A slurry trench cut-off was constructed for the Duncan Dam. Two failures of the trench wall occurred during construction, one of which required relocation of a portion of the cut-off to avoid delaying construction of the dam. The relocated cut-off was successfully constructed from a higher ground elevation, using a denser slurry, produced by mixing in more silt. To analyze the failures a nomograph was developed which can be used to determine the required slurry density for trench wall stability. The stability analysis, first developed by Morgenstern and Amir-Tahmasseb was confirmed. The hydrostatic head drop across the cut-off varies from 85 to 90 % of the gross head on the dam. Differential settlement of almost 10 ft (3 m) has occurred along the cut-off after completion without detrimental effects. Readings over the last 3 years indicate no reduction in the efficiency of the cut-off.