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Influence of Boundary Seepage on the Erodibility of Overtopped Embankments: A Novel Measurement and Experimental Technique

Author(s): Mahmoud Al-Riffai; Ioan Nistor

Linked Author(s): Ioan Nistor, Mahmoud Al-Riffai

Keywords: Breach overtopping; Erosion; Supercritical flow; Seepage; Unsaturated soil

Abstract: This study presents a novel experimental technique for measuring the transient response of pore-water-pressures (PWP) through a sand bed on a steep slope, typical to homogenous non-cohesive earthfill dams or dikes, under supercritical flow using planar overtopping tests. The use ofseepage-controlelements in 3-D breach overtopping simulations of embankment breach failures carried out in laboratory flumes has been previously investigated by the authors. In this study, the real-time PWPswere recorded using inexpensive tensiometer-transducer probe assemblies (TTPAs), designed, assembled and testedfor planar overtopping tests in hydraulic flumes. The TTPAs, comprised of a 2 mm diameter and 2 cm long ceramic probe, were installed atseveral locations below the downstream embankment slope face to capture the transient PWP response throughout the saturated-unsaturated zone with minimal intrusion in the soil and water mediums. This paper also describes the installation process, calibration, and responsetests for the TTPAs conducted in the Geotechnical and Hydraulic Engineering Laboratories at the University of Ottawa, Canada. A transient flownet analysis was also developed using the measured PWPs to define the magnitude and direction of the exit hydraulic gradient at the soil-water boundary of the eroding downstream slope. The time-history of the slope erosion and its subsequent retreat was captured and analyzed by means of a photogrammetric technique. Using a time-series of bed and water surface elevations, the evolution of the bed slope profile was further converted into an erosion rate and sediment flux, using the 1-dimensional sediment continuity equation. The shear stress acting on the eroding bed, determined by spatial and temporal discretization of the flow parameters, was modified to account for the non-hydrostatic pressure distributionsubjected by upward and downward seepage determined fromthe transient flownet analysis. The dimensionless bed mobility parameters, bed shear stress and sediment transport rate, which are employed in classical sediment formulations, were modified to account for seepage across the porous boundary. The degree of saturation, and consequently the seepage through the embankment models, was varied using different drainage configurations. The results of the modified bed mobility relationship, obtained for one of the embankment configurations using the proposed novel technique, revealed the importance of the seepage mechanism in supercritical flow on steep slopesparticularly with near incipient fluidization conditions.


Year: 2013

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