Surficial failures of alluvial stream banks

Abstract

Erosion of alluvial stream banks is of scientific and economic significance. The principal focus of this investigation was stream bank erosion caused by piping of sand seams, which leads to collapse of overlying strata. Analyses were performed to determine which parameters of the riverbank system are most important in controlling the amount and rate at which piping occurs. Among the failure mechanisms studied were wedge failure of large masses and surficial shearing (slabbing failure) of bank faces. Analyses showed that the most important factors governing piping were permeability and capillary suction in sand seams, slope of sand seams, and water in tension cracks behind the bank face. Flood hydrograph parameters (height of rise and duration of event) were less important, though significant. Whereas unit weight, cohesion, angle of internal friction, and water in tension cracks influenced wedge failures, only unit weight and cohesion directly influenced slabbing failures; however, those parameters influencing piping indirectly also influenced slabbing failures. Results of the analyses agreed closely with field observations. Key words: alluvial stream bank, erosion, sand seam, slope stability, piping, tension cracks, slabbing.