Seepage caused tension failures and erosion undercutting of hillslopes

Summary Seepage has been suggested as an important factor in gully and river bank erosion. This study investigated the underlying mechanisms of instability by seepage in laboratory studies. A 25-cm tall, 50-cm wide, and 20-cm long soil block with a focused inflow reservoir was constructed to investigate seepage gradient forces and the three-dimensional nature of seepage particle mobilization (i.e., seepage erosion) and undercutting. Experiments included sand and loamy sand soil blocks packed at prescribed bulk densities (1.30–1.70 Mg m−3) and with an outflow face at various angles (90°, 75°, and 60°). Constant heads of 15, 25, 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 approximately 15–30 s intervals. The bulk density of 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 being concurrently reduced by increased soil pore-water pressure, or (2) particle entrainment in the seepage flow, particle mobilization, bank undercutting, and bank collapse when the initial seepage force gradient was less than the resistance 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), and this result was largely controlled by the bank angle. A five parameter Gaussian function was fitted to the measured three-dimensional 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. The parameters of this distribution can be useful in the development of improved sediment transport functions and the incorporation of this failure mechanism into hillslope stability models.

[1]  Kyoji Sassa,et al.  Failure process and hydrologic response of a two layer physical model: Implications for rainfall-induced landslides , 2006 .

[2]  J. Hooke.  An analysis of the processes of river bank erosion , 1979 .

[3]  E. Langendoen,et al.  Measuring streambank erosion due to ground water seepage: correlation to bank pore water pressure, precipitation and stream stage , 2007 .

[4]  B. Mech.,et al.  River Width Adjustment. II: Modeling , 1998 .

[5]  R. Whitlow,et al.  Basic Soil Mechanics , 1983 .

[6]  N. Casagli,et al.  Monitoring and modelling of pore water pressure changes and riverbank stability during flow events , 2004 .

[7]  Eric W. Weisstein,et al.  The CRC concise encyclopedia of mathematics , 1999 .

[8]  Stephen E. Darby,et al.  Numerical Simulation of Widening and Bed Deformation of Straight Sand-Bed Rivers. II: Model Evaluation , 1996 .

[9]  G. Wilson,et al.  Numerical Modeling of Bank Instability by Seepage Erosion Undercutting of Layered Streambanks , 2008 .

[10]  H. Faulkner.  Piping Hazard on Collapsible and Dispersive Soils in Europe , 2006 .

[11]  David B. McWhorter,et al.  Ground-water hydrology and hydraulics , 1977 .

[12]  Garey A. Fox,et al.  Sediment Transport Model for Seepage Erosion of Streambank Sediment , 2006 .

[13]  Giovanni B. Crosta,et al.  On slope instability induced by seepage erosion , 1999 .

[14]  R. F. Piest,et al.  Gully Wall Stability in Loess-derived Alluvium1 , 1977 .

[15]  Nicola Casagli,et al.  Stability of streambanks formed in partially saturated soils and effects of negative pore water pressures: the Sieve River (Italy) , 1999 .

[16]  A. Howard,et al.  Erosion of cohesionless sediment by groundwater seepage , 1988 .

[17]  S. V. Chitale,et al.  River Width Adjustment. I: Processes and Mechanisms , 2000 .

[18]  S. Darby,et al.  Bank and near-bank processes in an incised channel , 2000 .

[19]  Feike J. Leij,et al.  The RETC code for quantifying the hydraulic functions of unsaturated soils , 1992 .

[20]  N. Sultana,et al.  Triggering mechanisms of slope instability processes and sediment failures on continental margins : a geotechnical approach , 2005 .

[21]  J. Poesen,et al.  Soil erosion in Europe , 2006 .

[22]  G. Wilson,et al.  Erosion of Noncohesive Sediment by Ground Water Seepage : Lysimeter Experiments and Stability Modeling , 2007 .

[23]  D. Tang,et al.  Ground water hydrology and hydraulics: David B. McWhorter and Daniel K. Sunada Water Resources Publications, Fort Collins, 290 pp. , 1979 .

[24]  Garey A. Fox,et al.  Soil properties controlling seepage erosion contributions to streambank failure , 2007 .

[25]  A. E. Jensen,et al.  Threshold phenomena in erosion driven by subsurface flow , 2004, cond-mat/0502213.

[26]  Erosion of Noncohesive Sediment by Groundwater Seepage: Lysimeter Experiments and Modeling , 2007 .

[27]  Muniram Budhu,et al.  Slope Instability from Ground-Water Seepage , 1996 .

[28]  T. Abam.  Factors affecting distribution of instability of river banks in the Niger delta , 1993 .

[29]  D. Fredlund,et al.  Soil Mechanics for Unsaturated Soils , 1993 .