Embankment failures at Vernon, British Columbia

A section of Highway 97, west of Vernon, B.C., is located over a soft-to-firm, compressible, silty clay subsoil. In addition to an extensive site investigation, the performance of two test embankments was observed for 2 years before construction was begun on the highway grade between them. When the highway fill reached a maximum thickness of about 10 m a failure occurred. The design was then changed to include berms on either side, but a second failure occurred when the grade was rebuilt. An undrained back-analysis of the first failure shows the influence of various variables on the factor of safety and illustrates the difficulty of choosing appropriate strength values for design when the site has a strong crust overlying a weaker layer and there is potential for progressive failure. Observations of settlements, pore pressures, and lateral movements in the subsoil describe the performance of the embankment during construction and reveal the importance of strain softening as a factor in the failures. Comparisons with a variety of similar failures in Canada, Scandinavia, and southeast Asia provide some guidance for future construction over similar materials. Key words : case history, embankment failure, field observation, pore pressures, stability, strength, undrained analysis.