Effect of static shear stress on the cyclic liquefaction of anisotropic sands: a DEM study

To investigate the combined effects of static shear stress and fabric anisotropy on the liquefaction behavior of sand, a series of undrained cyclic triaxial tests were simulated using the Discrete Element Method. These simulations were performed on both vertical and horizontal specimens under a broad range of compressional and extensional static shear stresses. The results show that vertical specimens are more prone to liquefaction under negative static shear stress, while horizontal specimens tend to liquefy faster under positive static shear stress. Increasing static shear stress generally biases strain development towards compression, a trend that is more pronounced in horizontal specimens, which can be closely related to the orientation of the contact normal fabric relative to the loading direction. The redundancy index can serve as a reliable micro-indicator of liquefaction under various static shear stress conditions, independent of initial anisotropy. Initial liquefaction can occur during either loading or unloading, depending on the static shear stress. But it consistently occurs when the Fabric Anisotropy Variable of the contact normals transitions from negative to zero, typically following a preceding shearing stage during which the orientation of the contact normal fabric reverses. To account for the combined effects of static shear stress and fabric anisotropy, a joint invariant incorporating a static shear stress ratio tensor and the contact normal-based deviatoric fabric tensor is defined. The number of cycles to failure shows a nearly unique relationship with this joint invariant for both vertical and horizontal specimens.