The effect of clay particles in pore water on the critical shear stress of sand

An equation for τ_cr of sand beds in the presence of interstitial clay particles was developed based on the balance of forces on a sand particle at the surface of the bed. The equation was applied to data collected from a series of laboratory flume experiments on medium sand beds in order to examine the variation of τ_cr with clay weight fraction ψ. A quasi-hydrodynamic model of interstitial clay flow, possessing some of the features of hydrodynamic lubrication, was developed to explain the experimental results through changes in the lubricating effect of the clay suspension between the sand grains as the clay fraction increased.; Based on the application of the equation for τ_cr to experimental data, insight was gained on the behavior of ψ_min , the weight fraction at which the minimum τ_cr occurs at the point of maximum lubrication. It was shown that ψ_min was affected by the frictional resistance offered by the clay suspension and by pore water salinity, but not sand size.; For a bed of 0.83 mm sand to which kaolinite in fresh water was added in varying amounts, measurements yielded ψ_min = 0.04. This result was explained through ψ_min being the fine weight fraction at which the thickness of the clay layer is close to the size of sand grain asperities (∼20 μm) at which a transition from thin-film to boundary layer lubrication occurs. It was thus shown that ψ_min is not the space-filling clay weight fraction as postulated by previous investigators.; The space-filling clay weight fraction was found to be ψ_r, the weight fraction at which the shear stress once again equals τ _co and the point at which the sediment starts to be dominated by the cohesive component. For a bed of 0.83 mm sand to which kaolinite in fresh water was added in varying amounts, measurements yielded ψ _r, = 0.13 which was found to be the point at which the pore volume not occupied by clay becomes negligible (<1% of the total volume).; Through the application of a shear resistance model to the experimental data collected in a Schulze ring shear tester, it was concluded that the strongest lubricating effect occurred when the over-all confining stress was carried by both the sand matrix and the thin interstitial clay suspension.; The effect of salinity on τ_cr and ψ _min was explained through a change in flocculation and a consequent change in the number of edge-to-face, particle-particle contacts.; The two models for clay lubrication presented in this study gave differing values for ψ_min, and this discrepancy warrants further study. (Abstract shortened by UMI.)...