion, was used to determine the critical velocity and shear stress for each soil sample. The results indicate that the critical shear stress increases with the fines content and effective cohesion. The soils with the optimum initial water contents demonstrate a higher erosion resistance than those with the initial water contents dry of optimum. The higher relative density appears to overshadow the effects of the fines content such that the critical shear stress of the denser soils remains relatively insensitive to the soil composition. The denser soils compacted at the optimum initial water content show the highest resistance against erosion. The critical Shields parameter is modified to include the fines content, relative density, and initial water content.