Comments on the teton dam failure: problems with the use of loess material in earth dam structures, by I. J. Smalley and T.A. Dijkstra

Abstract

The premise of the paper by Smalley and Dijkstra is that loess is not suitable for use as a core material in an earth dam because it does not have the mineralogy and properties required for this application. As noted by Smalley and Dijkstra, the Independent Panel pointed out several design flaws in the Teton dam: (a) complete dependence for seepage control on a combination of deep key trenches filled with loess and a grout curtain; (b) geometric configuration for the key trench that encouraged arching, cracking, and hydraulic fracturing; (c) reliance on special compaction of the loess as the only protection against piping of materials into open joints in abutment rock; (d) inadequate provisions for collection and safe discharge of seepage, which inevitably would occur through the foundation rock and cutoff systems. The highly erosive character of loess is well known (Gibbs, 1962). The prevention of piping by a cracked core constructed of loess requires a proper downstream filter material. It has been documented that the use of concrete sand as a drain for loess will prevent piping (Sanchez, Strutynsky and Silver, 1983). Therefore, whenever loess is used as a construction material in an embankment dam (core, cutoff trench, etc.) a proper drain material must be provided. Many dams, for example Ririe Dam located about 20 miles from Teton Dam, have been constructed with a loess core and properly designed drains (US Army Engineer District, Walla Walla, 1969). Therefore, the authors' premise that loess is not suitable for use as a core material in an earth dam because it does not have the mineralogy and properties required for this application is debatable. Loess is an acceptable material when proper drains are provided.