Drained shear strength of stiff clays for slope stability analyses

An investigation has been conducted to gain a better understanding of the drained residual and fully softened shear strengths of cohesive soils, the shear strength mobilized in field case histories involving previous and no previous sliding in stiff clays, and develop empirical relationships that describe the stress-dependent nature of the residual and fully softened shear strengths in terms of soil index properties. To investigate the drained residual strength, a Bromhead ring shear apparatus was modified to allow testing of overconsolidated, presheared, remolded soil specimens of different plasticity and clay-size fraction. Residual shear strengths measured using the modified ring shear apparatus were compared to shear strengths measured in conventional reversal direct shear tests on precut remolded specimens and shear strength values back-calculated from well documented field case histories of slopes that underwent previous sliding. The modified ring shear, direct shear, and triaxial compression apparatuses were used to investigate the fully softened shear strength using normally consolidated, remolded specimens. The natural soils used in the residual and fully softened research have a range of liquid limit of 24 to 286%, plasticity index of 8 to 244%, and clay-size fraction of 10 to 88%.; New relationships describing the residual and fully softened friction angles developed from the extensive ring shear testing were presented in terms of the liquid limit, clay-size fraction, and effective normal stress. Most of the existing residual and fully softened strengths relationships are based on only one soil index property and provide a friction angle that is independent of effective normal stress. The importance of incorporating the liquid limit, clay-size fraction, and effective normal stress in estimating the residual and fully softened friction angles was illustrated using field case histories.; Several first-time slides through stiff fissured clays were also studied to estimate the shear strength mobilized during failure. Based on these studies and laboratory test results, recommendations for estimating the field shear strength of stiff fissured clay involved in first-time slides were presented.; A reanalysis of the 1981 upstream slide in San Luis Dam was conducted to evaluate the shear strength reduction caused by the displacement induced by repeated loading. The effect of plasticity and degree of overconsolidation on this shear strength reduction was investigated.