Soil Failure Modes in Undrained Cyclic Loading

The failure modes of saturated sand in undrained cyclic loading are explored. A typical soil element subject to dynamic load exists initially under a nonhydrostatic state of stress. Anisotropically-consolidated samples in triaxial tests with a superimposed cyclic deviator stress represent such elements, except for the rotation of principal stress directions during cycling. The response of these samples depends on the degree of anisotropy and whether shear stress reversal occurs. For isotropically-consolidated undrained tests on saturated sand samples, the pore pressure increases with each cycle, thus decreasing the effective confining stress. The axial cyclic strain also increases as the effective stresses approach the failure state. However, the static axial strain should remain close to zero. For anisotropically-consolidated undrained tests with no shear stress reversal, the pore pressure generally, but not always, increases. In this case, the static axial strain will increase with each cycle, and the cyclic strain will remain smaller than the static strain.