An embankment failure on soft fissured clay

Construction at the Genesee Power Project near Edmonton required 13 m high approach fills on soft clay for a roadway overpass structure. During construction, the north approach fill failed at a height of about 12 m. An investigation of the failure was carried out and a monitoring program instigated for further fill construction and piling for the overpass structure.The in situ mass strength of the soft clay was found to be controlled by defects such as slickensides rather than the intact strength. A translational mode of failure is considered most likely to have occurred, with computations based on c′ = 0 and determined from normally consolidated samples yielding the appropriate factors of safety to account for the failure. "Tearing" of the fill and overconsolidated crust due to the translational failure mechanism, with associated reduced average shear strengths, also occurred.Monitoring data for construction after the failure are presented and discussed. The effective stress path/yield envelope approach was used to assess the data. An empirical performance criterion for the site was developed for the overpass structure based on the observed deformation rates and computed factors of safety. Key words: embankment, stability, clay, failure, translational failure, effective stress path, fissured.