Effect of orientation of principal stress axes on cyclic liquefaction potential of soils

Soil liquefaction is a large loss of strength and stiffness induced by pore pressure build up often triggered by cyclic motions such as earthquakes. Its occurrence has historically created major collapses and life losses worldwide. Different laboratory techniques have been employed to estimate the cyclic stress ratio to produce soil liquefaction, imposing variation of either shear and normal stresses. However, rotation of principal stress axes invariably occurs during cyclic earthquake motion but this aspect, which has been noted to have a major effect, has not been yet appropriately investigated. Using the Hollow Cylinder Torsional Apparatus, this research has investigated how varying the orientation of principal stress axes (with respect to the material axes) can affect the liquefaction potential of soils. The results of experimental programme demonstrate that there is a critical orientation of principal stress axes, different from the commonly employed triaxial or simple shear con-ditions, for which a minimum cyclic stress ratio is obtained.