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ARS Home » Southeast Area » Oxford, Mississippi » National Sedimentation Laboratory » Watershed Physical Processes Research » Research » Publications at this Location » Publication #223200
Title: Seepage caused tension failures and erosion undercutting of hillslopes

Author
item CHU-AGOR, M. - OKLAHOMA STATE UNIVERSITY
item FOX, G. - OKLAHOMA STATE UNIVERSITY
item CANCIENNE, R. - OKLAHOMA STATE UNIVERSITY
item Wilson, Glenn

Submitted to: Journal of Hydrology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/4/2008
Publication Date: 9/30/2008
Citation: Chu-Agor, M., Fox, G.A., Cancienne, R., Wilson, G.V. 2008. Seepage caused tension failures and erosion undercutting of hillslopes. Journal of Hydrology. 359(3-4):247-259.

Interpretive Summary: Although seepage erosion occurs in all three directions, experiments used in previous research to analyze the soil properties controlling this mechanism of hillslope, gully, and bank instability have been made in only two directions. A 50 cm by 50 cm by 50 cm soil box 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 two bulk densities (1.30 to 1.70 g cm-3) and with an outflow face at three 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 face and undercut surfaces at approximately 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 which was concurrent with shear strength reduction by increased soil pore-water pressure, or (2) particle suspension in the seepage flow, particle mobilization, bank undercutting, and bank collapse when the seepage force gradient was less than the initial resistance force of the soil block. For cases experiencing particle mobilization and undercutting, seepage erosion initiated at a single area or as multiple areas across the bank face, largely controlled by the bank angle. A mathematical model was fit to the shapes of the undercut area to describe the maximum depth of undercutting, position of the center of the peak, and the vertical and lateral spreads of the undercut. Trends from this analysis will assist in the development of improved sediment transport functions and the incorporation of this failure mechanism into stability models.

Technical Abstract: Abstract 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 face and undercut surfaces at approximately 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 which was concurrent with shear strength reduction by increased soil pore-water pressure, 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. Trends from this analysis will assist in the development of improved sediment transport functions and the incorporation of this failure mechanism into stability models.