Stress Anisotropy Effects on Clay Strength

Abstract

For ease and economy, most commercial laboratories perform consolidated undrained triaxial shear tests using an initial isotropic state of stress. The insitu state of stress for most clay soils, however, is anisotropic. How different are the undrained shear strengths and effective stress friction angles measured under “field conditions” (CAU) as opposed to routine laboratory conditions (CIU)? This study reviews available published data from over 40 different clays consolidated under both isotropic and anisotropic conditions before triaxial shear. Most of these clays were normally‐consolidated, although onethird were also tested at overconsolidated states. Important factors such as strain rates, soil structure, and differences in laboratories were considered to be beyond the scope of this study. For triaxial compression, it is suggested that the anisotropic undrained srength, on the average, may be estimated as 87% of the isotropic strength. Based on few data, a tentative correction factor of 0.60 is recommended for isotropic extension to account for stress anisotropy. The effective stress friction angle appears little affected by initial stress state.