Following the discovery of sinkholes in the WAC Bennett Dam, British Columbia, in 1996, investigations showed that there had apparently been movement of fine material out of the core of the dam. The internal erosion (suffusion) is caused by continuing slow seepage but the concern is for the long term mechanical consequences. In order to be able to predict the result of such movement of material, a class of soil model is required which is able to accommodate changes in both density and grading of the soil. An outline of features of the Severn-Trent sand model – which incorporates effects of density variation – is presented and a suggestion is made for ways in which this model might be extended to include effects of changing particle size distribution. The modelling of erosion (narrowing grading) is complementary to the modelling of effects of particle breakage (broadening grading): simulations for both are shown and remaining uncertainties identified.
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