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Response of the self-potential method to changing seepage conditions in embankment dams Sheffer, Megan Rae

Abstract

Internal erosion is of great concern to owners and operators of earthfill dams worldwide. The migration of fine-grained soil particles from the dam core in the direction of seepage leads to the development of preferential zones of increased fluid flow, which can compromise the structural stability of the embankment. Standard dam safety monitoring techniques provide sparse sampling of subsurface hydraulic conditions and may not be sufficient to effectively detect the onset of internal erosion. Consequently, there is a clear need for a comprehensive monitoring tool that is sensitive to changing seepage conditions. The self-potential (SP) method is a non-invasive geophysical technique that responds directly to seepage through the phenomenon of streaming potential: the voltage gradient induced by the flow of water through a porous medium. However, current standard methods of SP data interpretation do not provide information about soil properties and seepage flow rates required by geotechnical engineers to assess dam performance. A three-dimensional numerical modelling tool is presented for predicting the SP response to fluid flow based on a comprehensive seepage analysis. Both the hydraulic regime and the resultant electrical potential distribution are calculated based on the distribution of hydraulic and electrical properties within the subsurface. Effective characterization of these parameters is fundamental in achieving an accurate numerical solution. An examination of the influence of internal erosion on hydraulic conductivity, cross-coupling conductivity and electrical conductivity is achieved through theoretical analyses based on published parametric data. The numerical procedure is validated against a theoretical closed-form solution, and is further verified through a comparison with measured values in a controlled physical model of an embankment dam. The capacity for threedimensional analysis and interpretation of the SP response to seepage is illustrated through preliminary models of a zoned embankment dam located in British Columbia.

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