Seepage Force on Interfacial Bed Particles

The effective seepage force on cohesionless interfacial bed particles that comprise a plane bed was determined from the results of slope instability tests and from extrapolation of the results of erosion tests. The seepage flow was perpendicular to the bed and either into or out of the bed for the slope instability tests. Tests were conducted using two columns, one square and the other circular, in cross-section. For uniform sand particles ranging from 0.46 mm diam. to 0.72 mm diam., the seepage force on the interfacial sand grains was determined from slope instability tests to be one-half that on particles well within the bed. Subsequent to the condition of instability for seepage flow vertically upward and into a bed, erosion of the sand bed was measured. From an extrapolation of the results of the erosion tests to a condition of no erosion (which was assumed to correspond to the condition of incipient instability), it is concluded that the seepage force on the interfacial grains is approximately 35% to 40% of that on grains well within the bed.