Some practical limitations of total stress and effective stress analyses are discussed. For clays having a liquidity index of 0.36 or greater, φ-equal-zero analyses based on laboratory undrained shear strengths give factors of safety close to the actual factor of safety. However, based on field vane strengths, φ-equal-zero analyses may yield factors of safety which may be too high. The difference between field vane and calculated shear strengths increased as the plasticity index increased. For clays having a liquidity index less than 0.36, φ-equal-zero analyses using laboratory undrained shear strengths give factors of safety that are much too high; but the strength parameters can be corrected by the empirical relationship presented herein. An empirical relationship for correcting vane shear strength is also presented. A method is proposed for predicting the probable success of a φ-equal-zero analysis.
Data suggest that overconsolidated clays and clay shales or clays having a liquidity index less than 0.36 pose the greatest slope design dilemma. An effective stress analysis based on peak triaxial shear strength parameters generally yields factors of safety which are too high; residual shear strength parameters frequently yield factors of safety which are too low. To approximate the theoretical strength of an overconsolidated clay which has undergone a process of softening, the effective stress parameters might be obtained from triaxial tests performed on remolded, normally consolidated clay. It is suggested the soil be remolded to a moisture content equal to the plastic limit plus the product of 0.36 and the plasticity index
