Dyke 1 of the Bersimis 2 project in northern Quebec, Canada, is an earthfill structure founded on glacial deposits. The stability assessment of a natural slope downstream from dyke 1 was deemed necessary following a dam safety evaluation. Data needed for this assessment is scarce and can only be estimated. The investigations required to reduce the uncertainties can be expensive and have detrimental effects on the safety of the dyke. A practical method for evaluating the influence of uncertainties in the parameters affecting the factor of safety of the slope is described. Investigations to reduce uncertainties are proposed along with costs and possible impacts on the safety of dyke 1. The preferred option would have the greatest effect in reducing uncertainty for the minimum cost and impact on the safety of the structure. For dyke 1, topographic measurements of the external geometry of the slope proved to be the most cost effective option to reduce the overall uncertainty.
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