Seepage erosion mechanisms of bank collapse: three-dimensional seepage particle mobilization and undercutting

Authors: FOX, G - Oklahoma State University; CHU-AGOR, M - Oklahoma State University; CANCIENNE, R - Oklahoma State University; Wilson, Glenn

Submitted to: Proceedings of the World Environmental and Water Resources Congress Conference
Publication Type: Proceedings
Publication Acceptance Date: 2/1/2008
Publication Date: 5/12/2008
Citation: Fox, G.A., Chu-Agor, M.L., Cancienne, R.M., Wilson, G.V. 2008. Seepage erosion mechanisms of bank collapse: three-dimensional seepage particle mobilization and undercutting. Proceedings of the World Environmental and Water Resources Congress Conference. pp. 1-10. doi: 10.1061/40976(316)292.

Interpretive Summary: Seepage out of streambanks causes undercutting when soil particles flow with the water out of the bank. Undercutting can subsequently cause mass failure (slumping) of the bank. Although seepage erosion is a three-dimensional effect, two-dimensional (soil depth and lateral distance into the bank) experiments using soil beds have been used in previous research to analyze for the hydraulic and geotechnical controls on this mechanism of hillslope, gully, and bank instability. A 50 cm 3-dimensional (cube) soil block with a focused inflow reservoir was constructed to investigate the mechanisms of seepage erosion and the three-dimensional nature of seepage undercutting. Experiments included 25-cm tall, sand and loamy sand soil blocks packed at set bulk densities (1.30 to 1.70 g cm-3) and with an outflow face at various angles (90, 75, and 60 degrees). Constant water levels of 15 cm, 25 cm, and 35 cm were imposed on the soil to induce flow. A laser scanner was utilized to obtain the three-dimensional coordinates of the bank and undercut surfaces at 15 to 30 s intervals. The bulk density for the two different soil types controlled which seepage failure mechanism occurred: (1) tension or “pop-out” failures due to the seepage force exceeding the soil shear strength, or (2) particle entrainment in the seepage flow, particle mobilization, bank undercutting, and bank collapse when the seepage force gradient was initially less than the initial resistance force of the soil block. For cases experiencing particle mobilization and undercutting, seepage erosion initiated as single (i.e., concentrated at one point) or as multiple locales (i.e., initiating at several locations across the bank face), largely controlled by the bank angle. As a first approximation, a three-dimensional, five-parameter Gaussian distribution was fit to the undercut shapes to obtain values that describe the maximum depth of undercutting, position of the center of the peak, and the vertical and lateral spreads of the undercut.

Technical Abstract: Seepage flow initiates undercutting, similar to development and headward migration of internal gullies, by liquefaction of soil particles, followed by mass wasting of the bank. Although seepage erosion has three-dimensional characteristics, two-dimensional lysimeters have been used in previous research to analyze for the hydraulic and geotechnical controls on this mechanism of hillslope, gully, and bank instability. A 50 cm cubic soil block with a focused inflow reservoir was constructed to investigate the mechanisms of seepage erosion and the three-dimensional nature of seepage undercutting. Experiments included 25-cm tall, sand and loamy sand soil blocks packed at prescribed bulk densities (1.30 to 1.70 g cm-3) and with an outflow face at various angles (90, 75, and 60 degrees). Constant heads of 15 cm, 25 cm, and 35 cm were imposed on the soil to induce flow. A laser scanner was utilized to obtain the three-dimensional coordinates of the bank and undercut surfaces at 15 to 30 s intervals. The bulk density for the two different soil types controlled which seepage failure mechanism occurred: (1) tension or “pop-out” failures due to the seepage force exceeding the soil shear strength, or (2) particle entrainment in the seepage flow, particle mobilization, bank undercutting, and bank collapse when the seepage force gradient was initially less than the initial resistance force of the soil block. For cases experiencing particle mobilization and undercutting, seepage erosion initiated as unimodal (i.e., concentrated at one point) or as multimodal (i.e., initiating at several locations across the bank face), largely controlled by the bank angle. As a first approximation, a three-dimensional, five-parameter Gaussian distribution was fit to the undercut shapes to derive parameters for the maximum depth of undercutting, position of the center of the peak, and the vertical and lateral spreads of the undercut.