Internal Erosion Phenomena in Embankment Dams Throughflow and internal erosion mechanisms

University dissertation from Stockholm : KTH Royal Institute of Technology

Abstract: Internal erosion phenomena occur in granular material when particles within the porous matrix are transported by seeping fluid due to a hydraulic load that exceeds the erosion resistance of the material. Such phenomena pose a particular threat to embankment dams since in most cases they are concealed processes taking place within the dam’s body or its foundation.  However, these processes can evolve, cause sink holes and even lead to dam failure. This PhD thesis aims to accomplish a better quantitative understanding of two major internal erosion initiation processes, suffusion and concentrated leak mechanisms, which lead to both defect formation in a dam’s body and its foundation and also high throughflow in dams subjected to internal erosion. This understanding has the potential to facilitate numerical modelling and expedite dam safety assessment studies. One of the most likely breach mechanisms for large, zoned embankment dams is the unravelling and instability of the downstream slope of the dam initiated by internal erosion and leakage through the core of the dam. Leakage through an erosion tunnel results in extreme flows in the rockfill downstream of the tunnel and it is therefore of crucial importance to determine the flow resistance in such rockfill under these extremely high throughflows. For this purpose, the throughflow properties of coarse rockfill material were studied under flow conditions similar to those prevailing under assumed leakage scenarios in zoned embankment dams by 1) analysing pump test data from Trängslet rockfill dam in Sweden, 2) performing extensive laboratory experiments with a large-scale apparatus and 3) numerically simulating the three-dimensional flow through coarse, cobble-sized to boulder-sized rock materials, replicating the material used in the laboratory experiments by using Flow-3D software.Results from the field and laboratory tests demonstrate that the parameters of the nonlinear momentum equation of the flow depend on the Reynolds number for pore Reynolds numbers lower than 60,000.  The laboratory experiments in this study covered pore Reynolds numbers as large as 220,000 for diameter distributions in the range 100-160 mm and as large as 320,000 for the range 160-240 mm.Numerical Computational Fluid Dynamics (CFD) studies were also carried out to model the complex flow through the cobbles by overcoming the limits of the Reynolds number in the laboratory experiments (i.e. pore Reynolds number range of 10 to 106). The novelty of the numerical approach lies in the use of results from the large-scale experiments to constrain the three-dimensional numerical simulations with calibration and validation. The validated numerical three-dimensional model was used to conduct numerical experiments which extended the investigation of the parameters of the flow law to Reynolds numbers as large as 106. By applying a Lagrangian particle tracking method, a model for estimating the lengths of the flow channels in the porous media was also developed.  Additionally, the shear forces exerted on the coarse particles in the porous media were found to be significantly dependent on the inertial forces of the flow. These shear forces could be estimated using the proposed equation for developed turbulent flows in porous media.Suffusion and concentrated leak mechanisms were studied for this PhD thesis by means of laboratory experiments to develop a theoretical framework for continuum-based numerical modelling of the evolution of these mechanisms over time. An erosion apparatus was designed and constructed with the capability of applying simultaneous hydraulic and mechanical loading on the test specimens and providing continuous monitoring with mounted instrumentation. Results from the experiments were then used to develop constitutive laws of the soil erosion rate as a function of the applied hydromechanical load for both suffusion and concentrated leak mechanisms. These constitutive laws can be used to define both the initiation of erosion as well as the mass-removal rate. Both the initiation and mass removal rate of suffusion and concentrated leak mechanisms were found to be dependent on the soil in-situ stresses. To reflect this dependency, a new, modified hydromechanical envelope model was developed based on the Mohr-Coulomb criterion for defining the initiation of internal erosion.A three-dimensional electrical-resistivity-based tomography method was also adopted for the internal erosion apparatus and was found to be successful in visualising the porosity evolution occurring in the soil due to suffusion in the test soil. These three-dimensional topographical data could also be used to validate numerical models of the suffusion phenomenon.