Effective stress analysis of the stability of embankments on soft soils: Discussion

Abstract

In their paper, authors have presented a very interesting study of stability of four embankments on soft soils by effective stress analysis. The authors have concluded that the method of effective stress analysis seems to give better results than total stress analysis and that this method may present important advantages when control of stability is of primary concern. The writer of this discussion agrees with their conclusions and authors must be commended for having presented such well documented investigations. The writer wishes to discuss phenomenon that initial failure defined by effective stress analysis does not coincide with final failure plane. The failures of three test embankments at Narbonne, Lanester, and Cubzac and that of North Ridge Dam in Alberta have clearly demonstrated this phenomenon. The writer believes that final failure is result of progressive failure (retrogressive Failure) that can occur in soft soils that undergo significant strain softening when a localized zone becomes overstressed. The overstressed zone may initiate an internal slipping that results in a transfer of some stress to adjacent areas, which in turn become overstressed. The initial movement may also reduce strength of soil due to strain softening effect. If stress level is sufficiently high and/or strain softening is significant, progressive failure can occur. The simplified Bishop method can be used to analyze critical slip circle but cannot locate final failure zone if failure is caused by progressive failure. Progressive failure could be modelled by finite element method. Although finite element method is a very useful and powerful technique of analysing a wide range of engineering problems, it has its drawback in terms of practical applications: model may be too complicated for practicing geotechnical engineer; limitations and cryptic features may be built in that engineer is not aware of; engineer who is not familiar with model and finite element method may have difficulty in interpreting results; and realistic stress-strain and strength parameters are difficult to obtain. It is believed that it may take some time before practicing engineers can get used to