Foundation Failures of Four Rockfill Slopes

Failures of four rockfill slopes were interpreted as large-scale shear tests on the foundation strata of stiff fissured clay. Analyses of the slope failures together with in situ and laboratory shear tests on the clay, have provided information on the relation between the strength in bulk of a stiff fissured clay and that measured by small-scale tests. Two of the failures were progressive while two were non-progressive. For the non-progressive failures and one of the progressive failures, the strength in bulk was approximately equal to the strength measured in small-scale field and laboratory tests. The strength in bulk found from the remaining progressive slide was, however, less than strengths measured in small-scale tests. At all four sites in situ effective horizontal stresses in the clay strata were about equal to the passive pressure resistance of the clay. This fact was used to provide an alternative assessment of the bulk strength of the clay.