Effect of seepage flow on incipient motion of sand particles in a bed subjected to surface flow

摘要

Understanding the interaction of the groundwater seepage flow and sediment transport along permeable boundaries is essential for managing soil beds and bank erosion. A significant bank failure may occur due to sediment transport under the boiling condition. In the present study, an experiment is conducted to investigate the effects of injection or upward seepage on the critical shear stress of a cohesionless soil bed subjected to surface flow. The upward seepage force acting on the soil bed particles may reduce the virtual weight of the particles. In turn, it may also reduce the critical shear stress acting on the particles. A theoretical analysis examines how the upward seepage force acts to modify the critical shear stress for the sediment entrainment due to the increase in the hydraulic gradient of the seepage flow. A force analysis reveals that the ratio of the critical shear stress with seepage to that without seepage is dependent on the ratio of the hydraulic gradient of seepage to its value at the quicksand condition. A test apparatus was built, and the effects of injection on the critical shear stress of cohesionless soil bed particles were measured. An acrylic channel with a rectangular cross section was prepared, and a section of a cohesionless soil bed subjected to an upward seepage flow was installed at the bottom of the channel. The experiment demonstrates that the induced seepage affects the dynamics of the channel flow, even though the magnitude of the seepage flow velocity is significantly smaller than that of the free surface water flow. The test results show that the dimensionless critical shear stress of the cohesionless soil bed particles decreases slightly as the hydraulic gradient of the seepage flow is increased when critical shear stress is determined at the upstream edge of the seepage zone. Furthermore, the dimensionless critical shear stress decreases sharply with the increment of the hydraulic gradient of the seepage flow when critical shear stress is determined in the seepage zone. However, it does not decrease in either case, as was theoretically assumed, even when the hydraulic gradient approaches to the critical hydraulic gradient or the quicksand condition.