The operator of an industrial facility desired to drain one of its process water supply lakes to excavate material from the bottom of the reservoir. This required lowering the lake further than had ever been done before. The lake is impounded by a zoned embankment dam founded on claystone bedrock. Investigations did not disclose the presence of weak clay layers in the bedrock at the site, but weak layers are known to exist in this rock formation at other locations in the general vicinity. If a sufficiently weak layer is postulated in the foundation, stability analyses indicated that the embankment would be unstable with an empty reservoir. Dam failure would disrupt the planned reservoir excavation project and have negative business impacts on facility operations, but would not threaten lives or offsite property. To address the uncertainty concerning foundation strengths, a simplified probabilistic stability analysis approach proposed by Duncan (2000) was applied to supplement conventional, deterministic stability analyses. In this method, estimates of conceivable low, conceivable high, and most likely values are made for key stability analysis parameters. Stability analyses are then completed using these ranges of parameters and the results are combined using simple equations to calculate the probability of a factor of safety less than 1.0. Based on the resulting probability estimate, combined with the results of deterministic stability analyses, the plan to lower the lake was implemented, while carefully monitoring the embankment for early signs of movement and being prepared to implement contingency measures, if necessary. Measured embankment movements were minimal when the lake was lowered, and the project was completed with no instability problems. This paper will discuss the embankment dam, foundations, stability analyses, and monitoring results.
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