Analysis of slope stability of an earth dam due to rapid drawdown effects

A number of shallow slips have occurred in the embankments, which form the Headrace canal for an hydro-electric scheme in Western Ireland. The scheme is of significant strategic importance. The embankments are between 7.0m to 18.0m high and were constructed in the 1920's, using locally available glacial deposits directly on natural soft soils. Field data on the slips has been compared with predictions made by various methods of analysis. These include conventional limit state stability analyses, using both drained and undrained parameters and the Duncan, Wright and Wong (1990) method which is the favoured approach of by U S Army Corps of Engineers. The latter method allows for the variation of undrained strength with in situ stress. It was found that the Duncan et al. Method predicts a factor of safety greater than unity for the failed sections. It is concluded that a conventional limit state analysis using effective stress strength parameters, obtained from high quality laboratory tests, yields the most satisfactory comparison between the measured and predicted results.