The wet seam and the Teton Dam failure

Abstract

During the discussion of the June 1976 Teton Dam failure at the 13th ICOLD Congress in New Delhi in October 1979 (1), I concluded that the most probable trigger mechanism for the failure of that dam was the construction defect represented by the wet seam. This conclusion was based on the fact that even after intensive post-failure investigations, the previously postulated trigger mechanisms of hydraulic fracturing and differential settlement could not be substantiated, while the wet seam was found to occur throughout the core material of the dam remnant close to and parallel with the 1974--1975 winter shutdown surface. This wet seam was characterized by low density, high moisture content, and significantly lower temperature than the normal compacted silt (6°C cooler in drill hole PR 100) (2). I stated that the temperature difference implied that frozen ground or frozen borrow soil was present near the 1974--1975 winter shutdown surface and that when this layer melted an avenue of easy access of reservoir water through the core resulted. Since that time the Final IRG Report of the failure has been published, and it attributes the wet seam to causes other than frost action. This International Workshop on Dam Failures presents an oppor tuni ty for me to take another look at the wet seam as a failure mechanism. I find that additional data and information were available which had not been adequately considered in the IRG Final Report or which postulated that Report. This discussion is concerned with this new or not-adequately considered data. Also I feel obliged to respond to some of the criticism of the Teton Dam design, which I believe to have been unfair, by providing pertinent facts that should be considered.