Erosion of noncohesive sediment by ground water seepage: Lysimeter experiments and stability modeling

Seepage may be a significant mechanism of streambank erosion and failure in numerous geographical locations. Previous research investigated erosion by lateral subsurface flow and developed a sediment transport model similar to an excess shear stress equation. As a continuation of this earlier research, slope destabilization driven by lateral, subsurface flow was studied to further verify the recently proposed sediment transport model. Laboratory experiments were performed using a two-dimensional soil lysimeter. The experiments were conducted on two sandy soils: a field soil (loamy sand) and sieved sand with greater sand content and less cohesion. A series of seven lysimeter experiments were performed for the two different sands by varying the bank slope (90, 60, 45, 36, and 26 degrees). Flow and sediment concentrations were measured at the outflow flume. Pencil-size tensiometers were used to measure soil pore-water pressure. A slight modification of the existing seepage sediment transport model adequately simulated lysimeter experiments for both noncohesive soils without modifying the seepage parameters of the excess shear stress equation, especially for bank angles >45 degrees. The research then determined whether integrated finite element and bank stability models were capable of capturing both small- and large-scale sapping failures. The models predicted large-scale failures for bank angles >45 degrees in which tension cracks formed on the bank surface. The models failed to predict collapses for bank angles <45 degrees in which tension cracks formed on the seepage face. The failure to predict collapse was hypothesized to be due to the assumption of circular arc slip surfaces. More analytically complex stability approaches are needed to capture bank slope undermining.