Meaning, measurement, and field application of fully softened shear strength of stiff clays and clay shales

Fully softened shear strength mobilized in first-time slope failures, introduced by Skempton in 1970, corresponds to a random edge-face arrangement and interaction of clay particles in an entirely destructured fabric of stiff clays and clay shales. A series of triaxial compression tests was conducted on reconstituted normally consolidated specimens of 15 stiff clay and clay shale compositions. Based on the laboratory results an empirical correlation for secant fully softened friction angle, [ phi'(fs)]sigma'(s) (n), was developed for clay compositions with plasticity index in the range of 10%-250%, in effective normal stress range of 10-700 kPa. The laboratory measurements confirm an empirical equation for fully softened shear strength in terms of parameters [ phi'(fs)](100) (s) and mfs. The field application of secant fully softened friction angle was examined by stability analyses of 63 first-time slope failures in 38 geologic materials. These include 45 slope failures with a segment of observed slip surface at residual condition and the back-scarp mobilizing fully softened shear strength, and 18 slope failures with entire observed slip surface at fully softened condition. The back-calculated fully softened secant friction angles for first-time slope failures are in good agreement with [ phi'(fs)]sigma'(s) (n) correlation based on laboratory tests.